arXiv subjects

C. J. Walcher

Publications and source records attributed to C. J. Walcher.

89 records · Page 5Linked to original sources

arXiv2008

Physical properties of galaxies and their evolution in the VIMOS VLT Deep Survey. I. The evolution of the mass-metallicity relation up to z~0.9

We derive the mass-metallicity relation of star-forming galaxies up to $z\sim0.9$, using data from the VIMOS VLT Deep Survey. Automatic measurement of emission-line fluxes and equivalent widths have been performed on the full spectroscopic sample. This sample is divided into two sub-samples depending on the apparent magnitude selection: wide ($I_{\mathrm{AB}}<22.5$) and deep $I_{\mathrm{AB}}<24$). These two samples span two different ranges of stellar masses. Emission-line galaxies have been separated into star-forming galaxies and active galactic nuclei using emission line ratios. For the star-forming galaxies the emission line ratios have also been used to estimate gas-phase oxygen abundance, using empirical calibrations renormalized in order to give consistent results at low and high redshifts. The stellar masses have been estimated by fitting the whole spectral energy distributions with a set of stellar population synthesis models. We assume at first order that the shape of the mass-metallicity relation remains constant with redshift. Then we find a stronger metallicity evolution in the wide sample as compared to the deep sample. We thus conclude that the mass-metallicity relation is flatter at higher redshift. The observed flattening of the mass-metallicity relation at high redshift is analyzed as an evidence in favor of the open-closed model.

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-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↗

arXiv2008

A new method to separate star forming from AGN galaxies at intermediate redshift: The submillijansky radio population in the VLA-COSMOS survey

We explore the properties of the submillijansky radio population at 20 cm by applying a newly developed optical color-based method to separate star forming (SF) from AGN galaxies at intermediate redshifts (z<1.3). Although optical rest-frame colors are used, our separation method is shown to be efficient, and not biased against dusty starburst galaxies. This classification method has been calibrated and tested on a local radio selected optical sample. Given accurate multi-band photometry and redshifts, it carries the potential to be generally applicable to any galaxy sample where SF and AGN galaxies are the two dominant populations. In order to quantify the properties of the submillijansky radio population, we have analyzed ~2,400 radio sources, detected at 20 cm in the VLA-COSMOS survey. 90% of these have submillijansky flux densities. We classify the objects into 1) star candidates, 2) quasi stellar objects, 3) AGN, 4) SF, and 5) high redshift (z>1.3) galaxies. We find, for the composition of the submillijansky radio population, that SF galaxies are not the dominant population at submillijansky flux levels, as previously often assumed, but that they make up an approximately constant fraction of 30-40% in the flux density range of ~50 microJy to 0.7 mJy. In summary, based on the entire VLA-COSMOS radio population at 20 cm, we find that the radio population at these flux densities is a mixture of roughly 30-40% of SF and 50-60% of AGN galaxies, with a minor contribution (~10%) of QSOs.

astro-ph↗

arXiv2008

The VIMOS VLT Deep Survey: The K-band follow-up in the 0226-04 field

AIMS. We present a new Ks-band survey that represents a significant extension to the previous wide-field Ks-band imaging survey within the 0226-04 field of the VIMOS-VLT deep survey (VVDS). The new data add ~ 458 arcmin^2 to the previous imaging program, thus allowing us to cover a total contiguous area of ~ 600 arcmin^2 within this field. METHODS. Sources are identified both directly on the final K-band mosaic image and on the corresponding, deep chi^2-g'r'i' image from the CFHT Legacy Survey in order to reduce contamination while ensuring us the compilation of a truly K-selected catalogue down to the completeness limit of the Ks-band. The newly determined Ks-band magnitudes are used in combination with the ancillary multiwavelength data for the determination of accurate photometric redshifts. RESULTS. The final catalogue totals ~ 52000 sources, out of which ~ 4400 have a spectroscopic redshift from the VVDS first epoch survey. The catalogue is 90% complete down to K_Vega = 20.5 mag. We present K_s-band galaxy counts and angular correlation function measurements down to such magnitude limit. Our results are in good agreement with previously published work. We show that the use of K magnitudes in the determination of photometric redshifts significantly lowers the incidence of catastrophic errors. The data presented in this paper are publicly available through the CENCOS database.

astro-ph↗

arXiv2007

The Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy

We report the discovery of an almost complete Einstein ring of diameter 10" in Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Spectroscopic data from the 6m telescope of the Special Astrophysical Observatory reveals that the deflecting galaxy has a line-of-sight velocity dispersion in excess of 400 km/s and a redshift of 0.444, whilst the source is a star-forming galaxy with a redshift of 2.379. From its color and luminosity, we conclude that the lens is an exceptionally massive Luminous Red Galaxy (LRG) with a mass within the Einstein radius of 5 x 10^12 solar masses. This remarkable system provides a laboratory for probing the dark matter distribution in LRGs at distances out to 3 effective radii, and studying the properties of high redshift star-forming galaxies.

astro-ph↗

arXiv2007

Geometrical tests of cosmological models. III. The cosmology-evolution diagram at z=1

The rotational velocity of distant galaxies, when interpreted as a size (luminosity) indicator, may be used as a tool to select high redshift standard rods (candles) and probe world models and galaxy evolution via the classical angular diameter-redshift or Hubble diagram tests. We implement the proposed testing strategy using a sample of 30 rotators spanning the redshift range 0.2<z<1 with high resolution spectra and images obtained by the VIMOS/VLT Deep Redshift Survey (VVDS) and the Great Observatories Origins Deep Survey (GOODs). We show that by applying at the same time the angular diameter-redshift and Hubble diagrams to the same sample of objects (i.e. velocity selected galactic discs) one can derive a characteristic chart, the cosmology-evolution diagram, mapping the relation between global cosmological parameters and local structural parameters of discs such as size and luminosity. This chart allows to put constraints on cosmological parameters when general prior information about discs evolution is available. In particular, by assuming that equally rotating large discs cannot be less luminous at z=1 than at present (M(z=1) < M(0)), we find that a flat matter dominated cosmology (Omega_m=1) is excluded at a confidence level of 2sigma and an open cosmology with low mass density (Omega_m = 0.3) and no dark energy contribution is excluded at a confidence level greater than 1 sigma. Inversely, by assuming prior knowledge about the cosmological model, the cosmology-evolution diagram can be used to gain useful insights about the redshift evolution of the structural parameters of baryonic discs hosted in dark matter halos of nearly equal masses.

astro-ph↗

arXiv2007

The SWIRE-VVDS-CFHTLS surveys: stellar mass assembly over the last 10 Gyears. Evidence for a major build up of the red sequence between z=2 and z=1

(abridged abstract) We present an analysis of the stellar mass growth over the last 10 Gyrs using a large 3.6$μ$ selected sample. We split our sample into active (blue) and quiescent (red) galaxies. Our measurements of the K-LFs and LD evolution support the idea that a large fraction of galaxies is already assembled at $z\sim 1.2$. Based on the analysis of the evolution of the stellar mass-to-light ratio (in K-band) for the spectroscopic sub-sample, we derive the stellar mass density for the entire sample. We find that the global evolution of the stellar mass density is well reproduced by the star formation rate derived from UV dust corrected measurements. Over the last 8Gyrs, we observe that the stellar mass density of the active population remains approximately constant while it gradually increases for the quiescent population over the same timescale. As a consequence, the growth of the stellar mass in the quiescent population must be due to the shutoff of star formation in active galaxies that migrate into the quiescent population. From $z=2$ to $z=1.2$, we observe a major build-up of the quiescent population with an increase by a factor of 10 in stellar mass, suggesting that we are observing the epoch when an increasing fraction of galaxies are ending their star formation activity and start to build up the red sequence.

astro-ph↗

arXiv2007

The VIMOS VLT Deep Survey. The Assembly History of the Stellar Mass in Galaxies: from the Young to the Old Universe

We present a detailed analysis of the Galaxy Stellar Mass Function of galaxies up to z=2.5 as obtained from the VVDS. We estimate the stellar mass from broad-band photometry using 2 different assumptions on the galaxy star formation history and show that the addition of secondary bursts to a continuous star formation history produces systematically higher (up to 40%) stellar masses. At low redshift (z=0.2) we find a substantial population of low-mass galaxies (<10^9 Msun) composed by faint blue galaxies (M_I-M_K=0.3). In general the stellar mass function evolves slowly up to z=0.9 and more significantly above this redshift. Conversely, a massive tail is present up to z=2.5 and have extremely red colours (M_I-M_K=0.7-0.8). We find a decline with redshift of the overall number density of galaxies for all masses (59+-5% for M>10^8 Msun at z=1), and a mild mass-dependent average evolution (`mass-downsizing'). In particular our data are consistent with mild/negligible (<30%) evolution up to z=0.7 for massive galaxies (>6x10^10 Msun). For less massive systems the no-evolution scenario is excluded. A large fraction (>=50%) of massive galaxies have been already assembled and converted most of their gas into stars at z=1, ruling out the `dry mergers' as the major mechanism of their assembly history below z=1. This fraction decreases to 33% at z=2. Low-mass systems have decreased continuously in number and mass density (by a factor up to 4) from the present age to z=2, consistently with a prolonged mass assembly also at z<1.

astro-ph↗

arXiv2007

VVDS-SWIRE: Clustering evolution from a spectroscopic sample of galaxies with redshift 0.2<z<2.1 selected from Spitzer IRAC 3.6 micron and 4.5 micron photometry

By combining the VVDS with the SWIRE data, we have built the currently largest spectroscopic sample of galaxies selected in the rest-frame near-infrared. These allow us to investigate, for the first time using spectroscopic redshifts, the clustering evolution of galaxies selected from their rest-frame near-infrared luminosity in the redshift range 0.2<z<2.1. Therefore we use the projected two-point correlation function wp(rp) to study the three dimensional clustering properties of galaxies detected at 3.6 and 4.5 microns with IRAC and for which we have spectroscopic redshifts from first epoch VVDS. We find that in the flux limited samples at 3.6 and 4.5 microns, the clustering length does not change from redshift ~2 to the present. In addition, we find that the clustering lengths are systematically higher when galaxy samples are selected from increasingly redder wavelengths. These values are larger than the typical values found for I-band selected galaxies in the same redshift range, but we find that the difference in clustering length between I-band and 3.6/4.5 microns selected samples is decreasing with increasing redshift to become comparable at z~1.5. We interpret this as evidence that galaxies with older stellar populations and galaxies actively forming stars reside in comparably overdense environments at epochs earlier than z~1.5. The increasing difference in clustering length observed between rest-frame UV-optical and infrared selected samples could then be an indication that star formation is gradually shifting to lower density regions as cosmic time increases.

astro-ph↗

arXiv2007

The VVDS type-1 AGN sample: The faint end of the luminosity function

In a previous paper (Gavignaud et al. 2006), we presented the type-1 Active Galactic Nuclei (AGN) sample obtained from the first epoch data of the VIMOS-VLT Deep Survey (VVDS). The sample consists of 130 faint, broad-line AGN with redshift up to z=5 and 17.5< I <24.0, selected on the basis of their spectra. In this paper we present the measurement of the Optical Luminosity Function up to z=3.6 derived from this sample, we compare our results with previous results from brighter samples both at low and at high redshift. Our data, more than one magnitude fainter than previous optical surveys, allow us to constrain the faint part of the luminosity function up to high redshift. By combining our faint VVDS sample with the large sample of bright AGN extracted from the SDSS DR3 (Richards et al., 2006b) and testing a number of different evolutionary models, we find that the model which better represents the combined luminosity functions, over a wide range of redshift and luminosity, is a luminosity dependent density evolution (LDDE) model, similar to those derived from the major X-surveys. Such a parameterization allows the redshift of the AGN space density peak to change as a function of luminosity and explains the excess of faint AGN that we find at 1.0< z <1.5. On the basis of this model we find, for the first time from the analysis of optically selected samples, that the peak of the AGN space density shifts significantly towards lower redshift going to lower luminosity objects. This result, already found in a number of X-ray selected samples of AGN, is consistent with a scenario of "AGN cosmic downsizing", in which the density of more luminous AGN, possibly associated to more massive black holes, peaks earlier in the history of the Universe, than that of low luminosity ones.

astro-ph↗

arXiv2007

The VIMOS-VLT Deep Survey. Color bimodality and the mix of galaxy populations up to z~2

In this paper we discuss the mix of star-forming and passive galaxies up to z~2, based on the first epoch VIMOS-VLT Deep Survey (VVDS) data.In agreement with previous works we find that the galaxy rest-frame color distribution follows a bimodal distribution at z<=1, and we establish that this bimodality holds up to z~2. The details of the rest-frame color distribution depend however on redshift and on galaxy luminosity: faint galaxies are bluer than the luminous ones over the whole redshift range of our data, and galaxies become bluer as redshift increases. This blueing trend does not depend, to a first approximation, on galaxy luminosity. Using our spectroscopic dataset we can also separate galaxies based on a star-formation, estimated combining the equivalent width of the [OII] emission line and the strength of the Dn(4000) break. The comparison between this spectral classification and the rest-frame colors shows that about 35-40 % of the red objects are star forming galaxies. Hence we conclude that the red sequence cannot be used to effectively isolate a sample of purely passively evolving objects within a cosmological survey. We also find that the color-magnitude relations derived for the color and for the spectroscopically selected early-type galaxies have remarkably similar properties, with the contaminating star-forming galaxies within the red sequence objects introducing no significant offset in the rest frame colors. Therefore the average color of the red objects does not appear to be a very sensitive indicator for measuring the evolution of the early-type galaxy population.Finally, we use the multi-band VVDS photometric data and SED fitting to derive multi-color galaxy types, which have a slightly higher efficiency than rest-frame color in isolating the passive, non star-forming galaxies within the VVDS sample.

astro-ph↗

arXiv2006

The VIRMOS-VLT Deep Survey: the last 10 billion years of evolution of galaxy clustering

We discuss the evolution of clustering of galaxies in the Universe from the present epoch back to z ~ 2, using the first-epoch data from the VIMOS-VLT Deep Survey (VVDS). We present the evolution of the projected two-point correlation function of galaxies for the global galaxy population, as well as its dependence on galaxy intrinsic luminosities and spectral types. While we do not find strong variations of the correlation function parameters with redshift for the global galaxy population, the clustering of objects with different intrinsic luminosities evolved significantly during last 8-10 billion years. Our findings indicate that bright galaxies in the past traced higher density peaks than they do now and that the shape of the correlation function of most luminous galaxies is different from observed for their local counterparts, which is a supporting evidence of a non-trivial evolution of the galaxy vs. dark matter bias.

astro-ph↗

arXiv2006

The VIMOS VLT Deep Survey:The ultraviolet galaxy luminosity function and luminosity density at 3<z<4

We study the LF of the high-z galaxy population with 3<z<4 using a purely I-band magnitude-selected spectroscopic sample obtained in the framework of the VVDS. We determine the LF from the VVDS, taking care to add as few assumptions and as simple corrections as possible, and compare our results with those obtained from photometric studies, based on Lyman-break selections or photo-z measurements. We find that in the range 3<z<4, the VVDS LF is parameterized by phi*=1.24+-0.50 10-3 mag-1 Mpc-3 and M*=-21.49+-0.19, assuming a slope alpha=-1.4 consistent with most previous studies. While phi* is comparable to previously found values, M* is significantly brighter by about 0.5 mag at least. Using the conservative slope -1.4, we find a LD at 1700A rho(M<-18.5)=2.4 10 19 W Mpc-3 and rho Tot=3.1 10 19 W Mpc-3, comparable to that estimated in other studies. The unexpectedly large number of very bright galaxies found in the VVDS indicates that the color-selection and photo-z techniques that are generally used to build high-z galaxy samples may be affected by a significant fraction of color-measurement failures or by incomplete modelling of the mix of stellar emission, AGN contribution, dust absorption and intergalactic extinction assumed to identify high-z galaxies, making pure magnitude selection better able to trace the full population. Because of the difficulty to identify all low-luminosity galaxies in a spectroscopic survey, the LD could still be significantly underestimated. We also find that the relative contribution of the most luminous galaxies compared to the fainter ones is at least twice as large in the VVDS compared to former estimates. Therefore, the VVDS paints a quite different picture of the role of the most actively star-forming galaxies in the history of star formation.

astro-ph↗

arXiv2006

NIR Follow-Up of the VVDS 02hr Field

We present a new K-band survey covering 623 arcmin$^2$ in the VVDS 0226-0430 deep field down to a limiting magnitude K$_{\rm{Vega}}$ $\leq$ 20.5. We use the spectroscopic sample extracted from this new K-band catalogue to assess the effectiveness of optical-near infrared color selections in identifying extreme classes of objects at high redshift.

astro-ph↗

arXiv2004

Masses of star clusters in the nuclei of bulge-less spiral galaxies

In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (10^6 - 10^8 Lsun) but compact (Re ~ 5 pc) nuclear star clusters in very late type spirals with UVES at the VLT. We find the velocity dispersions of the nine clusters in our sample to range from 13 to 34 km/s. Using photometric data from the HST/WFPC2 and spherically symmetric dynamical models we determine masses between 8*10^5 and 6*10^7 Msun. The mass to light ratios range from 0.2 to 1.5 in the I band. This indicates a young mean age for most clusters, in agreement with previous studies. Given their high masses and small sizes we find that nuclear clusters are among the objects with the highest mean surface density known (up to 10^5 Msun pc^-2). From their dynamical properties we infer that, rather than small bulges, the closest structural kin of nuclear clusters appear to be massive compact star clusters. This includes such different objects as globular clusters, "super star clusters", ultra compact dwarf galaxies and the nuclei of dwarf elliptical galaxies. It is a challenge to explain why, despite the wildly different current environments, all different types of massive star clusters share very similar and structural properties. A possible explanation links UCDs and massive globular clusters to nuclear star clusters through stripping of nucleated dwarf galaxies in a merger event. The extreme properties of this type of clusters would then be a consequence of their location in the centers of their respective host galaxies.

astro-ph↗

arXiv2003

About the morphology of dwarf spheroidal galaxies and their dark matter content

The morphological properties of the Carina, Sculptor and Fornax dwarfs are investigated using new wide field data with a total area of 29 square degrees. The stellar density maps are derived, hinting that Sculptor possesses tidal tails indicating interaction with the Milky Way. Contrary to previous studies we cannot find any sign of breaks in the density profiles for the Carina and Fornax dwarfs. The possible existence of tidal tails in Sculptor and of King limiting radii in Fornax and Carina are used to derive global M/L ratios, without using kinematic data. By matching those M/L ratios to kinematically derived values we are able to constrain the orbital parameters of the three dwarfs. Fornax cannot have M/L smaller than 3 and must be close to its perigalacticon now. The other extreme is Sculptor that needs to be on an orbit with an eccentricity bigger than 0.5 to be able to form tidal tails despite its kinematic M/L.

astro-ph↗