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O. Le Fevre

Publications and source records attributed to O. Le Fevre.

At least 127 records · Page 7Linked to original sources
arXiv2009

Integral field spectroscopy with SINFONI of VVDS galaxies. II. The mass-metallicity relation at 1.2 < z < 1.6

This work aims to provide a first insight into the mass-metallicity (MZ) relation of star-forming galaxies at redshift z~1.4. To reach this goal, we present a first set of nine VVDS galaxies observed with the NIR integral-field spectrograph SINFONI on the VLT. Oxygen abundances are derived from empirical indicators based on the ratio between strong nebular emission-lines (Halpha, [NII]6584 and [SII]6717,6731). Stellar masses are deduced from SED fitting with Charlot & Bruzual (2007) population synthesis models, and star formation rates are derived from [OII]3727 and Halpha emission-line luminosities. We find a typical shift of 0.2-0.4 dex towards lower metallicities for the z~1.4 galaxies, compared to the MZ-relation in the local universe as derived from SDSS data. However, this small sample of eight galaxies does not show any clear correlation between stellar mass and metallicity, unlike other larger samples at different redshift (z~0, z~0.7, and z~2). Indeed, our galaxies lie just under the relation at z~2 and show a small trend for more massive galaxies to be more metallic (~0.1 logarithmic slope). There are two possible explanations to account for these observations. First, the most massive galaxies present higher specific star formation rates when compared to the global VVDS sample which could explain the particularly low metallicity of these galaxies as already shown in the SDSS sample. Second, inflow of metal-poor gas due to tidal interactions could also explain the low metallicity of these galaxies as two of these three galaxies show clear signatures of merging in their velocity fields. Finally, we find that the metallicity of 4 galaxies is lower by ~0.2 to 0.4 dex if we take into account the N/O abundance ratio in their metallicity estimate.

astro-ph.CO↗
arXiv2009

On the cosmic evolution of the scaling relations between black holes and their host galaxies: Broad Line AGN in the zCOSMOS survey

(Abriged) We report on the measurement of the rest frame K-band luminosity and total stellar mass of the hosts of 89 broad line Active Galactic Nuclei detected in the zCOSMOS survey in the redshift range 1<z<2.2. The unprecedented multiwavelength coverage of the survey field allows us to disentangle the emission of the host galaxy from that of the nuclear black hole in their Spectral Energy Distributions. We derive an estimate of black hole masses through the analysis of the broad Mg II emission lines observed in the medium-resolution spectra taken with VIMOS/VLT as part of the zCOSMOS project. We found that, as compared to the local value, the average black hole to host galaxy mass ratio appears to evolve positively with redshift, with a best fit evolution of the form (1+z)^{0.68 \pm0.12 +0.6 -0.3}, where the large asymmetric systematic errors stem from the uncertainties in the choice of IMF, in the calibration of the virial relation used to estimate BH masses and in the mean QSO SED adopted. A thorough analysis of observational biases induced by intrinsic scatter in the scaling relations reinforces the conclusion that an evolution of the MBH-M* relation must ensue for actively growing black holes at early times: either its overall normalization, or its intrinsic scatter (or both) appear to increase with redshift. This can be interpreted as signature of either a more rapid growth of supermassive black holes at high redshift, a change of structural properties of AGN hosts at earlier times, or a significant mismatch between the typical growth times of nuclear black holes and host galaxies.

astro-ph.CO↗
arXiv2009

The nonlinear biasing of the 10k zCOSMOS galaxies up to z~1

We use the overdensity field reconstructed in the volume of the COSMOS area to study the nonlinear biasing of the zCOSMOS galaxies. The galaxy overdensity field is reconstructed using the current sample of ~8500 accurate zCOSMOS redshifts at I(AB)<22.5 out to z~1 on scales R from 8 to 12 Mpc/h. By comparing the probability distribution function (PDF) of galaxy density contrast delta_g to the lognormal approximation of the PDF of the mass density contrast delta, we obtain the mean biasing function b(delta,z,R) between the galaxy and matter overdensity field and its second moments b(hat) and b(tilde) up to z~1. Over the redshift interval 0.4 = b(delta,z,R) delta is of the following characteristic shape. The function vanishes in the most underdense regions and then sharply rises in a nonlinear way towards the mean densities. is almost a linear tracer of the matter in the overdense regions, up to the most overdense regions in which it is nonlinear again and the local effective slope of vs. delta is smaller than unity. The function is evolving only slightly over the redshift interval 0.4<z<1. The linear biasing parameter increases from b(hat)=1.24+/-0.11 at z=0.4 to b(hat)=1.64+/-0.15 at z=1 for the M_B<-20-z sample of galaxies. b(hat) does not show any dependence on the smoothing scale from 8 to 12 Mpc/h, but increases with luminosity. The measured nonlinearity parameter b(tilde)/b(hat) is of the order of a few percent (but it can be consistent with 0) and it does not change with redshift, the smoothing scale or the luminosity. By matching the linear bias of galaxies to the halo bias, we infer that the M_B<-20-z galaxies reside in dark matter haloes with a characteristic mass of about 3-6 x 10^12 Msol, depending on the halo bias fit.

astro-ph.CO↗
arXiv2009

The 10k zCOSMOS: morphological transformation of galaxies in the group environment since z~1

We study the evolution of galaxies inside and outside of the group environment since z=1 using a large well defined set of groups and galaxies from the zCOSMOS-bright redshift survey in the COSMOS field. The fraction of galaxies with early-type morphologies increases monotonically with M_B luminosity and stellar mass and with cosmic epoch. It is higher in the groups than elsewhere, especially at later epochs. The emerging environmental effect is superposed on a strong global mass-driven evolution, and at z~0.5 and log(M*/Msol)~10.2, the "effect" of group environment is equivalent to (only) about 0.2 dex in stellar mass or 2 Gyr in time. The stellar mass function of galaxies in groups is enriched in massive galaxies. We directly determine the transformation rates from late to early morphologies, and for transformations involving colour and star formation indicators. The transformation rates are systematically about twice as high in the groups as outside, or up to 3-4 times higher correcting for infall and the appearance of new groups. The rates reach values, for masses around the crossing mass 10^10.5 Msol, as high as (0.3-0.7)/Gyr in the groups, implying transformation timescales of 1.4-3 Gyr, compared with less than 0.2/Gyr, i.e. timescales >5 Gyr, outside of groups. All three transformation rates decrease at higher stellar masses, and must decrease also at the lower masses below 10^10 Msol which we cannot well probe. The rates involving colour and star formation are consistently higher than those for morphology, by a factor of about 50%. Our conclusion is that the transformations which drive the evolution of the overall galaxy population since z~1 must occur at a rate 2-4 times higher in groups than outside of them.

astro-ph.CO↗
arXiv2009

Integral field spectroscopy with SINFONI of VVDS galaxies. I. Galaxy dynamics and mass assembly at 1.2 < z < 1.6

Context. Identifying the main processes of galaxy assembly at high redshifts is still a major issue to understand galaxy formation and evolution at early epochs in the history of the Universe. Aims. This work aims to provide a first insight into the dynamics and mass assembly of galaxies at redshifts 1.2 10^11Msun subject to at least one major merger over a 3 Gyr period as well as for continuous accretion feeding strong star formation. Conclusions. These results point towards a galaxy formation and assembly scenario which involves several processes, possibly acting in parallel, with major mergers and continuous gas accretion playing a major role. Well controlled samples representative of the bulk of the galaxy population at this key cosmic time are necessary to make further progress.

astro-ph.CO↗
arXiv2009

The Dependence of Star Formation Activity on Stellar Mass Surface Density and Sersic Index in zCOSMOS Galaxies at 0.5<z<0.9 Compared with SDSS Galaxies at 0.04<z<0.08

One of the key unanswered questions in the study of galaxy evolution is what physical processes inside galaxies drive the changes in the SFRs in individual galaxies that, taken together, produce the large decline in the global star-formation rate density (SFRD) to redshifts since z~2. Many studies of the SFR at intermediate redshifts have been made as a function of the integrated stellar mass of galaxies but these did not use information on the internal structural properties of the galaxies. In this paper we present a comparative study of the dependence of SFRs on the average surface mass densities (SigmaM) of galaxies of different morphological types up to z~1 using the zCOSMOS and SDSS surveys. The main findings about the evolution of these relatively massive galaxies are: 1) There is evidence that, for both SDSS ans zCOSMOS galaxies, the mean specific SFR within a given population (either disk-dominated or bulge-dominated) is independent of SigmaM; 2) The observed SSFR - SigmaM step-function relation is due, at all investigated redshifts, to the changing mix of disk-dominated and bulge-dominated galaxies as surface density increases and the strong difference in the average SSFR between disks and bulges. We also find a modest differential evolution in the size-mass relations of disk and spheroid galaxies; 3) The shape of the median SSFR - SigmaM relation is similar, but with median SSFR values that are about 5-6 times higher in zCOSMOS galaxies than for SDSS, across the whole range of SigmaM, and in both spheroid and disk galaxies. This increase matches that of the global SFRD of the Universe as a whole, emphasizing that galaxies of all types are contributing, proportionally, to the global increase in SFRD in the Universe back to these redshifts (abridged).

astro-ph.GA↗
arXiv2009

The zCOSMOS Redshift Survey: the role of environment and stellar mass in shaping the rise of the morphology-density relation from z~1

For more than two decades we have known that galaxy morphological segregation is present in the Local Universe. It is important to see how this relation evolves with cosmic time. To investigate how galaxy assembly took place with cosmic time, we explore the evolution of the morphology-density relation up to redshift z~1 using about 10000 galaxies drawn from the zCOSMOS Galaxy Redshift Survey. Taking advantage of accurate HST/ACS morphologies from the COSMOS survey, of the well-characterised zCOSMOS 3D environment, and of a large sample of galaxies with spectroscopic redshift, we want to study here the evolution of the morphology-density relation up to z~1 and its dependence on galaxy luminosity and stellar mass. The multi-wavelength coverage of the field also allows a first study of the galaxy morphological segregation dependence on colour. We further attempt to disentangle between processes that occurred early in the history of the Universe or late in the life of galaxies. The zCOSMOS field benefits of high-resolution imaging in the F814W filter from the Advanced Camera for Survey (ACS). We use standard morphology classifiers, optimised for being robust against band-shifting and surface brightness dimming, and a new, objective, and automated method to convert morphological parameters into early, spiral, and irregular types. We use about 10000 galaxies down to I_AB=22.5 with a spectroscopic sampling rate of 33% to characterise the environment of galaxies up to z~1 from the 100 kpc scales of galaxy groups up to the 100 Mpc scales of the cosmic web. ABRIDGED

astro-ph.CO↗
arXiv2009

HST/ACS Morphology of Lyman Alpha Emitters at Redshift 5.7 in the COSMOS Field

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 Lyαemission 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.

astro-ph.CO↗
arXiv2009

The zCOSMOS Survey. The dependence of clustering on luminosity and stellar mass at z=0.2-1

We study the dependence of galaxy clustering on luminosity and stellar mass at redshifts z ~ [0.2-1] using the first zCOSMOS 10K sample. We measure the redshift-space correlation functions xi(rp,pi) and its projection wp(rp) for sub-samples covering different luminosity, mass and redshift ranges. We quantify in detail the observational selection biases and we check our covariance and error estimate techniques using ensembles of semi-analytic mock catalogues. We finally compare our measurements to the cosmological model predictions from the mock surveys. At odds with other measurements, we find a weak dependence of galaxy clustering on luminosity in all redshift bins explored. A mild dependence on stellar mass is instead observed. At z~0.7, wp(rp) shows strong excess power on large scales. We interpret this as produced by large-scale structure dominating the survey volume and extending preferentially in direction perpendicular to the line-of-sight. We do not see any significant evolution with redshift of the amplitude of clustering for bright and/or massive galaxies. The clustering measured in the zCOSMOS data at 0.5 =10 is only marginally consistent with predictions from the mock surveys. On scales larger than ~2 h^-1 Mpc, the observed clustering amplitude is compatible only with ~1% of the mocks. Thus, if the power spectrum of matter is LCDM with standard normalization and the bias has no unnatural scale-dependence, this result indicates that COSMOS has picked up a particularly rare, ~2-3 sigma positive fluctuation in a volume of ~10^6 h^-1 Mpc^3. These findings underline the need for larger surveys of the z~1 Universe to appropriately characterize the level of structure at this epoch.

astro-ph.CO↗
arXiv2009

The density field of the 10k zCOSMOS galaxies

We use the current sample of ~10,000 zCOSMOS spectra of sources selected with I(AB) < 22.5 to define the density field out to z~1, with much greater resolution in the radial dimension than has been possible with either photometric redshifts or weak lensing. We apply new algorithms that we have developed (ZADE) to incorporate objects not yet observed spectroscopically by modifying their photometric redshift probability distributions using the spectroscopic redshifts of nearby galaxies. This strategy allows us to probe a broader range of galaxy environments and reduce the Poisson noise in the density field. The reconstructed overdensity field of the 10k zCOSMOS galaxies consists of cluster-like patterns surrounded by void-like regions, extending up to z~1. Some of these structures are very large, spanning the ~50 Mpc/h transverse direction of the COSMOS field and extending up to Delta z~0.05 in redshift. We present the three dimensional overdensity maps and compare the reconstructed overdensity field to the independently identified virialised groups of galaxies and clusters detected in the visible and in X-rays. The distribution of the overdense structures is in general well traced by these virialised structures. A comparison of the large scale structures in the zCOSMOS data and in the mock catalogues reveals an excellent agreement between the fractions of the volume enclosed in structures of all sizes above a given overdensity between the data and the mocks in 0.2<z<1.

astro-ph.CO↗
arXiv2009

The Vimos VLT Deep Survey: Stellar mass segregation and large-scale galaxy environment in the redshift range 0.2<z<1.4

Hierarchical models of galaxy formation predict that the properties of a dark matter halo depend on the large-scale environment surrounding the halo. As a result of this correlation, we expect massive haloes to be present in larger number in overdense regions than in underdense ones. Given that a correlation exists between a galaxy stellar mass and the hosting dark matter halo mass, the segregation in dark matter halo mass should then result in a segregation in the distribution of stellar mass in the galaxy population. In this work we study the distribution of galaxy stellar mass and rest-frame optical color as a function of the large-scale galaxy distribution using the VLT VIMOS Deep Survey sample, in order to verify the presence of segregation in the properties of the galaxy population. We use the VVDS redshift measurements and multi-band photometric data to derive estimates of the stellar mass, rest-frame optical color, and of the large-scale galaxy density, on a scale of approximately 8 Mpc, for a sample of 5619 galaxies in the redshift range 0.2 0.7. However, when we consider only galaxies in narrow bins of stellar mass, in order to exclude the effects of the stellar mass segregation on the galaxy properties, we do not observe any more any significant color segregation.

astro-ph.CO↗
arXiv2008

The Environments of Active Galactic Nuclei within the zCOSMOS Density Field

The impact of environment on AGN activity up to z~1 is assessed by utilizing a mass-selected sample of galaxies from the 10k catalog of the zCOSMOS spectroscopic redshift survey. We identify 147 AGN by their X-ray emission as detected by XMM-Newton from a parent sample of 7234 galaxies. We measure the fraction of galaxies with stellar mass M_*>2.5x10^10 Msun that host an AGN as a function of local overdensity using the 5th, 10th and 20th nearest neighbors that cover a range of physical scales (~1-4 Mpc). Overall, we find that AGNs prefer to reside in environments equivalent to massive galaxies with substantial levels of star formation. Specifically, AGNs with host masses between 0.25-1x10^11 Msun span the full range of environments (i.e., field-to-group) exhibited by galaxies of the same mass and rest-frame color or specific star formation rate. Host galaxies having M_*>10^11 Msun clearly illustrate the association with star formation since they are predominantly bluer than the underlying galaxy population and exhibit a preference for lower density regions analogous to SDSS studies of narrow-line AGN. To probe the environment on smaller physical scales, we determine the fraction of galaxies (M_*>2.5x10^10 Msun) hosting AGNs inside optically-selected groups, and find no significant difference with field galaxies. We interpret our results as evidence that AGN activity requires a sufficient fuel supply; the probability of a massive galaxy to have retained some sufficient amount of gas, as evidence by its ongoing star formation, is higher in underdense regions where disruptive processes (i.e., galaxy harrassment, tidal stripping) are lessened.

astro-ph↗
arXiv2008

The spatial clustering of X-ray selected AGN in the XMM-COSMOS field

We study the spatial clustering of 538 X-ray selected AGN in the 2 deg^2 XMM-COSMOS field that are spectroscopically identified to I_{AB}<23 and span the redshift range z=0.2-3.0. The median redshift and luminosity of the sample are z = 0.98 and L_{0.5-10}=6.3 x 10^{43} erg/s, respectively. A strong clustering signal is detected at ~18sigma level, which is the most significant measurement obtained to date for clustering of X-ray selected AGN. By fitting the projected correlation function w(r_p) with a power law on scales of r_p=0.3-40 Mpc/h, we derive a best fit comoving correlation length of r_0 = 8.6 +- 0.5 Mpc/h and slope of gamma=1.88 +- 0.07 (Poissonian errors; bootstrap errors are about a factor of 2 larger). An excess signal is observed in the range r_p~5-15 Mpc/h, which is due to a large scale structure at z ~ 0.36 containing about 40 AGN. When removing the z ~ 0.36 structure, or computing w(r_p) in a narrower range around the peak of the redshift distribution (e.g. z=0.4-1.6), the correlation length decreases to r_0 ~ 5-6 Mpc/h, which is consistent with that observed for bright optical QSOs at the same redshift. We investigate the clustering properties of obscured and unobscured AGN separately. Within the statistical uncertainties, we do not find evidence that AGN with broad optical lines (BLAGN) cluster differently from AGN without broad optical lines (non-BLAGN). The correlation length measured for XMM-COSMOS AGN at z~1 is similar to that of massive galaxies (stellar mass M_*> 3 x 10^{10} M_sun) at the same redshift. This suggests that AGN at z~1 are preferentially hosted by massive galaxies, as observed both in the local and in the distant (z~2) Universe. (shortened)

astro-ph↗
arXiv2008

Testing gravity on large scales. The skewness of the galaxy distribution at z~1

We study the evolution of the low-order moments of the galaxy overdensity distribution over the redshift interval 0.7<z<1.5. We find that the variance and the normalized skewness evolve over this redshift interval in a way that is remarkably consistent with predictions of first- and second-order perturbation theory. This finding confirms the standard gravitational instability paradigm over nearly 9 Gyrs of cosmic time and demonstrates the importance of accounting for the non-linear component of galaxy biasing to avoid disagreement between theory and observations.

astro-ph↗
arXiv2008

Physical properties of galaxies and their evolution in the VIMOS VLT Deep Survey. II. Extending the mass-metallicity relation to the range z=0.89-1.24

Aims. We present a continuation of our study about the relation between stellar mass and gas-phase metallicity in the VIMOS VLT Deep Survey (VVDS). In this work we extend the determination of metallicities up to redshift = 1.24 for a sample of 42 star-forming galaxies with a mean redshift value of 0.99. Methods. For a selected sample of emission-line galaxies, we use both diagnostic diagrams and empirical calibrations based on [OII] emission lines along with the empirical relation between the intensities of the [OIII] and [NeIII] emission lines and the theoretical ratios between Balmer recombination emission lines to identify star-forming galaxies and to derive their metallicities. We derive stellar masses by fitting the whole spectral energy distribution with a set of stellar population synthesis models. Results. These new methods allow us to extend the mass-metallicity relation to higher redshift. We show that the metallicity determinations are consistent with more established strong-line methods. Taken together this allows us to study the evolution of the mass-metallicity relation up to z = 1.24 with good control of systematic uncertainties. We find an evolution with redshift of the average metallicity of galaxies very similar to those reported in the literature: for a given stellar mass, galaxies at z = 1 have, on average, a metallicity = 0.3 dex lower than galaxies in the local universe. However we do not see any significant metallicity evolution between redshifts z = 0.7 (Paper I) and z = 1.0 (this paper). We find also the same flattening of the mass-metallicity relation for the most massive galaxies as reported in Paper I at lower redshifts, but again no apparent evolution of the slope is seen between z = 0.7 and z = 1.0.

astro-ph↗
arXiv2008

The VVDS-VLA Deep Field - IV: Radio-optical properties

(abridged) We use the 1.4 GHz VIMOS-VLA Deep Survey and the optical VVDS and the CFHT-LS to compare the properties of radio loud galaxies with respect to the whole population of optical galaxies. The availability of multiband photometry and high quality photometric redshifts allows to derive rest frame colors and radio luminosity functions down to a limit of a B rest-frame magnitude of M=-20. Galaxy properties and luminosity functions (LFs) are estimated up to z~1 for radio loud and radio quiet early and late type galaxies. Radio loud late type galaxies are redder than radio quiet objects of the same class and this is an effect related to the presence of more dust in stronger star forming galaxies. Moreover, we estimate optical LFs, stellar masses and star formation rate distributions for radio sources and compare them with those derived for a well defined control sample, finding that the probability for a galaxy to be a radio emitter significantly increases at high values of these parameters. Radio loud early type galaxies show luminosity evolution of their bivariate radio-optical LF, due to an evolution in the radio-optical ratio. The lack of evolution of the mass function of radio loud early type galaxies means that no new AGN are formed at z<1. On the contrary, radio loud late type objects show a strong evolution, both in luminosity and in density, of the radio LF for z>0.7. This evolution is the direct effect of the strong optical evolution of this class and no significant change with redshift of the radio-optical ratio is required. With the knowledge of the radio-optical ratio and the optical and radio LFs for late type galaxies, we estimated the star formation history of the Universe up to z~1.5, using optical galaxies as tracers of the global radio emission.

astro-ph↗
arXiv2008

The VVDS-SWIRE-GALEX-CFHTLS surveys: Physical properties of galaxies at z below 1.2 from photometric data

We intend to show that it is possible to derive the physical parameters of galaxies from their broad-band spectral energy distribution out to a redshift of 1.2. This method has the potential to yield the physical parameters of all galaxies in a single field in a homogeneous way. We use an extensive dataset, assembled in the context of the VVDS survey, which reaches from the UV to the IR and covers a sample of 84073 galaxies over an area of 0.89 deg$^2$. We also use a library of 100000 model galaxies with a large variety of star formation histories (in particular including late bursts of star formation). We find that we can determine the physical parameters stellar mass, age and star formation rate with good confidence. We validate the star formation rate determinations in particular by comparing it to a sample of spectroscopically observed galaxies with an emission line measurement. We use our sample to build the number density function of galaxies as a function of stellar mass, specific star formation rate and redshift. We then study whether the stellar mass function at a later time can be predicted from the stellar mass function and star formation rate distribution at an earlier time. We find that the predicted growth in stellar mass from star formation agrees with the observed one. However, the predicted stellar mass density for massive galaxies is lower than observed, while the mass density of intermediate mass galaxies is overpredicted. When comparing with a direct measurement of the major merger rate from the VVDS survey we find that major mergers are sufficient to explain about a third of the mass build-up at the massive end, while the rest is likely contributed through minor mergers.

astro-ph↗