Search

Dust-enshrouded, starbursting, submillimeter galaxies (SMGs) at z >= 3 have been proposed as progenitors of z >= 2 compact quiescent galaxies (cQGs). To test this connection, we present a detailed spatially resolved study of the stars, dust, and stellar mass in a sample of six submillimeter-bright starburst galaxies at z similar to 4.5. The stellar UV emission probed by HST is extended and irregular and shows evidence of multiple components. Informed by HST, we deblend Spitzer/IRAC data at rest-frame optical, finding that the systems are undergoing minor mergers with a typical stellar mass ratio of 1:6.5. The FIR dust continuum emission traced by ALMA locates the bulk of star formation in extremely compact regions (median r(e) = 0.70 +/- 0.29 kpc), and it is in all cases associated with the most massive component of the mergers (median log(M-*/M-circle dot)= 10.49 +/- 0.32). We compare spatially resolved UV slope (beta) maps with the FIR dust continuum to study the infrared excess (IRX = L-IR/L-UV)-beta relation. The SMGs display systematically higher IRX values than expected from the nominal trend, demonstrating that the FIR and UV emissions are spatially disconnected. Finally, we show that the SMGs fall on the mass-size plane at smaller stellar masses and sizes than the cQGs at z = 2. Taking into account the expected evolution in stellar mass and size between z = 4.5 and z = 2 due to the ongoing starburst and mergers with minor companions, this is in agreement with a direct evolutionary connection between the two populations.

International audience; We examine the behaviour of the infrared‐radio correlation (IRRC) over the range 0 < z less than or similar to 6 using new, highly sensitive 3 GHz observations with the Karl G. Jansky Very Large Array (VLA) and infrared data from the Herschel Space Observatory in the 2 deg(2) COSMOS field. We distinguish between objects where emission is believed to arise solely from star‐formation, and those where an active galactic nucleus (AGN) is thought to be present. We account for non‐detections in the radio or in the infrared using a doublycensored survival analysis. We find that the IRRC of star‐forming galaxies, quantified by the infrared‐to‐1.4 GHz radio luminosity ratio (qTIR), decreases with increasing redshift: q(TIR)(z) = (2.88 +/‐ 0.03)(1 + z)(‐0.19 +/‐ 0.01). This is consistent with several previous results from the literature. Moderate‐to‐high radiative luminosity AGN do not follow the same q(TIR)(z) trend as star‐forming galaxies, having a lower normalisation and steeper decrease with redshift. We cannot rule out the possibility that unidentified AGN contributions only to the radio regime may be steepening the observed q(TIR)(z) trend of the star‐forming galaxy population. We demonstrate that the choice of the average radio spectral index directly affects the normalisation, as well as the derived trend with redshift of the IRRC. An increasing fractional contribution to the observed 3 GHz flux by free‐free emission of star‐forming galaxies may also affect the derived evolution. However, we find that the standard (M82‐based) assumption of the typical radio spectral energy distribution (SED) for star‐forming galaxies is inconsistent with our results. This suggests a more complex shape of the typical radio SED for star‐forming galaxies, and that imperfect K corrections in the radio may govern the derived trend of decreasing q(TIR) with increasing redshift. A more detailed understanding of the radio spectrum is therefore required for robust K corrections in the radio and to fully understand the cosmic evolution of the IRRC. Lastly, we present a redshift‐dependent relation between rest‐frame 1.4 GHz radio luminosity and star formation rate taking the derived redshift trend into account.

International audience; We present results on the dust attenuation of galaxies at redshift similar to 3‐6 by studying the relationship between the UV spectral slope (beta(UV)) and the infrared excess (IRX; L‐IR/L‐UV) using Atacama Large Millimeter/submillimeter Array (ALMA) far‐infrared continuum observations. Our study is based on a sample of 67 massive, star‐forming galaxies with a median mass of M‐* similar to 10(10.7) M‐circle dot spanning a redshift range z = 2.6‐3.7 (median z = 3.2) that were observed with ALMA at lambda(rest) = 300 mu m. Both the individual ALMA detections (41 sources) and stacks including all galaxies show the IRX‐beta(UV) relationship at z similar to 3 is mostly consistent with that of local starburst galaxies on average. However, we find evidence for a large dispersion around the mean relationship by up to +/‐ 0.5 dex. Nevertheless, the locally calibrated dust correction factors based on the IRX‐beta(UV) relation are on average applicable to main‐sequence z similar to 3 galaxies. This does not appear to be the case at even higher redshifts, however. Using public ALMA observations of z similar to 4‐6 galaxies we find evidence for a significant evolution in the IRX‐beta(UV) and the IRX‐M‐* relations beyond z similar to 3 towards lower IRX values. We discuss several caveats that could affect these results, including the assumed dust temperature. ALMA observations of larger z > 3 galaxy sample spanning a wide range of physical parameters (e.g. lower stellar mass) will be important to investigate this intriguing redshift evolution further.

The XXL survey currently covers two 25deg2 patches with XMM observations of ∼10ks. We summarize the scientific results associated with the first release of the XXL dataset, which occurred in mid-2016. We review several arguments for increasing the survey depth to 40ks during the next decade of XMM operations. X-ray (z1 cluster density. It will eventually constitute a reference study and an ideal calibration field for the upcoming eROSITA and Euclid missions.

International audience; We explore the multiwavelength properties of AGN host galaxies for different classes of radio‐selected AGN out to z less than or similar to 6 via a multiwavelength analysis of about 7700 radio sources in the COSMOS field. The sources were selected with the Very Large Array (VLA) at 3 GHz (10 cm) within the VLA‐COSMOS 3 GHz Large Project, and cross‐matched with multiwavelength ancillary data. This is the largest sample of high‐redshift (z less than or similar to 6) radio sources with exquisite photometric coverage and redshift measurements available. We constructed a sample of moderate‐to‐high radiative luminosity AGN (HLAGN) via spectral energy distribution decomposition combined with standard X‐ray and mid‐infrared diagnostics. Within the remainder of the sample we further identified low‐to‐moderate radiative luminosity AGN (MLAGN) via excess in radio emission relative to the star formation rates in their host galaxies. We show that at each redshift our HLAGN have systematically higher radiative luminosities than MLAGN and that their AGN power occurs predominantly in radiative form, while MLAGN display a substantial mechanical AGN luminosity component. We found significant differences in the host properties of the two AGN classes, as a function of redshift. At z < 1 : 5, MLAGN appear to reside in significantly more massive and less star‐forming galaxies compared to HLAGN. At z > 1., we observed a reversal in the behaviour of the stellar mass distributions with the HLAGN populating the higher stellar mass tail. We interpret this finding as a possible hint of the downsizing of galaxies hosting HLAGN, with the most massive galaxies triggering AGN activity earlier than less massive galaxies, and then fading to MLAGN at lower redshifts. Our conclusion is that HLAGN and MLAGN samples trace two distinct galaxy and AGN populations in a wide range of redshifts, possibly resembling the radio AGN types often referred to as radiative‐and jet‐mode (or high‐and low‐excitation), respectively, whose properties might depend on the different availability of cold gas supplies.

International audience; We make use of the deep Karl G. Jansky Very Large Array (VLA) COSMOS radio observations at 3 GHz to infer radio luminosity functions of star‐forming galaxies up to redshifts of z similar to 5 based on approximately 6000 detections with reliable optical counterparts. This is currently the largest radio‐selected sample available out to z similar to 5 across an area of 2 square degrees with a sensitivity of rms approximate to 2.3 mu Jy beam(‐1). By fixing the faint and bright end shape of the radio luminosity function to the local values, we find a strong redshift trend that can be fitted with a pure luminosity evolution L‐1.4 (GHz) proportional to /(1 + z)((3.16 +/‐ 0.2)‐(0.32 +/‐ 0.07)z). We estimate star formation rates (SFRs) from our radio luminosities using an infrared (IR)‐radio correlation that is redshift dependent. By integrating the parametric fits of the evolved luminosity function we calculate the cosmic SFR density (SFRD) history since z similar to 5. Our data suggest that the SFRD history peaks between 2 < z < 3 and that the ultraluminous infrared galaxies (100 M‐circle dot yr(‐1) < SFR < 1000 M‐circle dot yr(‐1)) contribute up to similar to 25% to the total SFRD in the same redshift range. Hyperluminous infrared galaxies (SFR > 1000 M fi yr 1) contribute an additional less than or similar to 2% in the entire observed redshift range. We find evidence of a potential underestimation of SFRD based on ultraviolet (UV) rest‐frame observations of Lyman break galaxies at high redshifts (z greater than or similar to 4) on the order of 15‐20%, owing to appreciable star formation in highly dust‐obscured galaxies, which might remain undetected in such UV observations.

We examine the behaviour of the infrared-radio correlation (IRRC) over the range $0, Comment: 17 pages, 22 figures, 3 tables. Accepted for publication in A&A

We present the COSMOS2015 catalog which contains precise photometric redshifts and stellar masses for more than half a million objects over the 2deg$^{2}$ COSMOS field. Including new $YJHK_{\rm s}$ images from the UltraVISTA-DR2 survey, $Y$-band from Subaru/Hyper-Suprime-Cam and infrared data from the Spitzer Large Area Survey with the Hyper-Suprime-Cam Spitzer legacy program, this near-infrared-selected catalog is highly optimized for the study of galaxy evolution and environments in the early Universe. To maximise catalog completeness for bluer objects and at higher redshifts, objects have been detected on a $��^{2}$ sum of the $YJHK_{\rm s}$ and $z^{++}$ images. The catalog contains $\sim 6\times 10^5$ objects in the 1.5 deg$^{2}$ UltraVISTA-DR2 region, and $\sim 1.5\times 10^5$ objects are detected in the "ultra-deep stripes" (0.62 deg$^{2}$) at $K_{\rm s}\leq 24.7$ (3$��$, 3", AB magnitude). Through a comparison with the zCOSMOS-bright spectroscopic redshifts, we measure a photometric redshift precision of $��_{��z/(1+z_s)}$ = 0.007 and a catastrophic failure fraction of $��=0.5$%. At $3, Accepted for publication in ApJ

International audience; We used the Plateau de Bure Interferometer (PdBI) to map a sample of 15 submillimetre galaxies (SMGs) in the COSMOS field at the wavelength of 1.3 mm. The target SMGs were originally discovered in the James Clerk Maxwell Telescope (JCMT)/AzTEC 1.1 mmcontinuum survey at S/N-1.1mm = 4-4.5. This paper presents, for the first time, interferometric millimetre-wavelength observations of these sources. The angular resolution of our observations, similar to 1 `'.8, allowed us to accurately determine the positions of the target SMGs. Using a detection threshold of S/N-1.3mm \textgreater 4.5 regardless of multiwavelength counterpart association, and 4 \textless S/N-1.3 mm \textless= 4.5 if a multiwavelength counterpart within 1 `'.5 is also present, the total number of detections in our survey is 22. The most significant PdBI detection of S/N-1.3 (mm) = 10.3 is towards AzTEC19. Three of our detected SMGs (AzTEC21, 27, and 28; which corresponds to 20%) are marginally resolved at our angular resolution, and these sources are found to have elongated or clumpy morphologies and/or multiple components. Using optical to near-infrared photometric redshifts, available spectroscopic redshifts, and redshifts estimated from the radio-to-submm spectral index we infer a median redshift of \textless(z)over tilde\textgreater = 3.20 +/- 0.25 for our sample. To study the overall multiplicity and redshift distribution of flux-limited samples of SMGs we combined these sources with the 15 brightest JCMT/AzTEC SMGs detected at 1.1 mm, AzTEC1-15, and studied previously. This constitutes a complete, flux-and S/N-limited 1.1-mm selected sample. We find that the median redshift for the 15 brightest JCMT/AzTEC SMGs ((z) over tilde = 3.05 +/- 0.44) is consistent with that for AzTEC16-30. This conforms to recent observational findings that SMGs do not exhibit any significant trend between the redshift and (sub) mm flux density. For the combined AzTEC1-30 sample we derive a median redshift of (z) over tilde = 3.17 +/- 0.27, consistent with previous results based on mm-selected samples. We further infer that within the combined AzTEC1-30 sample similar to 25 +/- 9% of sources separate into multiple components.

International audience; We investigate the physical properties of a sample of six submillimeter galaxies (SMGs) in the COSMOS field, spectroscopically confirmed to lie at redshifts z \textgreater 4. While the redshifts for four of these SMGs were previously known, we present here two newly discovered z(spec) \textgreater 4 SMGs. For our analysis we employ the rich (X-ray to radio) COSMOS multi-wavelength datasets. In particular, we use new data from the Giant Meterwave Radio Telescope (GMRT) 325 MHz and 3 GHz Jansky Very Large Array (VLA) to probe the rest-frame 1.4 GHz emission at z = 4, and to estimate the sizes of the star formation regions of these sources, respectively. We find that only one SMG is clearly resolved at a resolution of 0.” 6 x 0.” 7 at 3 GHz, two may be marginally resolved, while the remaining three SMGs are unresolved at this resolution. Combining this with sizes from high-resolution (sub-) mm observations available in the literature for AzTEC 1 and AzTEC 3 we infer a median radio-emitting size for our z \textgreater 4 SMGs of (0.” 63 +/- 0.” 12) x (0.” 35 +/- 0.” 05) or 4.1 x 2.3 kpc(2) (major x minor axis; assuming z = 4.5) or lower if we take the two marginally resolved SMGs as unresolved. This is consistent with the sizes of star formation regions in lower-redshift SMGs, and local normal galaxies, yet higher than the sizes of star formation regions of local ultraluminous infrared galaxies (ULIRGs). Our SMG sample consists of a fair mix of compact and more clumpy systems with multiple, perhaps merging, components. With an average formation time of similar to 280 Myr, as derived through modeling of the UV IR spectral energy distributions, the studied SMGs are young systems. The average stellar mass, dust temperature, and IR luminosity we derive are M-star similar to 1.4 x 10(11) M-circle dot, T-dust similar to 43 K, and L-IR similar to 1.3 x 10(13) L-circle dot, respectively. The average L-IR is up to an order of magnitude higher than for SMGs at lower redshifts. Our SMGs follow the correlation between dust temperature and IR luminosity as derived for Herschel-selected 0.1 \textless z \textless 2 galaxies. We study the IR-radio correlation for our sources and find a deviation from that derived for z \textless 3 ULIRGs (\textless q(IR)\textgreater = 1.95 +/- 0.26 for our sample, compared to q approximate to 2.6 for IR luminous galaxies at z \textless 2). In summary, we find that the physical properties derived for our z \textgreater 4 SMGs put them at the high end of the L-IR-T-dust distribution of SMGs, and that our SMGs form a morphologically heterogeneous sample. Thus, additional in-depth analyses of large, statistical samples of high-redshift SMGs are needed to fully understand their role in galaxy formation and evolution.

Evolution in the measured rest frame ultraviolet spectral slope and ultraviolet to optical flux ratios indicate a rapid evolution in the dust obscuration of galaxies during the first 3 billion years of cosmic time (z>4). This evolution implies a change in the average interstellar medium properties, but the measurements are systematically uncertain due to untested assumptions, and the inability to measure heavily obscured regions of the galaxies. Previous attempts to directly measure the interstellar medium in normal galaxies at these redshifts have failed for a number of reasons with one notable exception. Here we report measurements of the [CII] gas and dust emission in 9 typical (~1-4L*) star-forming galaxies ~1 billon years after the big bang (z~5-6). We find these galaxies have >12x less thermal emission compared with similar systems ~2 billion years later, and enhanced [CII] emission relative to the far-infrared continuum, confirming a strong evolution in the interstellar medium properties in the early universe. The gas is distributed over scales of 1-8 kpc, and shows diverse dynamics within the sample. These results are consistent with early galaxies having significantly less dust than typical galaxies seen at z, Submitted to Nature, under review after referee report. 22 pages, 4 figures, 4 Extended Data Figures, 5 Extended Data tables

We present first results of a study aimed to constrain the star formation rate (SFR) and dust content of galaxies at z approximate to 2. We use a sample of BzK-selected star-forming galaxies, drawn from the Cosmic Evolution Survey, to perform a stacking analysis of their 1.4 GHz radio continuum as a function of different stellar population properties, after cleaning the sample from contamination by active galactic nuclei. Dust unbiased SFRs are derived from radio fluxes assuming the local radio-IR correlation. The main results of this work are: (1) specific star formation rate (SSFR) s are constant over about 1 dex in stellar mass and up to the highest stellar mass probed, (2) the dust attenuation is a strong function of galaxy stellar mass with more massive galaxies being more obscured than lower mass objects, (3) a single value of the UV extinction applied to all galaxies would lead to a gross underestimate of the SFR in massive galaxies, (4) correcting the observed UV luminosities for dust attenuation based on the Calzetti recipe provides results in very good agreement with the radio derived ones, (5) the mean SSFR of our sample steadily decreases by a factor of similar to 4 with decreasing redshift from z = 2.3 to 1.4 and a factor of similar to 40 down the local universe. These empirical SFRs would cause galaxies to dramatically overgrow in mass if maintained all the way to low redshifts; we suggest that this does not happen because star formation is progressively quenched, likely starting from the most massive galaxies.

The relation between the stellar mass and the star formation rate characterizes how the instantaneous star formation is determined by the galaxy past star formation history and by the growth of the dark matter structures. We deconstruct the M-SFR plane by measuring the specific SFR functions in several stellar mass bins from z=0.2 out to z=1.4. Our analysis is primary based on a MIPS 24$\mu m$ selected catalogue combining the COSMOS and GOODS surveys. We estimate the SFR by combining mid- and far-infrared data for 20500 galaxies. The sSFR functions are derived in four stellar mass bins within the range 9.5, Comment: 24 pages, 16 figures, 3 tables, published version in A&A

We study the physical properties of a sample of 6 SMGs in the COSMOS field, spectroscopically confirmed to lie at z>4. We use new GMRT 325 MHz and 3 GHz JVLA data to probe the rest-frame 1.4 GHz emission at z=4, and to estimate the sizes of the star-forming (SF) regions of these sources, resp. Combining our size estimates with those available in the literature for AzTEC1 and AzTEC3 we infer a median radio-emitting size for our z>4 SMGs of (0.63"+/-0.12")x(0.35"+/-0.05") or 4.1x2.3 kpc^2 (major times minor axis; assuming z=4.5) or lower if we take the two marginally resolved SMGs as unresolved. This is consistent with the sizes of SF regions in lower-redshift SMGs, and local normal galaxies, yet higher than the sizes of SF regions of local ULIRGs. Our SMG sample consists of a fair mix of compact and more clumpy systems with multiple, perhaps merging, components. With an average formation time of ~280 Myr, derived through modeling of the UV-IR SEDs, the studied SMGs are young systems. The average stellar mass, dust temperature, and IR luminosity we derive are M*~1.4x10^11 M_sun, T_dust~43 K, and L_IR~1.3x10^13L_sun, resp. The average L_IR is up to an order of magnitude higher than for SMGs at lower redshifts. Our SMGs follow the correlation between dust temperature and IR luminosity as derived for Herschel-selected 0.1=1.95+/-0.26 for our sample, compared to q~2.6 for IR luminous galaxies at z4 SMGs put them at the high end of the L_IR-T_dust distribution of SMGs, and that our SMGs form a morphologically heterogeneous sample. Thus, further in-depth analyses of large, statistical samples of high-redshift SMGs are needed to fully understand their role in galaxy formation and evolution., 16 pages, 8 figures, 6 tables; to appear in A&A

The relation between the stellar mass and the star formation rate characterizes how the instantaneous star formation is determined by the galaxy past star formation history and by the growth of the dark matter structures. We deconstruct the M-SFR plane by measuring the specific SFR functions in several stellar mass bins from z=0.2 out to z=1.4. Our analysis is primary based on a MIPS 24$��m$ selected catalogue combining the COSMOS and GOODS surveys. We estimate the SFR by combining mid- and far-infrared data for 20500 galaxies. The sSFR functions are derived in four stellar mass bins within the range 9.5, 24 pages, 16 figures, 3 tables, published version in A&A

Previous clustering analysis of low-power radio AGN has indicated that they preferentially live in massive groups. The X-ray surveys of the COSMOS field have achieved a sensitivity at which these groups are directly detected out to z=1.3. Making use of Chandra-, XMM- and VLA-COSMOS surveys we identify radio AGN members (10**23.6 < L_1.4GHz/(W/Hz) < 10**25) of galaxy groups (10**13.2 < M_200/M_sun < 10**14.4; 0.1, Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letters

One of the main achievements in modern cosmology is the so-called `unified model', which successfully describes most classes of active galactic nuclei (AGN) within a single physical scheme. However, there is a particular class of radio-luminous AGN that presently cannot be explained within this framework -- the `low-excitation' radio AGN (LERAGN). Recently, a scenario has been put forward which predicts that LERAGN, and their regular `high-excitation' radio AGN (HERAGN) counterparts represent different (red sequence vs. green valley) phases of galaxy evolution. These different evolutionary states are also expected to be reflected in their host galaxy properties, in particular their cold gas content. To test this, here we present CO(1-0) observations toward a sample of 11 of these systems conducted with CARMA. Combining our observations with literature data, we derive molecular gas masses (or upper limits) for a complete, representative, sample of 21 z, Comment: 8 pages, 4 figures, 4 tables; accepted for publication in ApJ

One of the main achievements in modern cosmology is the so-called `unified model', which successfully describes most classes of active galactic nuclei (AGN) within a single physical scheme. However, there is a particular class of radio-luminous AGN that presently cannot be explained within this framework -- the `low-excitation' radio AGN (LERAGN). Recently, a scenario has been put forward which predicts that LERAGN, and their regular `high-excitation' radio AGN (HERAGN) counterparts represent different (red sequence vs. green valley) phases of galaxy evolution. These different evolutionary states are also expected to be reflected in their host galaxy properties, in particular their cold gas content. To test this, here we present CO(1-0) observations toward a sample of 11 of these systems conducted with CARMA. Combining our observations with literature data, we derive molecular gas masses (or upper limits) for a complete, representative, sample of 21 z, 8 pages, 4 figures, 4 tables; accepted for publication in ApJ

We quantify the importance of the mechanical energy released by radio-galaxies inside galaxy groups. We use scaling relations to estimate the mechanical energy released by 16 radio-AGN located inside X-ray detected galaxy groups in the COSMOS field. By comparing this energy output to the host groups' gravitational binding energy, we find that radio galaxies produce sufficient energy to unbind a significant fraction of the intra-group medium. This unbinding effect is negligible in massive galaxy clusters with deeper potential wells. Our results correctly reproduce the breaking of self-similarity observed in the scaling relation between entropy and temperature for galaxy groups., Accepted for publication in the Astrophysical Journal. 12 Pages

VLA 1.4 GHz (rms noise ~0.012 mJy) and MIPS 24 and 70 micron (rms noise ~0.02 and ~1.7 mJy, respectively) observations covering the 2 square degree COSMOS field are combined with an extensive multi-wavelength data set to study the evolution of the IR-radio relation at intermediate and high redshift. With ~4500 sources -- of which ~30% have spectroscopic redshifts -- the current sample is significantly larger than previous ones used for the same purpose. Both monochromatic IR/radio flux ratios (q24 & q70), as well as the ratio of the total IR and the 1.4 GHz luminosity (qTIR) are used as indicators for the IR/radio properties of star forming galaxies and AGN. Using a sample jointly selected at IR and radio wavelengths in order to reduce selection biases, we provide firm support for previous findings that the IR-radio relation remains unchanged out to at least z~1.4. Moreover, based on data from ~150 objects we also find that the local relation likely still holds at 2.5, Comment: 52 pages, 23 figures (11 at reduced resolution). Accepted for publication in ApJS

Previous observational studies of the infrared (IR)-radio relation out to high redshift employed any detectable star forming systems at a given redshift within the restricted area of cosmological survey fields. Consequently, the evolution inferred relies on a comparison between the average IR/radio properties of (i) very IR-luminous high-z sources and (ii) more heterogeneous low(er)-z samples that often lack the strongest IR emitters. In this report we consider populations of objects with comparable luminosities over the last 10 Gyr by taking advantage of deep IR (esp. Spitzer 24 micron) and VLA 1.4 GHz observations of the COSMOS field. Consistent with recent model predictions, both Ultra Luminous Infrared Galaxies (ULIRGs) and galaxies on the bright end of the evolving IR luminosity function do not display any change in their average IR/radio ratios out to z~2 when corrected for bias. Uncorrected data suggested ~0.3 dex of positive evolution., 9 pages, 4 figures. Accepted for publication in ApJL.

Radio outflows of active galactic nuclei (AGN) are invoked in cosmological models as a key feedback mechanism in the latest phases of massive galaxy formation. Recently it has been suggested that the two major radio AGN populations -- the powerful high-excitation, and the weak low-excitation radio AGN (HERAGN and LERAGN, resp.) -- represent two earlier and later stages of massive galaxy build-up. To test this, here we make use of a local (0.04, Comment: 5 pages, 4 figures, 1 table, to appear in ApJL

We explore how well crowded field point-source photometry can be accomplished with SDSS data: We present a photometric pipeline based on DoPhot, and tuned for analyzing crowded-field images from the SDSS. Using Monte Carlo simulations we show that the completeness of source extraction is above 80% to i < 21 (AB) and a stellar surface density of about 200 sq.amin. Hence, a specialized data pipeline can efficiently be used for e.g. nearby resolved galaxies in SDSS images, where the standard SDSS photometric package Photo, when applied in normal survey mode, gives poor results. We apply our pipeline to an area of about 3.55sq.deg. around the dwarf spheroidal galaxy (dSph) Leo I, and construct a high S/N star-count map of Leo I via an optimized filter in color-magnitude space (g,r,i). Although the radial surface-density profile of the dwarf deviates from the best fit empirical King model towards outer radii, we find no evidence for tidal debris out to a stellar surface-density of 4*10^(-3) of the central value. We determine the total luminosity of Leo I, and model its mass using the spherical and isotropic Jeans equation. Assuming that 'mass follows light' we constrain a lower limit of the total mass of the dSph to be (1.7+/-0.2)*10^7 Msol. Contrary, if the mass in Leo I is dominated by a constant density dark-matter (DM) halo, then the mass within the central 12' is (2+/-0.6)*10^8 Msol. This leads to a mass-to-light ratio of >>6 (Ic_sol), and possibly >75 if the DM halo dominates the mass and extends further out than 12'. In summary, our results show that Leo I is a symmetric, relaxed and bound system; this supports the idea that Leo I is a dark-matter dominated system., 13 pages, 11 figures; accepted for publication in AJ

We present observations at 1.4 and 250 GHz of the $z\sim 5.7$ Ly$\alpha$ emitters (LAE) in the COSMOS field found by Murayama et al.. At 1.4 GHz there are 99 LAEs in the lower noise regions of the radio field. We do not detect any individual source down to 3$\sigma$ limits of $\sim 30\mu$Jy beam$^{-1}$ at 1.4 GHz, nor do we detect a source in a stacking analysis, to a 2$\sigma$ limit of $2.5\mu$Jy beam$^{-1}$. At 250 GHz we do not detect any of the 10 LAEs that are located within the central regions of the COSMOS field covered by MAMBO ($20' \times 20'$) to a typical 2$\sigma$ limit of $S_{250} < 2$mJy. The radio data imply that there are no low luminosity radio AGN with $L_{1.4} > 6\times 10^{24}$ W Hz$^{-1}$ in the LAE sample. The radio and millimeter observations also rule out any highly obscured, extreme starbursts in the sample, ie. any galaxies with massive star formation rates $> 1500$ M$_\odot$ year$^{-1}$ in the full sample (based on the radio data), or 500 M$_\odot$ year$^{-1}$ for the 10% of the LAE sample that fall in the central MAMBO field. The stacking analysis implies an upper limit to the mean massive star formation rate of $\sim 100$ M$_\odot$ year$^{-1}$., Comment: 11 pages AAStex format 3 figures. ApJ COSMOS Special Issue. Changes: Added 'Note added in proof' to reflect nine new sources in the LAE sample

We present observations at 1.4 and 250 GHz of the $z\sim 5.7$ Ly$��$ emitters (LAE) in the COSMOS field found by Murayama et al.. At 1.4 GHz there are 99 LAEs in the lower noise regions of the radio field. We do not detect any individual source down to 3$��$ limits of $\sim 30��$Jy beam$^{-1}$ at 1.4 GHz, nor do we detect a source in a stacking analysis, to a 2$��$ limit of $2.5��$Jy beam$^{-1}$. At 250 GHz we do not detect any of the 10 LAEs that are located within the central regions of the COSMOS field covered by MAMBO ($20' \times 20'$) to a typical 2$��$ limit of $S_{250} < 2$mJy. The radio data imply that there are no low luminosity radio AGN with $L_{1.4} > 6\times 10^{24}$ W Hz$^{-1}$ in the LAE sample. The radio and millimeter observations also rule out any highly obscured, extreme starbursts in the sample, ie. any galaxies with massive star formation rates $> 1500$ M$_\odot$ year$^{-1}$ in the full sample (based on the radio data), or 500 M$_\odot$ year$^{-1}$ for the 10% of the LAE sample that fall in the central MAMBO field. The stacking analysis implies an upper limit to the mean massive star formation rate of $\sim 100$ M$_\odot$ year$^{-1}$., 11 pages AAStex format 3 figures. ApJ COSMOS Special Issue. Changes: Added 'Note added in proof' to reflect nine new sources in the LAE sample

We discuss the panchromatic properties of 99 088 galaxies selected from the Sloan Digital Sky Survey (SDSS) Data Release 1 'main' spectroscopic sample (a flux-limited sample for 1360 deg2). These galaxies are positionally matched to sources detected by ROSAT, Galaxy Evolution Explorer (GALEX), two-Micron All-Sky Survey (2MASS), Infrared Astronomical Satellite (IRAS), Green Bank GB6 survey (GB6), Faint Images of the Radio Sky at Twenty-centimetres (FIRST), NRAO VLA Sky Survey (NVSS) and Westerbork Northern Sky Survey (WENSS). The matching fraction varies from

We discuss the panchromatic properties of 99,088 galaxies selected from the SDSS Data Release 1 spectroscopic sample (a flux-limited sample for 1360 deg^2). These galaxies are positionally matched to sources detected by ROSAT, GALEX, 2MASS, IRAS, GB6, FIRST, NVSS and WENSS. We find strong correlations between the detection fraction at other wavelengths and optical properties such as flux, colors, and emission-line strengths. Using GALEX, SDSS, and 2MASS data, we construct the UV-IR broad-band spectral energy distributions for various types of galaxies, and find that they form a nearly one-parameter family. For example, based on SDSS u- and r-band data, supplemented with redshift, the K-band 2MASS magnitudes can be "predicted" with an rms scatter of only 0.2 mag. When a dust content estimate determined from SDSS data by Kauffmann et al. (2003) is also utilized, this scatter decreases to 0.1 mag. We demonstrate that this dust content is indeed higher for galaxies detected by IRAS and that it can be used to "predict" measured IRAS 60 micron flux density within a factor of two using only SDSS data. We also show that the position of a galaxy in the emission-line-based Baldwin-Phillips-Terlevich diagram is correlated with the optical light concentration index and u-r color determined from the SDSS broad-band imaging data, and discuss changes in the morphology of this diagram induced by requiring detections at other wavelengths. We study the IR-radio correlation and find evidence that its slope may be different for AGN and star-forming galaxies and related to the H_alpha/H_beta line strength ratio., Comment: Accepted for publication in MNRAS; 25 pages, 32 color figures

We synthesize the rest-frame Stroemgren colors using SDSS spectra for 99,088 galaxies selected from Data Release 1. This narrow-band ~200 AA photometric system (uz, vz, bz, yz), first designed for the determination of effective temperature, metallicity and gravity of stars, measures the continuum spectral slope of galaxies in the rest-frame 3200-5800 AA wavelength range. Galaxies form a remarkably narrow locus (~0.03 mag) in the resulting color-color diagram. The Bruzual & Charlot population synthesis models suggest that the position of a galaxy along this locus is controlled by a degenerate combination of metallicity and age of the dominant stellar population. Galaxy distribution along the locus is bimodal, with the local minimum corresponding to an ~1 Gyr old single stellar population. The position perpendicular to the locus is independent of metallicity and age, and reflects the galaxy's dust content, as implied by both the models and the statistics of IRAS detections. A comparison of this locus with the galaxy locus in the H_delta-D_n(4000) diagram, utilized by Kauffmann et al. (2003) to estimate stellar masses, reveals a tight correlation, although the two analyzed spectral ranges barely overlap. Overall, the galaxy spectral energy distribution in the entire UV to near-IR range can be described as a single-parameter family with an accuracy of 0.1 mag, or better. This nearly one-dimensional distribution of galaxies in the multi-dimensional space of measured parameters strongly supports the conclusion of Yip et al. (2004), based on a principal component analysis, that SDSS galaxy spectra can be described by a small number of eigenspectra. Apparently, the contributions of stellar populations that dominate the optical emission from galaxies are combined in a simple and well-defined way., Accepted for publication in MNRAS; 19 pages, 28 color figures

We summarize the detection rates at wavelengths other than optical for \~99,000 galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 1 ``main'' spectroscopic sample. The analysis is based on positional cross-correlation with source catalogs from ROSAT, 2MASS, IRAS, GB6, FIRST, NVSS and WENSS surveys. We find that the rest-frame UV-IR broad-band galaxy SEDs form a remarkably uniform, nearly one parameter, family. As an example, the SDSS u and r band data, supplemented with redshift, can be used to predict K band magnitudes measured by 2MASS with an rms scatter of only 0.2 mag; when measurement uncertainties are taken into account, the astrophysical scatter appears not larger than ~0.1 mag., Comment: 2 pages, 1 figure. To appear in "The Interplay among Black Holes, Stars and ISM in Galactic Nuclei", Proc. IAU 222 (Gramado, Brazil), eds. Th. Storchi Bergmann, L.C. Ho, H.R. Schmitt

We discuss the density and radial velocity distributions of over 3000 candidate RR Lyrae stars selected by various methods using Sloan Digital Sky Survey data for about 1000 deg^2 of sky. This is more than 20 times larger sample than previously reported by SDSS (Ivezic et al. 2000), and includes candidate RR Lyrae stars out to the sample limit of 100 kpc. A cutoff in the radial distribution of halo RR Lyrae at ~50-60 kpc that was suggested by the early SDSS data appears to be a statistical anomaly confined to a small region (~100 deg^2). Despite the large increase in observed area, the most prominent features remain to be those associated with the Sgr dwarf tidal stream. We find multiple number density peaks along three lines of sight in the Sgr dwarf tidal stream plane, that may indicate several perigalactic passages of the Sgr dwarf galaxy., 4 pages, 2 color figures, uses newpasp.sty, ``Satellites and Tidal Streams, May 26-30, 2003, La Palma, Canary Island, Spain'', in press

We discuss a sample of over 3000 candidate RR Lyrae stars selected by various methods using Sloan Digital Sky Survey data for about 1000 deg^2 of sky. These stars probe the halo structure out to ~100 kpc from the Galactic center. Their spatial and radial velocity distributions are very inhomogeneous, with the most prominent features tracing the Sgr dwarf tidal stream. Outside the Sgr dwarf tidal stream, the spatial distribution in the 5-60 kpc range of Galactocentric radius R is well described by an R^{-3} power law., Comment: 7 pages, 5 figures, uses newpasp.sty, ``Milky Way Surveys: The Structure and Evolution of Our Galaxy'', Boston, June 14-17, 2003, in press

The XXL survey currently covers two 25 sq. deg. patches with XMM observations of ~10ks. We summarise the scientific results associated with the first release of the XXL data set, that occurred mid 2016. We review several arguments for increasing the survey depth to 40 ks during the next decade of XMM operations. X-ray (z1 cluster density. It will eventually constitute a reference study and an ideal calibration field for the upcoming eROSITA and Euclid missions., Proceeding of the XMM Next Decade Workshop held at ESAC, 9-11 May 2016

The equatorial 2 deg2 COSMOS area is the only large field for which a complete, deep, pan-chromatic data set exists, from an outstanding survey effort, and that all large telescopes can observe. Now, this pioneering and ambitious COSMOS survey is undergoing major extension, pushing its frontiers via the newly approved Chandra COSMOS Legacy Survey, the second largest Chandra proposal ever approved.'COSMOS-Legacy' will uniformly cover the 1.7 deg2 COSMOS/HST field with 2.8 Ms of Chandra ACIS-I imaging at ~160 ksec depth. This project expands the current deep C-COSMOS area by a factor of ~3 at ~3e-16 (1.45 vs 0.44 deg2). This will be achieved with 56x50 ks tiles covering a total area of 2.2 deg2, which will be observed during Chandra Cycle 14. The area and depth of COSMOS Legacy are designed to detect ~40 z>4, and ~4 z>5 Large Scale Structures on >15 arcmin scales. These structures have proven to connect luminous AGN (over 200 at z>3 will be detected) and sub-mm galaxies. COSMOS Legacy will also probe mini-quasars at z>7, using anistotropies of the unresolved X-ray Background, and the masses of the Dark Matter halos hosting X-ray AGN up to 3, via autocorrelation functions on ~30arcmin scales. To fully achieve these goals, COSMOS Legacy is complemented by spectroscopic follow-up with DEIMOS and MOSFIRE at Keck and KMOS at the VLT and FMOS at Subaru, just approved observations with Spitzer and JVLA, and with harder (5-80 keV) X-ray imaging with NuSTAR. In the near future, observations with Subaru HyperSuprimeCam (grizY) to r(AB)=28.2 are planned.

Over the past decade, several works have used the ratio between total (rest 8-1000 mu m) infrared and radio (rest 1.4 GHz) luminosity in star-forming galaxies (q(IR)), often referred to as the infrared-radio correlation (IRRC), to calibrate the radio emission as a star formation rate (SFR) indicator. Previous studies constrained the evolution of q(IR) with redshift, finding a mild but significant decline that is yet to be understood. Here, for the first time, we calibrate q(IR) as a function of both stellar mass (M-star) and redshift, starting from an M-star-selected sample of > 400 000 star-forming galaxies in the COSMOS field, identified via (NUV-r)/(r-J) colours, at redshifts of 0.1

(Abridged)One of the fundamental challenges for astrophysics in the 21st century is finding a way to untangle the physical processes that govern galaxy formation and evolution. Given the importance and scope of this problem, the multi-wavelength astronomical community has used the past decade to build up a wealth of information over specific extragalactic deep fields to address key questions in galaxy formation and evolution. These fields generally cover at least 10square degrees to facilitate the investigation of the rarest, typically most massive, galaxies and AGN. Furthermore, such areal coverage allows the environments to be fully accounted for, thereby linking the single halo to the two-halo terms in the halo occupation distribution. Surveys at radio wavelengths have begun to lag behind those at other wavelengths, especially in this medium-deep survey tier. However, the survey speed offered by the JVLA means that we can now reach a point where we can begin to obtain commensurate data at radio wavelengths to those which already exists from the X-ray through to the far-infrared over ~10 square degrees. We therefore present the case for a 10 square degree survey to 1.5uJy at L-band in A or B Array, requiring ~4000 hours to provide census of star-formation and AGN-accretion activity in the Universe. For example, the observations will allow galaxies forming stars at 10Msolar/yr to be detected out to z~1 and luminous infrared galaxies (1000Msolar/yr to be found out to z~6. Furthermore, the survey area ensures that we will have enough cosmic volume to find these rare sources at all epochs. The bandwidth will allow us to determine the polarisation properties galaxies in the high-redshift Universe as a function of stellar mass, morphology and redshift., 18 pages, 6 figures. White Paper submitted to the call for Very Large Array Sky Surveys

We present detailed morphological properties of Lyman alpha emitters (LAEs) at z~ 5.7 in the COSMOS field, based on {\it Hubble Space Telescope} Advanced Camera for Surveys (ACS) data. The ACS imaging in the F814W filter covered 85 LAEs of the 119 LAEs identified in the full two square degree field, and 47 LAEs of them are detected in the ACS images. Nearly half of them are spatially extended with a size larger than 0.15 arcsec (~0.88 kpc at z=5.7) up to 0.4 arcsec (~2.5 kpc at z=5.7). The others are nearly unresolved compact objects. Two LAEs show double-component structures, indicating interaction or merging of building components to form more massive galaxies. By stacking the ACS images of all the detected sources, we obtain a Sersic parameter of n~0.7 with a half-light radius of 0.13 arcsec (0.76 kpc), suggesting that the majority of ACS detected LAEs have not spheroidal-like but disk-like or irregular light profiles. Comparing ACS F814W magnitudes (I_814) with Subaru/Suprime-Cam magnitudes in the NB816, i', and z' bands, we find that the ACS imaging in the F814W band mainly probes UV continuum rather than Lyman alpha line emission. UV continuum sizes tend to be larger for LAEs with larger Ly\alpha emission regions as traced by the NB816 imaging. The non-detection of 38 LAEs in the ACS images is likely due to the fact that their surface brightness is even too low both in the UV continuum and Lyalpha emission. Estimating I_814 for the LAEs with ACS non-detection from the z' and NB816 magnitudes, we find that 16 of these are probably LAEs with a size larger than 0.15 arcsec in UV continuum. All these results suggest that our LAE sample contains systematically larger LAEs in UV continuum size than those previously studied at z~6., Comment: 71 pages, 18 figures. Accepted for pulication in ApJ. High resolution paper: http://cosmos.phys.sci.ehime-u.ac.jp/~murayama/COSMOS/cosmos-lae57-acs.pdf

49 pages, 18 figures; accepted for publication in the ApJS, COSMOS special issue. The data will be available from the COSMOS archive at IPAC/IRSA http://irsa.ipac.caltech.edu/Missions/cosmos.html, more information on the VLA-COSMOS survey can be found at http://www.mpia-hd.mpg.de/COSMOS/, full version available at http://www.mpia-hd.mpg.de/COSMOS/#paper The VLA-COSMOS large project is described and its scientific objective is discussed. We present a catalog of ~ 3,600 radio sources found in the 2deg^2 COSMOS field at 1.4 GHz. The observations in the VLA A and C configuration resulted in a resolution of 1.5'x1.4' and a mean rms noise of ~ 10.5(15) uJy/beam in the central 1(2)deg^2. 80 radio sources are clearly extended consisting of multiple components, and most of them appear to be double-lobed radio galaxies. The astrometry of the catalog has been thoroughly tested and the uncertainty in the relative and absolute astrometry are 130mas and

International audience We present the COSMOS2015(24) catalog, which contains precise photometric redshifts and stellar masses for more than half a million objects over the 2deg(2) COSMOS field. Including new YJHK(s) images from the UltraVISTA-DR2 survey, Y-band images from Subaru/Hyper-Suprime-Cam, and infrared data from the Spitzer Large Area Survey with the Hyper-Suprime-Cam Spitzer legacy program, this near-infrared-selected catalog is highly optimized for the study of galaxy evolution and environments in the early universe. To maximize catalog completeness for bluer objects and at higher redshifts, objects have been detected on a chi(2) sum of the YJHK(s) and z(++) images. The catalog contains similar to 6 x 10(5) objects in the 1.5 deg(2) UltraVISTA-DR2 region and similar to 1.5 x 10(5) objects are detected in the “ultra-deep stripes” (0.62 deg(2)) at K-s \textless= 24.7 (3 sigma, 3 `, AB magnitude). Through a comparison with the zCOSMOS-bright spectroscopic redshifts, we measure a photometric redshift precision of sigma(Delta z(1) (+ zs)) = 0.007 and a catastrophic failure fraction of eta = 0.5%. At 3 \textless z \textless 6, using the unique database of spectroscopic redshifts in COSMOS, we find sigma(Delta z(1) (+ zs)) = 0.021 and eta = 13.2%. The deepest regions reach a 90% completeness limit of 10(10)M(circle dot) to z = 4. Detailed comparisons of the color distributions, number counts, and clustering show excellent agreement with the literature in the same mass ranges. COSMOS2015 represents a unique, publicly available, valuable resource with which to investigate the evolution of galaxies within their environment back to the earliest stages of the history of the universe. The COSMOS2015 catalog is distributed via anonymous ftp and through the usual astronomical archive systems (CDS, ESO Phase 3, IRSA).

We report the discovery of a locus of stars in the SDSS $g-r$ vs. $u-g$ color-color diagram that connects the colors of white dwarfs and M dwarfs. While its contrast with respect to the main stellar locus is only $\sim$1:2300, this previously unrecognized feature includes 863 stars from SDSS Data Release 1. The position and shape of the feature is in good agreement with predictions of a simple binary star model that consists of a white dwarf and an M dwarf, with the components' luminosity ratio controlling the position along this binary system locus. SDSS DR1 spectra for 47 of these objects strongly support this model.

One of the key quests in astronomy is to study the growth and evolution of galaxies across cosmic time. Radio observations provide a powerful means of studying the formation of stars and subsequent buildup of distant galaxies, in a way that is unbiased by the presence of dust. This thesis provides a detailed view of faint, star-forming galaxies in the early Universe through sensitive radio observations, and compiles several studies probing distant star formation with both radio synchrotron and free-free emission. In Chapter 2, we detect a large number of galaxies using sensitive new radio data from the Very Large Array, allowing us to separate radio emission from star formation and active galactic nuclei in the faint radio sky. In Chapter 3, we calibrate synchrotron emission as a tracer of star formation in distant starburst galaxies, while in Chapters 4 & 5 we turn towards radio free-free emission — a faint but very powerful tracer of star formation. Using sensitive new radio data at high frequencies, we perform the first detailed studies of free-free emission in distant galaxies.

We show that the most distant X-ray detected cluster known to date, ClJ1001 at z=2.506, hosts a strong overdensity of radio sources. Six of them are individually detected (within 10") in deep 0.75" resolution VLA 3GHz imaging, with S(3GHz)>8uJy. Of the six, AGN likely affects the radio emission in two galaxies while star formation is the dominant source powering the remaining four. We searched for cluster candidates over the full COSMOS 2-square degree field using radio-detected 3GHz sources and looking for peaks in Sigma5 density maps. ClJ1001 is the strongest overdensity by far with >10sigma, with a simple z_phot>1.5 preselection. A cruder photometric rejection of z2.5. Samples of hundreds such high-redshift clusters could potentially constrain cosmological parameters and test cluster and galaxy formation models., ApJ Letters in press

Future radio surveys will generate catalogues of tens of millions of radio sources, for which redshift estimates will be essential to achieve many of the science goals. However, spectroscopic data will be available for only a small fraction of these sources, and in most cases even the optical and infrared photometry will be of limited quality. Furthermore, radio sources tend to be at higher redshift than most optical sources and so a significant fraction of radio sources hosts differ from those for which most photometric redshift templates are designed. We therefore need to develop new techniques for estimating the redshifts of radio sources. As a starting point in this process, we evaluate a number of machine-learning techniques for estimating redshift, together with a conventional template-fitting technique. We pay special attention to how the performance is affected by the incompleteness of the training sample and by sparseness of the parameter space or by limited availability of ancillary multi-wavelength data. As expected, we find that the quality of the photometric-redshift degrades as the quality of the photometry decreases, but that even with the limited quality of photometry available for all sky-surveys, useful redshift information is available for the majority of sources, particularly at low redshift. We find that a template-fitting technique performs best with high-quality and almost complete multi-band photometry, especially if radio sources that are also X-ray emitting are treated separately. When we reduced the quality of photometry to match that available for the EMU all-sky radio survey, the quality of the template-fitting degraded and became comparable to some of the machine learning methods. Machine learning techniques currently perform better at low redshift than at high redshift, because of incompleteness of the currently available training data at high redshifts., Comment: Submitted to PASP

We present the catalog of optical and infrared counterparts of the Chandra COSMOS-Legacy Survey, a 4.6 Ms Chandra program on the 2.2 square degrees of the COSMOS field, combination of 56 new overlapping observations obtained in Cycle 14 with the previous C-COSMOS survey. In this Paper we report the i, K, and 3.6 micron identifications of the 2273 X-ray point sources detected in the new Cycle 14 observations. We use the likelihood ratio technique to derive the association of optical/infrared (IR) counterparts for 97% of the X-ray sources. We also update the information for the 1743 sources detected in C-COSMOS, using new K and 3.6 micron information not available when the C-COSMOS analysis was performed. The final catalog contains 4016 X-ray sources, 97% of which have an optical/IR counterpart and a photometric redshift, while 54% of the sources have a spectroscopic redshift. The full catalog, including spectroscopic and photometric redshifts and optical and X-ray properties described here in detail, is available online. We study several X-ray to optical (X/O) properties: with our large statistics we put better constraints on the X/O flux ratio locus, finding a shift towards faint optical magnitudes in both soft and hard X-ray band. We confirm the existence of a correlation between X/O and the the 2-10 keV luminosity for Type 2 sources. We extend to low luminosities the analysis of the correlation between the fraction of obscured AGN and the hard band luminosity, finding a different behavior between the optically and X-ray classified obscured fraction., Comment: 28 pages, 14 figures, accepted to ApJ

X-ray surveys contain sizable numbers of star forming galaxies, beyond the AGN which usually make the majority of detections. Many methods to separate the two populations are used in the literature, based on X-ray and multiwavelength properties. We aim at a detailed test of the classification schemes and to study the X-ray properties of the resulting samples. We build on a sample of galaxies selected at 1.4 GHz in the VLA-COSMOS survey, classified by Smolcic et al. (2008) according to their optical colours and observed with Chandra. A similarly selected control sample of AGN is also used for comparison. We review some X-ray based classification criteria and check how they affect the sample composition. The efficiency of the classification scheme devised by Smolcic et al. (2008) is such that ~30% of composite/misclassified objects are expected because of the higher X-ray brightness of AGN with respect to galaxies. The latter fraction is actually 50% in the X-ray detected sources, while it is expected to be much lower among X-ray undetected sources. Indeed, the analysis of the stacked spectrum of undetected sources shows, consistently, strongly different properties between the AGN and galaxy samples. X-ray based selection criteria are then used to refine both samples. The radio/X-ray luminosity correlation for star forming galaxies is found to hold with the same X-ray/radio ratio valid for nearby galaxies. Some evolution of the ratio may be possible for sources at high redshift or high luminosity, tough it is likely explained by a bias arising from the radio selection. Finally, we discuss the X-ray number counts of star forming galaxies from the VLA- and C-COSMOS surveys according to different selection criteria, and compare them to the similar determination from the Chandra Deep Fields. The classification scheme proposed here may find application in future works and surveys., Comment: 15 pages, 7 figures, 3 tables