M Pandey-Pommier

J Richard

F Combes

K S Dwarakanath

B Guiderdoni

C Ferrari

S Sirothia

D Narasimha

Low-frequency radio emission in the massive galaxy

Keywords: Cosmology, large-scale structures of Universe-galaxies, clusters, individual, MACS J0717.5 + 3745radiation mechanisms, non-thermal

Introduction

Cluster of galaxies are the largest gravitationally bound systems in the Universe. They are formed by mergers of smaller clusters and galaxy groups, as well as through continuous accretion of gas. Within the cluster, dark matter represents ∼ 80% of the gravitating matter followed by ∼ 15% of diffuse hot gas and ∼ 5% is in luminous matter within the galaxies and the embedding Intra Cluster Medium, (ICM) (Feretti et al. 2012). Important progress in the study of ICM and their interactions in galaxy clusters have been made with multi-wavelength observations (Boselli & Gavazzi 2006;Markevitch & Vikhlinin 2007), however, less is known about the physical properties and the origin of non-thermal diffuse intra cluster components, that also play a vital role in the evolution of galaxy clusters (Dursi & Pfrommer 2008;Parrish et al. 2009) and constrain the large-scale structure formation scenario.

Send offprint requests to:

M. Pandey-Pommier, e-mail: mamta.pommier at univ-lyon1.fr ⋆ Present address: CRAL-l'Observatoire de Lyon, France

In fact, deep radio observations showed that in some clusters significant diffuse synchrotron emission is present, that indicate the existence of magnetic fields and cosmic rays within the cluster volume (Ferretti et al. 2005a, Ferrari et al. 2008, Kale & Dwarakanath 2009). These diffuse radio sources of different size scales (100 kpc to >Mpc) with surface brightness down to 0.1µJy arcsec -2 at 1.4 GHz are known in about 80 clusters to date, at different cluster locations (center, intermediate, periphery) (Feretti et al. 2012, van Weeren et al. 2010). They are further classified as relics, mini-halos, and halos. They are steep-spectrum (S (ν) ∝ ν -α , α > 1) sources and represent the best evidence for the presence of large-scale magnetic fields and relativistic particles at different cluster locations (relics at the periphery, Giovannini & Feretti 2004;Bonafede et al. 2009a, mini-halos in the central regions of relaxed clusters, Govoni et al. 2009, Murgia et al. 2010, and halos in non relaxed clusters, Feretti & Giovannini 2008).

Observational and theoretical evidence are favoring the hypothesis that the origin of relativistic particles in clusters is related to merging events (so called electron primary model accel- Notes. a- Cohen et al. 2007, b-23rd cycle observations, c-Rengelink et al. 1997, d-Condon et al. 1998, e-Becker et al. 1995 eration). There are several observational probes that agree with primary model predictions. In particular, Mpc scale radio halos have been detected only in merging clusters, showing a correlation between radio and X-ray luminosity, and, in a few cases, a spectral steepening as a function of frequency and distance from the cluster center (e.g. Giovannini et al. 2009;Venturi et al. 2007Venturi et al. , 2008;;Brunetti et al. 2008). The radio halo emission is centrally situated and typically unpolarized (except A2255, Govoni et al. 2005 andMACS J0717.5 + 3745, Bonafede et. al. 2009b). Further, radio halos are expected to result from electron acceleration due to intracluster turbulence (Brunetti et al. 2001;[START_REF] Petrosian | [END_REF]). On the other hand, radio relics are bright elongated or round structures (Feretti et al. 2012) situated at the peripheral regions with variable sizes up to megaparsecs scale. Electrons in radio relics are expected to be accelerated through diffusive shock acceleration (DSA), related to shock waves resulting from merging or accretion shocks in clusters (e.g. Ferrari et al. 2008). Relics are highly polarized (∼ 20 -30%) and irregular in nature with mostly steep spectrum (α ≥ 1) (Drury 1983;Ensslin et al. 1998;Finoguenov et al. 2010;Miniati et al. 2001;Bruggen et al. 2011). A magnetic field of µG level has been detected in radio relics with relativistic particles in cluster outskirts. Furthermore, there are 42 halos currently known, where 39 clusters show at least 1 or more relic source (for a total of 50 relics) (Feretti et al. 2012, Bagchi 2003).

MACS J0717.5 + 3745 is a massive X-ray luminous merging cluster, L X = (24.6 ± 0.3) × 10 44 ergs -1 with ICM temperature 11.6 ± 0.5 keV located at z = 0.5548, discovered in the Massive Cluster Survey (MACS; Ebeling et al. 2001;Edge et al. 2003;Ebeling et al. 2007). It has a very complex morphological distribution showing multiple mergers with shock regions, known at high redshift and hence serves as a most promising target for studies of physical mechanisms driving merger events. Ebeling et al. 2004 discovered a 6-Mpc filamentary substructure leading into the cluster from south-east with optical observations from Subaru 8 m telescope. Ma et al. (2008 and2009) presented optical (HubbleS paceT elescope (HST), ACS; Keck-II, DEIMOS) and X-ray (Chandra) observations, and identified four distinct components in MACS J0717.5 + 3745 undergoing a triple merger event, instead of typically seen two cluster mergers e.g. 1ES 0657 -56 (Markevitch et al. 2002). Ma et al. analysed the galaxy and gas distribution with radial velocity informations and derived that the merger events may have formed the large-scale filament to the south-east of the cluster. Further, the ongoing merger give rise to shock heating which inturn increases the ICM temperature as measured in X-ray (Chandra) observations. The contiguous accretion of gas along the cluster-filament interface may give rise to large size (∼ 1 Mpc) of the high temperature ICM region, as measured in MACS J0717.5 + 3745 X-ray observations. The clus-ter is known to have disturbed morphological distribution with the largest Einstein radius, θ e ∼ 55 ′′ and shallow surface mass distribution (Zitrin et al. 2009). In strong lensing analysis studies four mass peaks were located in agreement with optically identified regions by Ma et al. 2008and 2009(Limousin et al. 2012). The Sunyaev-Zel ′ dovich effect (SZE) studies on the cluster was performed by Mroczkowski et al. 2012 with MUS-TANG bolometer array on the Green Bank Telescope (GBT) between 90 -268 GHz. The MUSTANG SZE data confirm that a pressure enhancement is seen in the shock-heated gas (≥ 20 keV) immediately adjacent to the extended radio emission as seen in low-frequency radio maps of this cluster and as previously indicated from Chandra observations. The radio observations on the cluster have been equally interesting with the discovery of relic structure by (Edge et al. 2003), from the Faint Images of the Radio Sky (FIRS T ) at 20 cm (Becker et al. 1995) with no radio halo structure found in the data. Bonafede et al. 2009b conducted multi-frequency and multi-channel Very Large Array (VLA) observations at frequencies spanning 1.365-4.885 GHz to further probe the cluster morphology and discovered a giant radio halo of 1 Mpc at 4.885 GHz and 1.5 Mpc at 1.365 GHz along with the 20% polarized radio substructure. The lack of Faraday rotation in the observations is not consistent with the assumption of relic produced by a merger shock near the center of the cluster. Further, no steepening of the spectral index across the short axis of the substructure is seen, as would be expected of a radio relic following a merger. Bonafede et al. thus argue that the substructure is not a radio relic, but rather a bright, polarized filament connected with the radio halo. A global spectral index for the entire diffuse radio emission of α 1.42GHz 4.9GHz = -1.27 ± 0.02 with an equipartition magnetic field estimate of ∼ 1.2µG was derived assuming Jaffe-Perola emission model for the integrated radio spectra of the cluster (Jaffe & Perola 1973). van Weeren et al. 2009 further conducted GMRT observations at 0.614 GHz to study the diffuse radio emission within the cluster and found a radio relic of 700 kpc and giant radio halo of 1.2 Mpc with global spectral index of, α 0.61GHz 4.9GHz = -1.24 ± 0.05. The equipartition magnetic field of 5.8 µG was derived at the location of the radio halo from these observations. According to van Weeren et al. 2009, during the merger events the electrons within the cluster that are accelerated by Diffuse Shock Acceleration mechanism have formed the bright central radio substructure. However, the spectral index over the bright substructure varies roughly between -0.8 and -1.5, thus does not fit with merger shock scenario. In fact the Mach number values calculated using the spectral index information lies between 1.7 to 2.8, indicating relic might rather trace an accretion shock. Further, the high temperature regions of the ICM is co-spatial with the bright central radio substructure that is perpendicular to the merger axes of the cluster and large-scale filament, that doesnot rules out the possibility that the relic might trace the merger shock scenario either. However such merger shocks are seen in clusters typically with radii less than 1 Mpc, contrary to MACS J0717.5 + 3745 where a cluster radii of greater than 1 Mpc is measured (Bonafede et al. 2009, van Weeren et al. 2009). Feretti et. al. 2012 have carried out a statistical study of diffuse radio emission in galaxy clusters and have not classified the bright central radio substructure as a 'relic ′ due to its location in the cluster central region.

The main interest of our study for radio emission arising from galaxy clusters and ICM is to explore their spectral nature at very low frequencies (down to 0.235 GHz). With the high sensitivity data at 0.235 GHz we aim to get better estimates of luminosity and magnetic field, one of the fundamental ingredients in the physics of the ICM. By combining multi-frequency radio, Chandra and HS T archival data it is possible to derive important correlations between shock regions and non thermal emissions. Ferreti et al. 2012, Murgia et al. 2010, studied the link between the cluster X-ray properties, their evolutionary stage and their non-thermal emission. These studies have been crucial for a comprehensive physical description of the intracluster medium and suggests that non-thermal emission play an important role in the cluster formation and evolution.

In this paper we present 0.235 and 0.61 GHz continuum observations of MACS J0717.5 + 3745 with the Giant Metrewave Radio Telescope (GMRT). These observations are complemented by VLA and Westerbork Synthesis Radio Telescope (WSRT) archival observations. In Sect. 2 we give a brief overview of radio observations and data reduction. In Sect. 3, the radio images, spectral index maps, and spectra of discrete field sources are presented. Finally the discussion and conclusions are presented in Sects. 4 and 5. Throughout this paper we assume a ΛCDM cosmology with H 0 = 71kms -1 M pc -1 , Ω M = 0.3, and Ω Λ = 0.7. At the distance of MACS J0717.5+3745 (z = 0.5548), 1 ′′ corresponds to 6.348 kpc (www.astro.ucla.edu/ ∼ wright/CosmoCalc.html).

Observation and data reduction

We have observed the massive cluster MACS J0717.5 + 3745 in the 23rd observation cycle using the GMRT at 0.235, and 0.61 GHz. The antennas were pointed at RA = 07 h 17 m 32.63 s and Dec = +37 • 44 ′ 59.70 ′′ (J2000). The visibilities were acquired at 128 spectral channels in continuum mode, in order to have better signal to noise ratio (S/N) and thus reduce the narrow band radio frequency interferences (RFIs). The details of the observations are summarized in Table 1, where we provide the frequency, total bandwidth, observation date, total time on source, beamwidth (FWHM) of the full array, and rms level (1σ) in the full resolution images. The visibility data was processed (calibration, flagging, imaging) with the NRAO ′ s Astronomical Image Processing System (AIPS) software.

GMRT observations at 0.235 and 0.614 GHz

The 0.235 and 0.614 GHz observations consist of one data set each of 5.0h on source, collected on 5 November 2012. The observations were performed for both the Lower Side Band (LSB) and the Upper Side Band (USB) with a total bandwidth of 32 MHz splitted into 128 channels at 0.61 GHz and 8 MHz splitted into 60 channels at 0.235 GHz. The flux density scale was calibrated by the primary calibrators 3C 147 and 3C 286. The source 0735 + 331 was observed for 6 mins scan at intervals of ∼50 min and used as secondary phase and gain calibrator. The bandpass was calibrated using the sources 3C 286 and 3C 147. About 26% of the dataset that was affected by RFIs was flagged using task FLAGIT. Both data sets were averaged in frequency to eight channels to reduce bandwidth smearing effect and facilitate RFI flagging. Several cycles of imaging and phase i Fig. 2. Low resolution radio images of massive galaxy cluster MAS C J0717.5 + 3745 with contours starting at (3σ) and scaling by a factor of 2. Left to right: 0.074 GHz (VLSS; σ =73 mJy/beam; resolution = 80" × 80"), 0.234 GHz (GMRT; σ = 5mJy/beam; with resolution similar to WSRT), 0.325 GHz (WSRT, σ = 3 mJy/beam; resolution = 54"×84"; Coordinates in the map at BJ1950), 0.612 GHz (GMRT, σ = 0.69 mJy/beam; with resolution similar to WSRT), 1.400 GHz (VLA; σ = 0.41mJy/beam; resolution = 45" × 45").

self-calibration was applied to the data to produce initial phase corrected images. A final amplitude and phase self-calibration run was performed to obtain the final amplitude and phase corrected images. At 0.61 GHz both the IFs were simultaneously gridded, flagged, cleaned and imaged, while at 0.235 GHz only one IF was available for imaging. To minimize the effects of non-coplanar baselines while imaging in AIPS the polyhedron method [START_REF] Perley | Synthesis Imaging in Radio Astronomy[END_REF]Cornwell & Perley 1992) was used. To remove the sidelobes effect from the bright sources away from the field center, image size of twice the primary beam radius was cleaned using 3D-technique with multiple facets to account for the non-coplanarity of the incoming wavefront within the large primary beam. Natural weighting was applied to each dataset using robust parameter 0.5 (Briggs 1995), in order to give constant weights to all visibilities and pick up the diffuse structure. The contour levels of the full resolution 0. (Becker et al. 1994), with resolution similar to 0.61 GHz map. We have labeled the brightest radio sources close to the diffuse emission, alphabetically in order to further clarify the discussion on the association of these sources with the halo (ref. Fig. 1).

The theoretical noise in the image is given by

σ thermal = √ 2T sys G √ n(n -1)N IF ∆νt int (1)
where T sys , the system temperature for GMRT = 92 K at 0.61 GHz and 177 K at 0.235 GHz. N IF , the number of side bands recording both RR and LL polarizations = 1 at 0.61 GHz and 0.5 at 0.235 GHz. ∆ν, the bandwidth per sideband = 13.5 MHz at 0.61 GHz and 6.9 MHz at 0.235 GHz. n is the number of working antennas = 27 at both the frequencies. G is the antenna gain = 0.32K Jy -1 at 0.61 GHz and 0.33K Jy -1 at 0.235 GHz. t int is the net integration time = 5 hours on the target source. The expected theoretical thermal noise at 0.61 GHz is typically 31µJy and at 0.235 GHz is typically 115µJy. The noise recorded in our maps are 48µJy at 0.61 GHz and 650µJy at 0.235 GHz. Due to natural weighting applied to the data during imaging, a increase in the recorded noise level by typically 10% as compared to the theoretical noise is seen. Further, at 0.235 GHz a significant increase in the sky temperature and RFI is seen that leads to increase in the noise by a factor of ∼5.6 than the expected noise.

Archive data from VLA and WSRT

To investigate the multi-frequency extended radio emission from MACS J0717.5 + 3745 we have used the archival low resolution data available from the WENSS survey (Rengelink et al. 1997) at 0.325 GHz, the VLA at 0.074 GHz-the VLSS survey (Cohen et al. 2007) and 1.4 GHz VLA's NVSS survey (Condon et al. 1998). The details of the survey parameters are listed in table 1. The degraded resolution maps at lower frequencies increases the sensitivity to completely measure the arcmin size diffuse structure emission that lies completely within the primary beam. Fig. 2 shows the diffuse radio emission at various frequencies from MACS J0717.5 + 3745. The GMRT images at 0.235 and 0.614 GHz are convolved with poorer beam resolution in order to produce low resolution map comparable with 0.325 MHz map.

Results: Optical and radio properties-images and spectral analysis

We analyze the results of the GMRT observations with particular emphasis on the study of diffuse emission in our high sensitivity maps at 0.61 GHz and new image at 0.235 GHz. These observation will help us in determination of the total integrated radio spectrum down to 0.235 GHz for the cluster halo, bright central filament structure and discrete field sources (HT, F, B, C, C1, C2, C3 and C4, ref. Fig. 1) that are related to the cluster and the large-scale optical and X-ray filaments. The new 0.235 GHz map allows us to derive much better the spectral index, and therefore the various probable (re-)acceleration mechanisms, as steep spectra index implies aged radio plasma, with no recent re-acceleration activity. Furthermore, the low frequency image at 0.235 and 0.61 GHz allows us to explore the nature of two new contaminating steep spectrum field sources (C2 and C4) within the halo, that are often not detected at higher frequencies. These compact sources donot contribute into diffuse emission from the halo and have different origin, hence they were subtracted from the data during the flux density measurements. The flux density of radio halo, bright central filament structure and field sources (HT, B and F) at the different frequencies has been determined by using AIPS task IMSTAT, due to their extended nature. The task JMFIT was used to determine the flux density of unresolved sources (C, C1, C2, C3 and C4). We determined the flux density by integrating their radio brightness down to the 3σ level.

3.1. The 'Chairshaped ′ filament structure

The high resolution images at 0.61 GHz and 0.235 GHz (Fig. 1) reveal a bright elongated substructure or 'Chairshaped ′

The extended radio halo

With the high sensitivity and better resolution observation performed with the GMRT at 0.61 and 0.235 GHz, we have managed to map the most distant halo within the galaxy cluster MACS J0717.5 + 3745 (z = 0.5548) down to 0.235 GHz. The diffuse extended radio halo emission is detected all around the bright central radio substructure with a total size of about 1.50 Mpc at 0.61 GHz and 1.58 Mpc at 0.235 GHz. As the noise is high in the FIRST survey data at 1.4 GHz(Fig. 1 rightpanel), the diffuse structure is not detected, though the resolution is similar to GMRT at 0.61 GHz. However, the NVSS low resolution map at 1.4 GHz (Fig. 2 Panel 5) shows a diffuse halo structure of roughly ∼4 arcmin (corresponding to a linear size of 1.58 Mpc), while the apparent angular extent of the halo in the WNESS image is ∼4.67 arcmin, corresponding to ∼1.8 Mpc (Bonafede et al. 2009b). Table 2. shows the physical properties of the halo structures at multi-frequency that are derived from lower resolution maps that allows best to extract the diffuse structure.

The low resolution maps in Fig. 2 shows the extended halo structure clearly detected at 0.074, 0.235, 0.325, 0.61 and 1.4 GHz with resolution (∼ 54 ′′ × 84 ′′ ) similar to WSRT at 0.325 GHz or poorer. The 0.61 GHz map has a very good sensitivity to the extended emission as compared to 0.235 GHz map due to reduced noise level of about 0.69 mJy/beam. At 0.074 GHz less diffuse structure is detected around the central halo due to poorer sensitivity of 73 mJy/beam, while at 1.4 GHz the diffuse structure is slightly resolved out due to better resolution as compared to WSRT map.

The discrete field sources

In addition to the 'Chair-shaped ′ filament, source HT, compact sources C4 and halo structure, Figure 1 and Table 3, also shows the associated field sources (C1, C2, C3, F, B, C) and their detection at optical wavelengths. An elongated source F is detected at the south-west edge of the halo, below the cluster center. This source is identified at redshift, z = 0.1546 (van Weeren et al. 2009), as FR-I type (Fanaroff & Riley 1974) source with compact core and diffuse linear extension (on both sides) of the order of ∼ 1

′ from GMRT 0.61 GHz data (van Weeren et al. 2009). We have identified a radio galaxy, 2MASX J07173724+3744224, as a counterpart for source F from NED catalog. It is interesting to note that another radio galaxy [S GM2010]071741.44+374410.5 located at redshift, z = 0.55 is also identified in the radio map. We also confirm the identification of faint compact source C2 by van Weeren et al. 2009. This source is buried in the extended emission from source F and has AGN as counterparts. Two slightly resolved sources B and C are detected outside the halo in the south-west direction along the X-ray and optical large scale filament (Ebeling et al. 2004). Source B has a compact core with "headtail ′′ morphology and has two variable radio galaxies (NVSS J071741 + 374316, NVSS J071741 + 374313) identified as counterparts from the NED catalog. For the unresolved source C also two radio galaxies (CXO J071753.5 + 374210 and NVSS J071753.3 + 374209) are identified as NED counterparts. The main properties of the identified radio galaxies along with their position coordinates, flux densities, morphology and counterparts are listed in Table 3. We need to take into account the possible contamination from these radio galaxies embedded within the diffuse emission, in order to conduct a proper study of the diffuse emission associated with the ICM. The compact field sources (C1 and C3) are not listed in the table, as they dont show extended diffuse structures and hence donot contribute into the halo emission.

Furthermore, the low-resolution images (Fig. 2) reveal a bridge of diffuse radio emission, which connects the radio galaxies B and C with the cluster halo. This connecting diffuse radio emission bridge has an angular size of ∼ 9arcmin which transforms to linear scale size of 3.66 Mpc and is coincident with the large-scale filament funneling matter onto MACS J0717.5+3745 that has been detected at optical and X-ray wavelengths by Ebeling et al. 2004 andMa et al. 2009. This bridge emission is best detected at 5σ level in the 0.61 GHz low resolution map due to its high sensitivity and confirm the detection at 0.325 GHz by Bonafede et al. 2009b. At 0.234 GHz and 0.074 GHz, the bridge emission is partially detected as the noise is comparatively higher in the map. The faint bridge emission is not detected in higher resolution VLA map at 1.4 GHz as compared to 0.235, 0.325 and 0.61 GHz maps. The detection of radio emission in this area further indicates the presence of magnetic field initially within the filament before the amplification due to merger event. The diffuse bridge emission could also be a part of the whole diffuse halo structure; however, further observations at frequencies below 0.325 GHz with sensitivity below 0.69 mJy/beam are needed with radio facility like LOFAR in order to confirm this result and explain mechanism empowering relativistic electrons to emit at such large linear scales.

Integrated radio spectra for cluster and discrete sources

Radio spectra serves as an important tool for understanding the origin of the relativistic electrons emitting non-thermal emission in galaxy clusters. The non-thermal synchrotron emission detected in cluster of galaxies follow a typical power law decay with S (ν) ∝ ν -α , where S (ν) is the flux density, ν is the frequency and α is the spectral index. To understand the connection between the halo and discrete field sources it is important to understand their spectral nature. We analyzed the integrated radio spectra of the 'Chairshaped ′ filament, halo and sources HT, B, C, C2, C4 and F by complementing the GMRT measurements at 0.235 and 0.614 GHz with the flux densities available in literature and archival data.

The integrated spectral index was computed using the relation LogS (ν)

Log(ν) = α (2)
A global spectral index of, α 0.235 4.86 = -1.33 ± 0.01 was obtained for the 'Chairshaped ′ filament by excluding source HT and C4, between 0.235 and 4.86 GHz, from high resolution maps as shown in Fig. 3 and in agreement with Bonafede et al. 2009b. The error in the spectral index values is obtained from linear fit to the data over the frequency range 0.235 to 4.86 GHz. Overall, the GMRT measurements agree both with the adjacent data points taken from the literature and the straight line fit, thus providing a consistent check of the flux density scale in our im-ages. The 'Chairshaped ′ filament spectral index, α 0.235 0.61 = -1.26 ± 0.37 and α 1.4 4.86 = -1.35 ± 0.1 suggests steepening in the spectra at higher frequencies as compared to lower frequencies (Fig. 3 T op Panel). In the case of radio halo, a global spectral index, α 0.235 4.86 = -0.98 ± 0.03 was computed, from the lower resolution maps by excluding the source F, C2, C1, and C3. Bonafede et al. 2009b = -1.04 ± 0.13 and we have derived a spectral index, α 0.235 0.61 = -1.17 ± 0.37 from our observations (Fig. 3 T op Panel). The error in the spectral index value between 0.235 and 0.61 GHz was calculated using the noise in the image and flux density error measurements (Chandra et al. 2004). The analysis of spectral index suggests a steeping at higher frequencies due to spectral ageing that might lead to high frequency cut-off. Further, the steepness seen at lower frequencies could be due to more population of low energy emitting relativistic particles available within the cluster volume (van Weeren et al. 2012). The total flux of the radio halo at 0.61 GHz and 0.235 GHz are ∼ 162 ± 0.23 mJy and ∼ 492.5 ± 0.52 mJy respectively. Hence the monochromatic radio power at 1.42 GHz computed with α 0.235 0.61 = -1.17 is, P 1.42 = 9.88×10 25 WHz -1 . Thus, MACS J0717.7+3745 is one of the most powerful radio halo ever observed. The power estimate is slightly less then Bonafede et al. 2009b, as we have extrapolated the halo flux at 1.42 GHz using α 0.235 0.61 = -1.17 and might have missed some additional diffuse emission due to limited sensitivity for large angular scale. However, we have provided better flux and radio power estimates at 1.42 GHz as compared to van Weeren et al. 2009 due to our high sensitivity maps.

Important information about the magnetic field in this cluster can be derived from the spectral analysis performed above. The strength of the magnetic field can be estimated by assuming minimum energy densities in the radio sources. The minimum energy density (in units of erg/cm 3 ) is given by

U min = ζ(α, ν 1 , ν 2 )(1 + k) 4/7 ν -4α/7 0 (1 + z) (12-4α)/7 I 0 d -4/7
(3)

where ζ(α, ν 1 , ν 2 ), a constant tabulated in Govoni & Feretti 2004, is 2.29 × 10 -13 , α is -1.17, I 0 the surface brightness is 7.78 µJyarcsec -2 at frequency ν 0 = 1420 MHz, d the depth of the source is (kpc), and k the ratio of the energy in relativistic protons to that in electrons = 100. A volume filling factor of one has been assumed in the above equation. The equipartition magnetic field in terms of minimum energy is thus given as

B eq = ( 24π 7 U min ) (4)
Using the quantities above, we compute the minimum energy, U min = 10.87 × 10 -13 ergs/sec 3 and the equipartition magnetic field strength, B eq = 3.43µGauss. The value of magnetic field derived is slightly higher as compared to van Weeren et al. 2009 due to better flux and linear size of the halo derived in our observations down to 0.235 GHz. This equipartition magnetic field strength is calculated on a fixed range of frequency (10MHz -10GHz), however the upper and lower frequency limits are dependent on the energy of the radiating electrons (Brunetti et al. 1997;Beck & Krause 2005). Hence, considering the low and high energy cut-offs for the particle distribution and taking into account γ min ≤ γ max , the energy boundaries indicated by the Lorentz factor, the revised equipartition magnetic field strength (B ′ eq ) is

B ′ eq ∼ 1.1γ 1+2α 3-α min B 7 2(3-α)

eq

(5) where, γ min is 100. Thus the revised magnetic field strength, B ′ eq = 6.49µgauss and two times larger than the standard equipartition estimate as expected (Govoni & Feretti 2004).

The main goal of this work is to constrain the spectral index of the halo in the MACS J0717.5 + 3745 cluster down to 0.235 GHz. Due to the faint and extended nature of the halo, it is complicated to derive such a constrain on the spectral index distribution. Using the best comparison between sensitivity and resolution in the 0.235 GHz image at a 13.46 ′′ × 10.76 ′′ resolution, we have created the spectral index map for the halo. In the Fig. 4 we present the spectral index image between 0.235 and 0.61 GHz with 0.235 GHz radio contours overlaid at a resolution of 13.3 ′′ × 10.7 ′′ . The uncertainties in the computed spectral index map are also shown in Fig. 5 ranging from 0.09 to 0.9. The error in the spectral index estimate between 0.61 and 0.235 GHz as discussed earlier is consistent with the spec- tral index error map. The spectral index is shown only in the regions where the flux is greater that 3σ at both the frequencies. Additional errors in the spectral index could be noticed due to different calibration errors and UV-coverage at both the frequencies. The overall radio spectrum of the halo is steep. The spectral index ranges from α ∼ -0.85, close to the cluster center, up to about α ∼ -2.3 at the edge. The analysis of the spectral profile in the 'Chairshaped ′ filament structure shows a variation from -0.92 up to -1.5. The flatter spectral index in 'Chairshaped ′ filament is coincident with the high temperatures in merging region (Ma et al. 2008) and in agreement with the ICM temperature and flatness in the spectra correlation derived via Govoni et al. 2004. Source HT shows variation in spectral index from -0.95 near the head region to -1.8 towards the tail region. Such a spectral steepening from the head towards the lobes has been seen in radio galaxies and is believed to be due to spectral ageing due to synchrotron and Inverse Compton (IC) losses. As source HT has much steeper spectra as compared to the 'Chairshaped ′ filament, the radio emission from this region is expected to have different origin.

The extended sources F, B and C presents a power law radio spectrum with a spectral index of about α = -1.09 ± 0.16, α = -0.93 ± 0.03 and α = -0.73 ± 0.02 (Fig. 3 Bottom Panel). This is a quite usual value for active radio galaxies. The study of the spectral index suggest that radio emission in these galaxies are less steeper as compared to that of the halo and the 'Chairshaped ′ filament structure and hence produced by different population of relativistic electrons.

Discussion

Most galaxy clusters that host radio halos and bright filament also show signatures of recent or ongoing merger activities (Ferrari et al 2008). The massive cluster MACS J0717.5 + 3745 is a complex case involving triple merger (Ma et al. 2008(Ma et al. , 2009)). The implications to the spectral index trends in the diffuse radio emission in MACS J0717.5 + 3745 from the proposed origins in merger shocks is explored further here.

Cluster dynamics and ICM Temperature

A schematic representation of the Chandra map of MACS J0717.5 + 3745 (available from NAS A/CXC/I f A/C, Ma et al. 2008Ma et al. , 2009) ) is shown by the colored scale over HST map in (Fig. 6), where the blue colour indicates the hottest region and reddish purple colour indicates the cooler region and the temperatures in between are represented by purple colour. (Fig. 7) shows the radio contours overlaid on Chandra colour image. We have further identified 4 different regions of interest based on optical data, for further analysis within the halo marked as CL1, CL2, CL3, and CL4 shown in Fig. 7. The optical and X-ray studies (Ma et al. 2008(Ma et al. , 2009) ) suggest that the cluster region marked as CL1 has already passed through the main cluster previously in the opposite direction and is likely falling back, whereas the cluster CL2 has unclear origin and has higher speed along the line of sight as compared to the other clusters. The cluster region labeled as CL3 is identified as the core of the main cluster based on the good alignment between the galaxies and hot gas for cluster CL3, along with its motion compared to MACS J0717.5 as a whole. While, region CL4 is a cooler (redder) region of gas and has likely survived from before the collision in the cluster. Further, CL1 and CL4 are also suggested to be originated from the filament. The large region of relatively hot gas (shown in blue) is seen to extend from the left side of cluster CL3 to the right side of region CL4 and may be caused by heating as significant quantities of gas from the filament plough into the main cluster. Thus the cooler region is detected towards the outer edge while the hotter region is detected at the center where merging events occur. Further clusters CL2 and CL4 are identified as the cool cores of clusters merging with the main body of MACS J0717.5 + 3745 (Ma et al. 2008(Ma et al. , 2009)).

A spatial comparison of the low frequency radio image and the X-ray brightness image is shown in Fig. 7. The 'Chairshaped ′ filament structure with flat spectrum (α 0.61 0.235 = -0.92to-1.5) is well placed between the merging region of the 4 clusters where high temperature activities are detected (10.2±2.4 keV, Ma et al. 2009). The merging events causes heating of the thermal plasma due to shock, which inturn re-accelerates the relativistic particles and amplifies the magnetic field. The radio halo with steep spectra of α 0.61 0.235 = -0.85to -2.3 from the central region near the 'Chairshaped ′ filament structure towards the outer edge, traces the overall outer edge and cooler region of the cluster. A correlation between the radio halo integrated spectral index and the average X-ray gas temperature for a large sample of nearby radio halos has been derived recently by Giovannini et al. 2009. Hot clusters (T > 10keV) show an average spectral index = -1.1 to -1.2 and cold clusters (T ≤ 8 keV) show steep spectra (average spectral index = -1.7), according to the correlation derived by Giovannini et al. 2009, Venturi et al. 2013. The radio spectrum of present radio halo with α ∼ -1.17 derived from our low frequency observations further confirms the result that flatter spectra are present in high temperature merging clusters (Ma et al. 2009).

Further Venturi et al. 2013 based on the mass of the cluster derived that, the radio halos show different spectra depending on the energy release into particle re-acceleration during mergers. The trend of flatter spectra is seen in massive hot clusters (T > 8 keV) with halos, while moderately massive clusters (T ∼ 5 -8keV) shows halos with both steep and flat spectra (Cassano et al. 2006, Cassano et al. 2010). Fig. 8, reports the distribution of spectral index (α 235MHz 1400MHz ) in radio-halo clusters versus the X-ray temperature, kT (keV) adapted from Venturi et al. (2013). The position of MACS J0717.5 + 3745 is shown by red point with kT=10.2 ± 2.4 keV. Thus, it is the most distant massive cluster with radio halo showing spectra flatter than ∼ 1 as compared to other hotter clusters suggesting its complex nature. MACS J0717.5 + 3745, being most flattest among the massive galaxy clusters, is an interesting candidate that provides an upper limit for our understanding of mass-spectral index correlation of giant radio halos in distant massive hotter clusters with multiple mergers.

Spectral index distribution within the halo

We further carried out spectral index analysis of specific regions CL2, CL3 and CL4 where halo emission is detected at radio wavelengths and 3 merging clusters are detected from optical observations (Limousin et al. 2012). Fig. 9, shows Chandra contours overlaid on radio spectral index map between 0.61 and 0.235 GHz. A variation in the spectral index distribution is clearly seen to be coincident with the dynamical state of the cluster derived from X-ray images. The spectral index (α 0.61 0.235 ) map analysis for Region CL2 shows a variation of -1.3 towards the outer edge and gradually decreases to -1.65 towards the 'Chair-shaped ′ filament. In the case of region CL3, the spectral index varies from -1.2 from the outer edge to -2.3 towards the 'Chairshaped ′ filament. For region CL4, the spectral index is -1.3 at the outer edge and -0.98 towards the 'Chairshaped ′ filament. Thus, from the overall spectral index distribution analysis, we found that steep spectrum region is co-spatial with the colder region within MACS J0717.5 + 3745 and in confirmation with Ma et al. 2009.

In addition to the above spectral steepening trend, the diffuse radio emission in the halo shows additional spectral steepness at lower frequencies (Fig. 3 T op Panel) with α 0.61 0.235 = -1.17±0.37 as compared to higher frequencies α 0.61 1.42 = -1.04 ± 0.13. Such upturn in the spectrum of the halo at lower frequencies have been measured in A2256 (Kale & Dwarakanath 2010, van Weeren et al. 2012). This trend of upturn in the spectrum could be due to superposition of at least two or more spectra having different steepness that adds up to flatten the spectra at low frequencies. In the case of MACS J0717.5 + 3745 the triple merger events could be the three epochs at which the electrons were accelerated with different input energy giving rise to varied spectral steepness. However, detailed simulations are required to establish the exact mechanism of acceleration of electrons to relativistic energies and the chronology between the three merger events. Typically the synchrotron spectrum is expected to steepen at higher frequencies due to energy losses. Thus putting together the spectral information across the frequency range of 0.235 to 4.86 GHz for the halo region, a scenario consistent with a high frequency steepening due to the energy losses and a low frequency flattening due to a secondary electron population emerges as seen in A2256 (Kale & Dwarakanath 2010, van Weeren et al. 2012).

An alternative scenario could be that the emission of the 'Chairshaped ′ filament could be projected on the halo emission causing a flattening of the spectrum at higher frequencies. The complex spectrum may simply result from a superposition of two components ('Chairshaped ′ filament + halo). Further, the steeper spectral indices in the 1.42 -4.86 GHz integrated spectra could be the act of ageing of the synchrotron spectrum due to energy losses. New observations with LOFAR at lower frequencies (< 0.235 GHz) and complementary observations at high frequencies are needed to determine the shape of the spectrum of the halo (cut-off), to better constrain its origin and image the entire extent of the diffuse bridge emission in the cluster at better sensitivity.

Conclusion

In this work we present a study of the radio emission in MACS J0717.5 + 3745 with more sensitive observation at 0.61 GHz and new map 0.235 GHz with the GMRT. We have confirmed the detection of the radio halo within the massive galaxy cluster down to 0.235 GHz. It is the most powerful and distant cluster ever observed. The results achieved from these observation are summarized below:

-A bright 'Chairshaped ′ filament structure is detected at 0.235 GHz at a linear scale of 853 kpc and at 0.61 GHz at 772 kpc. This 'Chairshaped ′ filament is enclosed in a diffuse halo structure of 1.6 kpc at 0.235 GHz and 1.5 kpc at 0.61 GHz. The complex morphology of the radio cluster indicates the ongoing merger activity within the cluster, supported by optical and X-ray studies. -A radio power of 0.98 × 10 -26 WHz -1 at 1.4 GHz is estimated from the steep radio spectral index of -1.17 derived from GMRT observations with a hint of steepening at lower frequencies. -Using standard equipartition magnetic field assumption, we derived an equipartition magnetic field strength of 3.43 µG for the radio halo. A somewhat higher value of revised magnetic field strength of 6.49 µG is obtained by using lower and higher energy cutoffs instead of fixed frequency cutoffs. -We also confirm the presence of additional diffuse bridge emission of ∼ 3.7 Mpc connecting the central radio halo with the discrete field source B and C, indicating the presence of magnetized ICM at this redshift. -The location of the bright 'Chairshaped ′ filament coincides with regions in the cluster having higher temperatures.

The major axis of the 'Chairshaped ′ filament is located perpendicular to the merger axis of the system. This shows that the 'Chairshaped ′ filament is probably the result of a large-scale shock wave within the cluster where are particles are accelerated during merger events.. -In addition to the halo spectrum, we also analyzed the properties of the cluster discrete sources, specifically sources HT, B, C, C2, C4 and F. Sources HT and B are head-tail sources, while sources C, C2, C4 are unresolved and source F is extended with core and lobe morphology. The spectral index analysis along with NED counterparts suggests that these are radio galaxies with different origin of radio emission as compared to the cluster.

Fig. 1 .

 1 Fig. 1. High resolution radio images of massive galaxy cluster MAS C J0717.5 + 3745 (z = 0.5548) at 0.235 GHz (Top left) at FWHM beam of 13.46 ′′ × 10.76 ′′ resolution, 0.61 GHz (Top middle) at FWHM of 5.70 ′′ × 4.82 ′′ resolution and 1.42 GHz (Top right) at FWHM of 5 ′′ × 5 ′′ resolution. Contours start at (3σ) and scale by a factor of √ 2. Bottom panels: Finding charts from HST for source HT, F (z = 0.1546), B and C with overlaid 0.61 GHz radio contours.

  235 and 0.614 GHz image are shown in the Fig. 1. The radio image at 0.614 GHz has a FWHM beam of 5.70 ′′ × 4.82 ′′ and a noise level of 0.048 mJy/beam (1σ) and at 0.235 GHz has a FWHM beam of 13.46 ′′ × 10.76 ′′ and a noise level of 0.65 mJy/beam (1σ). Fig. 1 also shows the high resolution VLA map at 1.4 GHz obtained from the FIRS T survey archive

  Fig.1also shows the associated field source HT, located mid-way along the elongated central filament and its detection at optical wavelength. Source HT is identified as "headtail" source from radio observation byvan Weeren et al. 2009. We confirm with our observations the detection of the tail structure down to 0.235 GHz and steepening in the spectral index map (Fig.3), as expected at lower frequencies. We have identified radio galaxy NVSS J071734 + 374501, from the Nasa/IPAC Extra-galactic Database (NED) as a likely counterpart for source HT. No optical counterpart was detected in DSS2-red map byEdge et al. 2003. However, the core of the radio galaxy was identified to be coincident with the optical counterpart in the HS T image with a spectral index of ∼ 0.7(Bonafede et al. 2009b).van Weeren et al. 2009 further identified an elliptical galaxy located within the cluster at RA 07 h 17 m 35 s .5, Dec +37 • 45 ′ 0.5 ′′ .5 as a likely counterpart. A steep spectral index of -1.55 was derived from low resolution image byvan Weeren et al. 2009 indicating that the emission from the tail region is partially contaminated by the surrounding emission from the 'Chairshaped ′ filament region. Further, the unresolved source C4 located mid-way below the 'Chairshaped ′ structure is a new detection in our 0.235 GHz and not detected at 0.61 GHz. The source is associated to compact AGN from NED catalog and has highly steep spectrum with cut-off frequency at 0.61 GHz (re f.Fig.3Bottom Panel) .

Fig. 3 .

 3 Fig. 3. Top 1st panel: Integrated radio spectra of the halo, 'Chairshaped ′ filament source and total(halo+'Chairshaped ′ filament source). Top 2nd panel: spectral index variation with respect to frequency for the halo and 'Chairshaped ′ filament source. Bottom 1st panel: Integrated radio spectra of extended sources HT, B and F; Bottom 2nd panel: Integrated radio spectra of compact source C, C2 and C4. The down arrows represent the 3σ detection limit. Data from literature are plotted at 4.86, 1.4, 0.325 and 0.074 GHz along with 0.61 and 0.235 GHz GMRT data.

Fig. 4 .

 4 Fig. 4. Spectral index map between 0.235 and 0.61 GHz at a resolution of 13.3" × 10.7". Contours are from the 0.61 GHz image and drawn at a level of [1, 2, 3, 4, 16, 32, ..] × 0.15mJybeam -1 .

Fig. 5 .

 5 Fig. 5. Spectral index error map between 0.235 and 0.61 GHz image. The error map shows the uncertainties in the spectral index based on the rms noise in both the radio images.

Fig. 6 .

 6 Fig. 6. MACS J0717.5 + 3745: Chandra colour image on HST showing galaxies in 4 different clusters involved in collision, with direction of motion for three fastest moving clusters. The length of the arrow shows the approximate speed in a direction perpendicular to the line of sight. The direction of motion of the cluster appears roughly parallel to the direction of the filament (Credit: X-ray (NAS A/CXC/I f A/C.Maetal.2008, 2009); Optical (NAS A/S T S cI/I f A/C.Maetal.2008, 2009)).

Fig. 7 .

 7 Fig. 7. MACS J0717.5 + 3745: GMRT contours at 0.61 GHz overlaid on X-ray Chandra map with 4 different cluster regions involved in collision indicated by circles.

Fig. 8 .

 8 Fig. 8. Spectral index of radio halos in the 235 MHz-1.4 GHz interval as function of the cluster X-ray temperature (adapted from Venturi et al. 2013 and Giacintucci et al. 2013).The red points marks the position of MACS J0717.5 + 3745

Fig. 9 .

 9 Fig. 9. MACS J0717.5 + 3745: GMRT spectral index map between 0.61 and 0.235 GHz with overlaid X-ray contours from Chandra.

Table 1 .

 1 GMRT observations and data from archive

	Observation	VLS S a	GMRT b	WS RT c	GMRT	NVSS d FIRST e
	Frequency (MHz)	74	235	325	610	1420	1420
	Bandwidth (MHz)	1.56	2×16	120	2×16	2×50	50
	Channel width	122 KHz	125 kHz	5 MHz	125 kHz	50 MHz 3 MHz
	Observation dates	Archive 5 Nov, 2012	Archive	5 Nov, 2012 Archive Archive
	Total on-source time (hrs) Synthesized beam (arcsec)	-80×80	∼4.5 13×11	-54×54cosecδ	∼4.5 5.7×4.8	-45×45	-5×5
	rms noise, σ(mJy beam -1 )	73	0.65	3.1	0.048	0.42	15

Table 2 .

 2 Integrated spectra for cluster and 'Chairshaped ′ filament structureBonafede et al. 2009b filament that was discovered byEdge et al. 2003 as the 'relic ′ within the cluster. This bright elongated substructure runs from north to south with a twist towards south-west. At the north, center and south end of this structure enhanced radio emission is measured. The linear size of the entire structure is 853 kpc at 0.235 GHz and 772 kpc at 0.61 GHz in GMRT maps. We have provided better size estimates (about ∼70 kpc more) with our maps as compared tovan Weeren et al. 2009 due to better sensitivity achieved in our observations. The FIRS T survey map at 1.4 GHz shows the 'Chairshaped ′ filament with a total linear size of 731 kpc. The physical properties of this bright central filament derived from high resolution maps are listed in table 2. In addition to the 'Chairshaped ′ structure,

	Source Beam	ν	Flux density Size
	Relic Halo	(GHz) (mJy) 4.8 16.00 * 1.42 83.00 * 0.61 260±3 0.325 -13.46 × 10.76 0.235 844±8 ′′ × ′′ 18 × 14 5 × 4 5.70 × 4.82 84 × 54 0.074 -80 × 80 18 × 14 4.8 26±1 * 21 × 18 1.42 118±5 * 13.34 × 11.05 0.61 162±0.23 84 × 54 0.325 1200±0.5 * 25.45 × 22.31 0.235 492.5±0.52 (3.9 ′ =)1.58 (Mpc) -0.83 (1.9 ′ =)0.772 -(2.1 ′ =)0.853 -1 1.5 (3.7 ′ =)1.5 1.8 80 × 80 0.074 5800±0.6 1.3

Notes. *

Table 3 .

 3 Integrated spectra for field sources

	Source	RA		ν	Flux density morphology Counterparts	z
		DEC		(GHz) (mJy)		from NED
	Source-HT 07 h 17 m 32.64 s 4.8 37 • 44 ′ 59.66 " 1.42	3.9 * 22.4 *	head -tail	Elliptical galaxy NVSS J071734 + 374501	unknown unknown
				0.61	109.8±3.2
				0.325 -
				0.235 573±7
				0.074 -
	Source-F	07 h 17 m 36.86 s 4.8	-	core+lobes	2MASX J07173724 + 3744224 0.1546
		37 • 44	′ 19.66 "	1.42	9.08 *
				0.61	30.29±0.5
				0.325 10.17 *
				0.235 63.17±3
				0.074 219.24 a
	Source-B	07 h 17 m 41.07 s 4.8 37 • 43 ′ 14.65 " 1.42	6.40 * 18.3 *	head -tail	NVSS J071741 + 374316 NVSS J071741 + 374313	unknown unknown
				0.61	44.79±2.6
				0.325 61.22 *
				0.235 100.93±0.8
				0.074 219.24 a
	Source-C	07 h 17 m 53.50 s 4.8 37 • 42 ′ 09.55 " 1.42	14.70 * 32.30 *	unresolved	CXO J071753.5 + 374210 NVSS J071753.3 + 374209	unknown unknown
				0.61	75.96±1
				0.325 115.18 *
				0.235 109.75±0.6
				0.074 219.24 a
	Source-C2 07 h 17 m 36.37 s 4.8	-	unresolved	unknown
		37 • 44	′ 03.37 "	1.42	-
				0.61	0.98±0.05
				0.325 -
				0.235 1.95±0.5 a
				0.074 -
	Source-C4 07 h 17 m 36.26 s 4.8	-	unresolved	unknown
		37 • 44	′ 43.80 "	1.42 0.61	-0.15±0.1 a
				0.325 -
				0.235 29.64±0.7
				0.074 -

Notes. a = 3σ upper limit from the map,

* Bonafede et al. 2009b

  derived a spectral index, α 1.42 4.86 = -1.27 ±0.02, where as van Weeren et al. 2009 derived a value α 0.61 4.86 = -1.24 ± 0.05. Further, van Weeren et al. 2009 computed a spectral index, α 0.61

	1.42

Acknowledgements. We thank the staff of the GMRT, who made these observations possible. The GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. This research had made extensive use of the NASA/IPAC extragalactic sources data base NED, Data from the NASA/NVSS survey and the public archives from the HST and Chandra. Chiara Ferrari acknowledges financial support by the "Agence Nationale de la Recherche" through grant ANR-09-JCJC-0001-01.

Frequency (GHz)

Source C-alpha= -0.73+/-0.02 Source C2-alpha= -0.72+/-0.3 Source C4-alpha=-5.54+/-0.3


K Quenea

I Lamy
email: lamy@versailles.inra1fr

P Winterton
email: peter.winterton@univ-tlse3.fr

A Bermond
email: bermond@inapg.inra.fr

C Dumat

Interactions between metals and soil organic matter in various particle size fractions of soil contaminated with waste water

Keywords:

water

Only scarce field studies concern the consequences of natural soil organic matter (SOM) and metal interactions on SOM dynamics in soils. We investigated the interactions of four metals (Pb, Zn, Cu and Cd) with the SOM associated to five different size fractions (between 2000 µm and b2 µm) of a sandy top soil contaminated by waste water. Metal, organic carbon and nitrogen concentrations were measured and chemical extractions (with Na 4 P 2 O 7 and EDTA) were also performed to assess the variations of SOM-metal interactions irrespective of the size fraction. In addition, as in that selected contaminated site, maize (C 4 plant), replaced C 3 crops 15 years ago, natural isotopic 13 C labelling gave new insights into SOM turnover. First, the results suggest that metals influence the SOM dynamics in that sandy soil: a C 3 "old carbon" enrichment was observed in the small or clay size fractions, while the "new" C 4 carbon associated with sandy soil particles presents a rapid turnover. Metal accumulation in the clay fraction is attributed to particulate organic matter (poorly associated) and SOM decay which overtime accumulated metals and eventually these metal-SOM associations prevent the biological decomposition of such carbon pools. Moreover, the δ 13 C signals, C/N ratios and results from chemical extractions clearly showed differences in the origin, nature and reactivity of the SOM as a function of the size fraction with consequences on the metal behaviour. Differences were observed between metals studied: Zn seems to be mainly bound to SOM associated with clay particles, while Pb seems to prefer to interact directly with the mineral surfaces versus the SOM.

Introduction

Due to various anthropogenic activities, potentially toxic metals are accumulated in soils, with a risk of water and biota contamination [START_REF] Alloway | Heavy Metals in Soils[END_REF]. The design of pertinent soil criteria for environment protection and remediation relies on an understanding of the mechanisms controlling metal behaviour [START_REF] Cecchi | Multimetal contamination of a calcic cambisol by fallout from a lead-recycling plant[END_REF][START_REF] Arshad | A field study of lead phytoextraction by various scented Pelargonium cultivars[END_REF]. Among the various reactive soil constituents, soil organic matter (SOM) has a large sorption capacity towards metals [START_REF] Yin | The importance of organic matter distribution and exact soil: solution ratio on the desorption of heavy metals from soils[END_REF]. The metal-SOM interaction has various and complex consequences both on the solubility, mobility and bioavailability of metals [START_REF] Impellitteri | Correlation of the partitioning of dissolved organic matter fractions with the desorption of Cd, Cu, Ni, Pb and Zn from 18 Dutch soils[END_REF] and on SOM turnover [START_REF] Boucher | Decomposition of leaves of metallophyte Arabidopsis halleri in soil microcosms: fate of Zn and Cd from plant residues[END_REF].

Generally, solid phase SOM is associated with retention, decreased mobility and reduced bioavailability of trace metals [START_REF] Sauvé | Adsorption of free lead by pedogenic oxides, ferrihydrite and leaf compost[END_REF]. But cationic metals that would ordinarily precipitate at the pH values of most soils are sometimes maintained in solution through complexation with soluble organics [START_REF] Ferrand | Phytoavailability of zirconium in relation with its initial speciation and soil characteristics[END_REF]. Furthermore, metals participate and/or affect biogeochemical cycles in soils and influence the soil biota [START_REF] Barajas-Aceves | Soil microbial biomass and organic C in a gradient of zinc concentrations in soils around a mine spoil tip[END_REF].

Reactive soil components are obviously in interaction [START_REF] Dumat | The effect of removal of soil organic matter and iron on the adsorption of Cs[END_REF] and SOM consists of various components with a wide range of turn-over times, weight, reactivity and chemical nature: therefore the way such interactions are studied is complex [START_REF] Woomer | The importance and management of soil organic matter in the tropics[END_REF]. The studies concerning one extracted soil component are open to criticism, conversely, methods of soil particle-size fractionation can be used to assess the distribution of metals in soils [START_REF] Ducaroir | Evidence of trace metal association with soil organic matter using particle size fractionation after physical dispersion treatment[END_REF] and provide information on the SOM behaviour.

The metal distribution among the different soil components can be estimated using specific chemical reagents in order to discriminate the total metal content over various operationally defined fractions in the solid phase [START_REF] Tessier | Sequential extraction procedure for the speciation of particulate trace metals[END_REF][START_REF] Dumat | Metal ion geochemistry in smelter impacted soils and soil solutions[END_REF]. For instance, sodium pyrophosphate (Na 4 P 2 O 7 ) is used to dissolve SOM: pyrophosphate complexes the metals which stabilize the SOM and the hydrogen bonds between OM and minerals are broken [START_REF] Dumat | The effect of removal of soil organic matter and iron on the adsorption of Cs[END_REF]. In contrast, EDTA is a powerful but unspecific complexing agent: the metals adsorbed both by SOM and oxides are extracted [START_REF] Miller | Effect of sequence in extraction of trace metals from soils[END_REF]. The comparison of Na 4 P 2 O 7 and EDTA metal extractions should improve the estimation of metals specifically linked to SOM.

In soils where the vegetation changes from C 3 to C 4 (or vice versa), the variations in the natural abundance of 13 C ( 13 C/ 12 C ratio) in SOM over time can be used to identify organic carbon sources in the soil [START_REF] Schwartz | Present dynamics of the savanna-forest boundary in the Congolese Mayombe: a pedological, botanical and isotopic ( 13 C and 14 C) study[END_REF] or to determine the SOM turnover rate [START_REF] Balesdent | Measurement of soil organic matter turnover using 13 C natural abundance[END_REF]. Using that tool, [START_REF] Dumat | A study of the trace metal ion influence on the turn-over of soil organic matter in various cultivated contaminated soils[END_REF] highlighted the influence of metals on the SOM turnover on pseudogley brown leached soils strongly contaminated by inorganic atmospheric fallout from a smelter. However, only scarce field studies concern the consequences of natural SOM-metal interactions on SOM dynamics: supplementary studies in various pedologic and pollution contexts are therefore needed.

In the present work a contaminated sandy top soil under maize that had received waste water applications was used to investigate the interactions between four metals (Pb, Cd, Zn and Cu) and SOM. The tested hypothesis throughout these experiments was a significant influence of organic matter-metal interaction both on SOM dynamics and metals fate.

Materials and methods

Soil sample preparation and soil properties

The studied sandy luvisols, in the north of the Paris basin, has been precisely described by [START_REF] Lamy | Use of major-and trace-element correlations to assess metal migration in sandy Luvisols irrigated with wastewater[END_REF]. Moreover the site management history is widely exposed in several publications [START_REF] Bourennane | Enhancing spatial estimates of metal pollutants in raw wastewater irrigated fields using a topsoil organic carbon map predicted from aerial photography[END_REF][START_REF] Lamy | Use of major-and trace-element correlations to assess metal migration in sandy Luvisols irrigated with wastewater[END_REF][START_REF] Dère | Trace metal inputs reconstitution and migration assessment in a sandy Luvisol after 100 years of massive irrigation with raw wastewaters[END_REF][START_REF] Dère | Long-term fate of exogenous metals in a sandy Luvisol subjected to intensive irrigation with raw wastewater[END_REF]: wastewater irrigation of garden markets occurred from 1899 to 2002 (average 40,000 m 3 ha -1 year -1 ).

In comparison with experiments performed in controlled laboratory conditions, only few study dealing with organic matter-metals interactions concern field experimentations based on concrete case. Our study offers an original experimental design from this point of view. The studied site has been considered only after metal contamination has been noticed, which means: several years after that the spreading has been forbidden, thus the waste water was not available for analysis. However [START_REF] Dère | Trace metal inputs reconstitution and migration assessment in a sandy Luvisol after 100 years of massive irrigation with raw wastewaters[END_REF][START_REF] Dère | Long-term fate of exogenous metals in a sandy Luvisol subjected to intensive irrigation with raw wastewater[END_REF] observed that metals concentrations in the sandy topsoil have increased by tenfold compared to the pedogeochemical background and concluded to a soil metal pollution due to waste water spreading.

Field sampling was oriented to collect representative soil samples weighing 40 kg in closed bags. After mixing, all the samples were air dried and sieved to exclude particles larger than 2 mm to give the bulk soil. Thereafter trace metal, carbon and nitrogen concentrations were measured. The cation exchange capacity (CEC), determined by the ammonium acetate method at pH 7 (Metson's method, AFNOR NF X 31130) was 4.9 Cmol( + ) kg -1 and the pH water = 7.1.

Physical fractionation of the various soil size fractions

Before particle size fractionation, an initial physical dispersion of the soil samples was performed by end-to-end shaking overnight in pure de-ionised water in order to minimise the potential variations of metal speciation that could occur with chemical dispersion. The solid/ solution ratio was 1/5 (m/V) with the use of glass balls favouring the soil dispersion. The suspensions were then sieved through a series of grids (AFNOR French reference) with decreasing mesh size (2000, 200 and 50 µm). Then the fractions (20 µm b size b 50 µm) and (2 µm b size b 20 µm) were obtained by sedimentation at normal gravity (g), during times determined by Stokes' law. The b2 µm fractions were finally obtained after concentration of the suspensions by centrifuging at 39,200 g for 1 h at 15 °C on a Beckman Avanti J30I apparatus. All the particles recovered were air dried at 60 °C, weighed, ground (200 µm) and homogenised.

Measure of metal concentrations and chemical extractions

The total metal concentrations in the various soil size fractions were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES) on a Jobin Yvon JYESS 4.03 apparatus, after triple acid digestion (HF, HCl and HNO 3 ) of the solid samples. The digestion protocol and measurement accuracy were checked by reference to a known standard soil (GRX2). It indicated the efficiency of the digestion protocol and of the ICP-AES assay (within ±1%). All the results correspond to the mean of two replicates.

Two extractions were performed on separate sub-samples. The Na 4 P 2 O 7 extractions were performed with the procedure of [START_REF] Vieiriae | Sequential extraction of copper and zinc from sewage sludges. Use of organic solvents[END_REF]. The contact time was 16 h, with 0.1 M Na 4 P 2 O 7 at pH = 10: maximum efficiency is for pH N 9 [START_REF] Asher | Effects of pyrophosphate, EDTA and DTPA on zinc sorption by montmorillonite[END_REF]. The EDTA extraction conditions were chosen such that EDTA was in excess at equilibrium [START_REF] Gesthem | EDTA extractability of trace metals in polluted soil: a chemical-physical study[END_REF]. The contact time was 24 h, with 0.05 M EDTA at pH = 6.5. The various solid phase fractionations were performed at room temperature (20 °C), with a solid/solution ratio of 1/10, with two replicates per sample. In order to obtain a solution free of solid material before filtration, a strong centrifugation (50,000 g) was performed on the Na 4 P 2 O 7 extracts (20,000 g for EDTA extracts). After each extraction, the soil was rinsed with deionised water to limit precipitation of metals and to improve the extraction efficiency. The extracts and rinses were analysed after filtration (0.2 µm), for metals using ICP-AES. Analyses of soil extractions were calibrated using standard solutions made with the extracting agents.

Amounts of organic carbon and stable carbon isotopic signals (δ 13 C)

The ploughed contaminated topsoil (0-30 cm), which has been under maize (C 4 metabolism with δ 13 C from -10 to-12‰) for 15 years, was previously used for garden markets cultures, presenting a C 3 metabolism (δ 13 C from -27.6 to -30.5‰). That C 3 -C 4 sequence allows natural labelling of organic matter in relation with the dynamics of the carbon sources [START_REF] Bender | Variations in the 13 C/ 12 C ratios of plants in relation to the pathway of photosynthetic carbon dioxide fixation[END_REF][START_REF] O'leary | Carbon isotope fractionation in plants[END_REF][START_REF] Collister | Compound-specific δ 13 C analyses of leaf lipids from plants with differing carbon dioxide metabolisms[END_REF]. Organic carbon and nitrogen contents of the soil samples were determined by dry combustion in a Carlo Erba NA 1500 CHN elemental analyser. Stable carbon isotope ratios of SOM and plants were measured on a Fisons SIRA 10 Isotope Ratio Mass Spectrometer. 13 C natural abundance was expressed in δ units, by reference to the international standard PDB (marine carbonate: belemnite from the PEE Dee formation) [START_REF] Craig | Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analysis of carbon dioxide[END_REF], according to the following equation:

δ 13 Cx = 13 C= 12 C h i sample -13 C= 12 C h i PDB 13 C= 12 C h i PDB ×1000:
All samples were sieved to 1 mm, ground to 100 µm and homogenised. Organic carbon, nitrogen and δ 13 C were measured in duplicate, with previous elimination of carbonates by acidification. According to [START_REF] Balesdent | Measurement of soil organic matter turnover using 13 C natural abundance[END_REF] and [START_REF] Boutton | Stable carbon isotope ratio of soil organic matter and their use as indicators of vegetation and climate change[END_REF], mean δ 13 C (‰) reference values for carbon isotopic composition are around -27 for C 3 plants (mainly trees, rice, wheat, certain grass species), -14 for C 4 plants (mainly maize, tropical grasses and certain other grasses) and -8 for current atmospheric CO 2 .

Statistical data treatment

Data obtained were subjected to analysis of variance (ANOVA) with one factor, using the software Statistica, Edition'98 (StatSoft Inc., Tulsa, OK, USA). For each bioassay, mean values with different letters represent a significant difference (p b 0.05) as measured by the LSD Fisher test. Letters are reported on the figures. The results of total metal and organic carbon concentrations in the different soil particle size classes are given in Table 1. Whatever the metal considered, the concentration strongly increased as the size decreased according to [START_REF] Harter | Role of the metal-organic complexation in metal sorption by soils[END_REF]: 30-40 times for Zn, Pb and Cu and 375 times for Cd. The clay fraction is also that with the highest levels of organic carbon and nitrogen. The concentrations measured increased from 1.3% C and 0.04% N in the 2000-200 µm fraction to 7.2% C and 0.7% N in the clay fraction. The proportion of the different soil particle size classes was multiplied by the concentration of the metals in each class to give the actual quantity of each metal in each size class (Fig. 1). The determination of these different metal pools demonstrated the relevance of focussing both on the clay fraction and on the sum of the different coarse fractions in order to study the SOM-metal interaction.

Results

Amounts of trace metals, carbon and nitrogen in the various size fractions

Chemical extractions

The results of the chemical extractions performed on the sandy top soil size fractions for Zn (A), Cu (B), Pb (C) and Cd (D) are reported both in Fig. 2: quantities of metals extracted (mg of the metal per kg of soil fraction) by EDTA or Na 4 P 2 O 7 and in Fig. 3: ratios of the quantities of metals extracted by EDTA or Na 4 P 2 O 7 divided by the total metal content of that size fraction. The data of Fig. 2 indicate that the quantities extracted per unit weight of the fraction increased with decreasing particle size. However, for the ratios (Fig. 3), the reverse trend was generally observed, in particular with Na 4 P 2 O 7 extraction: a large proportion of metals extractable from the coarse fraction and in the 20-2 µm fraction.

In the clay fraction, more Zn and Cu were extracted with Na 4 P 2 O 7 than with EDTA (Fig. 2). Moreover, higher proportions of Zn and Cu were extracted by Na 4 P 2 O 7 than Cd and Pb (Fig. 3) suggesting two different patterns depending on the metal.

Use of the δ 13 C signal to study the organic matter dynamics in relation with metals

Fig. 4 gives: (a) δ 13 C and (b) C 4 /C: ratio between carbon of C 4 origin and total organic carbon for the different size fractions [START_REF] Balesdent | Measurement of soil organic matter turnover using 13 C natural abundance[END_REF][START_REF] Dumat | A study of the trace metal ion influence on the turn-over of soil organic matter in various cultivated contaminated soils[END_REF]. The value of δ 13 C for the topsoil was between -21.8‰ for the coarse fraction and -23.2‰ for the fine fraction. No δ 13 C values for organic matter from waste water are available from the literature, however a δ 13 C signature of -25.4‰ measured for anaerobically digested sludge from South West France [START_REF] Parat | The organic carbon derived from sewage sludge as a key parameter determining the fate of trace metals[END_REF] could give a general idea. In fact, the quantity of organic carbon in waste waters is generally very low compared to the carbon inputs from plants, therefore the influence of the organic matter from waste waters on the δ 13 C signature of the SOM can be ignored considering the much greater inputs of organic matter from C 3 and C 4 plants.

Discussion and conclusion

SOM and metal interaction

The comparison between EDTA (a powerful but unspecific metal chelating agent) and Na 4 P 2 O 7 (which mobilises metals bound to SOM) (Fig. 2) suggests that a significant part of all the metals is bound to the SOM. That hypothesis is moreover supported by total concentrations results: carbon from the clay fraction represented more than 25% of the total soil carbon and the highest proportion of metals was also Na 4 P 2 O 7 extracted from this fraction. For lead and zinc, two metals with high total concentrations in the sandy top soil, the following empirical equations between metal concentrations (expressed in mg kg -1 ) and organic carbon concentrations (expressed in percent However in relation to SOM influence, two different behaviours were observed for the four studied metals. For Zn and Cd, the quantities extracted by EDTA and Na 4 P 2 O 7 were fairly similar (except for Zn in the case of clay fraction: Na 4 P 2 O 7 N EDTA) suggesting that these two metals were preferentially linked to the SOM (Fig. 2). In contrast, for lead and cadmium, the quantities extracted by EDTA were significantly higher than the quantities extracted by Na 4 P 2 O 7 : other soil components (like clay minerals or oxides) significantly interact with these two metals [START_REF] Dumat | Metal ion geochemistry in smelter impacted soils and soil solutions[END_REF][START_REF] Cecchi | Multimetal contamination of a calcic cambisol by fallout from a lead-recycling plant[END_REF].

The general trend observed was that of a decrease of metal proportions extracted by Na 4 P 2 O 7 with the decrease of the soil particle size: the metal extraction yield was greater in sand and in the 20-2 µm fractions compared with the clay fraction (Fig. 3). The efficiency of Na 4 P 2 O 7 could be influenced by the nature of the SOM and by the interactions between SOM and other soil components. Indeed, with decreasing particle size, Table 1 shows a decrease in C/N ratio, in agreement with a change in the chemical nature of the SOM [START_REF] Dormaar | Monosaccharides in hydrolysates of water-stable aggregates after 67 years of cropping to spring wheat as determined by capillary gas chromatography[END_REF][START_REF] Besnard | Influence of organic amendments on copper distribution among particle-size and density fractions in Champagne vineyard soils[END_REF]. Two different pools of metals with contrasting behaviours (in relation with the SOM characteristics) could be present in the soil: metals in the coarse fractions and metals in the clay fraction.

The coarsest fraction was mainly composed of sand and poorly degraded particulate organic matter (POM) [START_REF] Cambaradella | Particulate soil organic matter changes across a grassland cultivation sequence[END_REF]. Minerals in the 2000-20 µm fraction were relatively inert, so the interaction between these minerals and metals should mainly be weak such as electrostatic bonding; POM (with high specific surface area) may therefore play a significant role [START_REF] Besnard | Influence of organic amendments on copper distribution among particle-size and density fractions in Champagne vineyard soils[END_REF]. Sequential extraction performed by [START_REF] Dère | Long-term fate of exogenous metals in a sandy Luvisol subjected to intensive irrigation with raw wastewater[END_REF] on the bulk fraction of the same sandy topsoil ranked lability (potential to be remobilized from solid phase to soil solution) as: Zn ≫ Cu N Pb N Cd. [START_REF] Doelsch | Impact of sewage sludge spreading on heavy metal speciation in tropical soils (Réunion, Indian Ocean)[END_REF] also observed the high lability of Zn in sewage sludge. In the coarse fractions of the soil, the metals associated with POM will therefore mainly be under relatively labile forms.

During biodegradation, the level of SOM oxidation increases [START_REF] Baldock | Assessing the extent of decomposition of natural organic materials using solid-state 13 C NMR spectroscopy[END_REF], leading to an increase of its reactivity towards metals. Since the level of humification increases as particle size decreases, SOM from the fine fraction constitutes a highly reactive pool for binding metals. Moreover, the metals arrived in the sandy top soil simultaneously with organic matter from the waste water, the metals could therefore have already been associated with dissolved organic matter [START_REF] Vaca-Paulín | Effect of sewage sludge or compost on the sorption and distribution of copper and cadmium in soil[END_REF][START_REF] Doelsch | Impact of sewage sludge spreading on heavy metal speciation in tropical soils (Réunion, Indian Ocean)[END_REF]. In addition, [START_REF] Wong | The sorption of Cd and Zn by different soils in the presence of dissolved organic matter from sludge[END_REF] concluded that dissolved organic matter interacts with the clay minerals becoming less accessible to microorganisms. Due to its higher stability against microorganism degradation, the SOM of the clay fraction could have a stabilization effect on the metals and reduce their bioavailability [START_REF] Dabkowska-Naskret | The role of organic matter in association with zinc in selected arable soils from Kujawy region, Poland[END_REF].

4.2. Use of the δ 13 C signal to study organic matter dynamics in relation with metals

The measurement for the topsoil (under maize for 15 years) of δ 13 C (‰) between -21.8 (coarse fraction) and -23.2 (clay fraction), whereas the value of -12.5‰ has been measured on a maize sample composed of crushed leaves and stalks by [START_REF] Jolivet | Organic carbon dynamics in soil particle-size separates of sandy spodosols when forest is cleared for maize cropping[END_REF], indicates that the SOM is relatively unaffected by the maize δ 13 C. From the C 4 /C ratios calculated, the amount of SOM of C 4 origin was seen to be under 40% and decreased with the soil size fraction: 0.4 for the 2000-200 µm fraction and 0.3 for the clay fraction. [START_REF] Balesdent | Measurement of soil organic matter turnover using 13 C natural abundance[END_REF] or [START_REF] Balesdent | Natural 13 C abundance as a tracer for studies of soil organic matter dynamics[END_REF] estimated the residence time of carbon in size fractions of different uncontaminated soils under maize for 13 years: the expected proportion of new C 4 carbon from maize in the coarse fraction (2000-200 µm) of these uncontaminated soils was 80%. [START_REF] Balesdent | Measurement of soil organic matter turnover using 13 C natural abundance[END_REF] concluded that turnover times were 4 ± 1.5 years for the 2000-200 µm fraction and 13 ± 2 years for the 200-50 µm fraction. In contrast all fractions finer than 10 µm exhibited a slow turnover. Our results are therefore consistent with a preferential accumulation of old C 3 organic matter in the polluted soil, particularly in the clay fraction. Even in heavily metal contaminated soils, biological activity occurs [START_REF] Valsecchi | Microbial biomass, activity and organic matter accumulation in soils contaminated with heavy metals[END_REF][START_REF] Aoyama | Comparison of the effects of Cu, Pb, As on plant residue decomposition, microbial biomass and soil respiration[END_REF], but alterations of organic matter decomposition are reported [START_REF] Berg | Reduction in the decomposition rates of Scots Pine needle litter due to heavy-metal pollution[END_REF][START_REF] Cotrufo | Effects of urban heavy metal pollution on organic matter decomposition in Quercus ilex L. woods[END_REF].

In comparison with more contaminated "old" C 3 plant residues in the clay fraction, the "new" C 4 maize residues left in the soil (after the harvest) were mineralised more quickly. The POM is poorly associated with the soil mineral matrix and has a relatively rapid turnover in comparison with the finest SOM in the clay fraction [START_REF] Cambaradella | Particulate soil organic matter changes across a grassland cultivation sequence[END_REF][START_REF] Balesdent | The significance of organic separates to carbon dynamics and its modelling in some cultivated soils[END_REF]. [START_REF] Parat | The organic carbon derived from sewage sludge as a key parameter determining the fate of trace metals[END_REF] suggest an accumulation of plant residues in the coarse fraction of a sludgecontaminated soil by physical protection as the sludge could enhance aggregate formation. However, the topsoil studied here had been intensively used for crops for several decades and its sandy texture limits the physical protection of SOM [START_REF] Guggenberger | Land-use effects on the composition of organic matter in particle-size separates of soils. II. CPMAS and solution 13 C NMR analysis[END_REF]. Moreover, the soil we studied received mainly waste water amendment, so the effect on aggregation was limited.

Some metals are progressively released during the decay of C 3 POM. Then, part of the metals was adsorbed by the reactive colloidal organic matter in the clay fraction [START_REF] Besnard | Influence of organic amendments on copper distribution among particle-size and density fractions in Champagne vineyard soils[END_REF][START_REF] Boucher | Decomposition of leaves of metallophyte Arabidopsis halleri in soil microcosms: fate of Zn and Cd from plant residues[END_REF]. According to [START_REF] Brookes | The use of microbial parameters in monitoring soil pollution by heavy metals[END_REF] or [START_REF] Mhatre | Bioindicators to detect contamination of soils with special reference to heavy metals[END_REF], decreased organic matter decomposition in polluted soils could result from the inhibition of microbial activity by bio-available toxic metals. However, [START_REF] Hattori | Decomposition of organic matter with previous cadmium adsorption in soils[END_REF] concluded that the recalcitrance of plant residues towards microorganism degradation should also be considered. The formation, in the clay fraction, of organo-metallic complexes relatively resistant to biodegradation [START_REF] Orlov | Nature and mechanisms of formation of metal-humus complexes[END_REF] could therefore result. This hypothesis is consistent with the lack of accumulation of highly contaminated coarse plant residues in the top soil i.e. the microorganisms degrade the fresh organic matter (relatively free of metals and not protected by clay minerals), but tend to leave the old C 3 organic matter residues stabilized by chemical and physical phenomena.

Finally from the results obtained in the present study, it can be concluded that the presence of metals carried by waste water modified the turnover rate of the SOM in the contaminated sandy top soil. A decrease of SOM turnover due to the influence of metals was previously observed for pseudogley brown leached soils strongly contaminated by inorganic atmospheric fallout from a smelter (Dumat

  For the bulk topsoil sample: (i) trace metal concentrations (mg kg -1 ) were [Pb] = 142, [Zn] = 1233, [Cu] = 68 and [Cd] = 1.8; (ii) organic carbon (%) and nitrogen (%) concentrations are respectively 1.75 and 0.15, with a C/N value of 11.7. These results are given as the means of the two replicates and the relative standard deviation never exceeded 7%. Compared with the natural pedogeochemical background values ([Pb] = 5.9 mg kg -1 , [Zn] = 13.4 mg kg -1 , [Cu] = 3.9 mg kg -1 and [Cd] = 0.1 mg kg -1 ), our results indicate a significant contamination of the top soil[START_REF] Lamy | Use of major-and trace-element correlations to assess metal migration in sandy Luvisols irrigated with wastewater[END_REF].Particle size fractionation of soil samples resulted in the recovery of 98 ± 0.5% of the initial sample mass. The size distribution observed was: 42.3% (2000µm N fraction N 200 µm), 40.4% (200 µm N fraction N 50 µm), 5.4% (50 µm N fraction N 20 µm), 4.5% (20 µm N fraction N 2 µm) and 7.4% fraction b 2 µm i.e. the sand fractions (fine and coarse) accounted for 80% of the total weight.

  ) were determined: [Pb] = 228.59 × [C]-294.93 with a correlation coefficient (r) of 0.96 and [Zn] = 1150.4 × [C]-1412.6 with r = 0.95.

Fig. 1 .

 1 Fig. 1. Metal pools (Pb: , Zn: , Cu: □ and Cd: ■) in the different size fractions of the topsoil.

Fig. 2 .

 2 Fig. 2. Chemical extractions performed on various size fractions of the sandy topsoil. Quantities of metals extracted (mg of the metal per kg of soil fraction) by EDTA or Na 4 P 2 O 7 . A) Zn-EDTA: • and Zn-Na 4 P 2 O 7 : ○; B) Cu-EDTA: ♦ and Cu-Na 4 P 2 O 7 : ◊. C) Pb-EDTA: ■ and Pb-Na 4 P 2 O 7 : □; D) Cd-EDTA: and Cd-Na 4 P 2 O 7 : --.

Fig. 3 .

 3 Fig. 3. Chemical extractions performed on various size fractions of the sandy topsoil. Ratios (%) of the quantities of metals extracted by EDTA (■) or Na 4 P 2 O 7 (□) divided by the total metal content of that size fraction. A) Zn, B) Cu, C) Pb and D) Cd.

Table 1

 1 Metal concentrations (mg kg -1 ), organic C (%), N (%) and C/N of the various size fractions of the contaminated sandy top soil

	Soil size	Concentration (mg kg -1 )
	fraction

Acknowledgement

The authors thank the French National Environment Program (PNSE 2005) for financial support.