J. P. Picat

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Aims. Our aim is to investigate the history of mass assembly for galaxies of different stellar masses and types.Methods. We selected a mass-limited sample of 4048 objects from the VIMOS VLT Deep Survey (VVDS) in the redshift interval . We then used an empirical criterion, based on the amplitude of the 4000 ÅBalmer break (D n4000), to separate the galaxy population into spectroscopically early- and late-type systems. The equivalent width of the [OII]3727 line is used as proxy for the star formation activity. We also derived a type-dependent stellar mass function in three redshift bins.Results. We discuss to what extent stellar mass drives galaxy evolution, showing for the first time the interplay between stellar ages and stellar masses over the past 8 Gyr. Low-mass galaxies have small D n4000 and at increasing stellar mass, the galaxy distribution moves to higher D n4000 values as observed in the local Universe. As cosmic time goes by, we witness an increasing abundance of massive spectroscopically early-type systems at the expense of the late-type systems. This spectral transformation of late-type systems into old massive galaxies at lower redshift is a process started at early epochs (z > 1.3) and continuing efficiently down to the local Universe. This is also confirmed by the evolution of our type-dependent stellar mass function. The underlying stellar ages of late-type galaxies apparently do not show evolution, most likely as a result of a continuous and efficient formation of new stars. All star formation activity indicators consistently point towards a star formation history peaked in the past for massive galaxies, with little or no residual star formation taking place in the most recent epochs. In contrast, most of the low-mass systems show just the opposite characteristics, with significant star formation present at all epochs. The activity and efficiency of forming stars are mechanisms that depend on galaxy stellar mass, and the stellar mass assembly becomes progressively less efficient in massive systems as time elapses. The concepts of star formation downsizing and mass assembly downsizing describe a single scenario that has a top-down evolutionary pattern in how the star formation is quenched, as well as how the stellar mass is grown. The role of (dry) merging events seems to be only marginal at z < 1.3, as our estimated efficiency in stellar mass assembly can possibly account for the progressive accumulation of observed passively evolving galaxies.

We have reconstructed the three-dimensional density fluctuation maps to using the distribution of galaxies observed in the VVDS-Deep survey. We use this overdensity field to measure the evolution of the probability distribution function and its lower-order moments over the redshift interval . We apply a self-consistent reconstruction scheme which includes a complete non-linear description of galaxy biasing and which has been thoroughly tested on realistic mock samples. We find that the variance and skewness of the galaxy distribution 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 Gyr of cosmic time and demonstrates the importance of accounting for the non-linear component of galaxy biasing to consistently reproduce the higher-order moments of the galaxy distribution and their evolution.

Aims.We present a measurement of the dependence of galaxy clustering on galaxy stellar mass at redshift , based on the first-epoch data from the VVDS-Deep survey. Methods.Concentrating on the redshift interval , we measured the projected correlation function, , within mass-selected sub-samples covering the range ~109 and ~. We explored and quantify in detail the observational selection biases due to the flux-limited nature of the survey, both from the data themselves and with a suite of realistic mock samples constructed by coupling the Millennium Simulation to semi-analytic models. We identify the range of masses within which our main conclusions are robust against these effects. Serious incompleteness in mass is present below , with about two thirds of the galaxies in the range that are lost due to their low luminosity and high mass-to-light ratio. However, the sample is expected to be 100% complete in mass above . Results.We present the first direct evidence for a dependence of clustering on the galaxy stellar mass at a redshift as high as . We quantify this by fitting the projected function with a power-law model. The clustering length increases from Mpc for galaxies with mass to Mpc when only the most massive () are considered. At the same time, we observe a significant increase in the slope, which over the same range of masses, changes from to . Comparison to the SDSS measurements at shows that the evolution of is significant for samples of galaxies with , while it is negligible for more massive objects. Considering the growth of structure, this implies that the linear bias b L of the most massive galaxies evolves more rapidly between these two cosmic epochs. We quantify this effect by computing the value of b L from the SDSS and VVDS clustering amplitudes and find that b L decreases from at to at , for the most massive galaxies, while it remains virtually constant () for the remaining population. Qualitatively, this is the kind of scenario expected for the clustering of dark-matter halos as a function of their total mass and redshift. Our result therefore seems to indicate that galaxies with the highest stellar mass today were originally central objects of the most massive dark-matter halos at earlier times, whose distribution was strongly biased with respect to the overall mass density field.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Aims.We study the luminosity function of the high-redshift galaxy population with redshifts using a purely I-band magnitude-selected spectroscopic sample obtained in the framework of the VIMOS VLT Deep Survey (VVDS).Methods.We determine the luminosity function 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 photometric-redshift measurements.Results.We find that in the redshift range , the VVDS luminosity function is parameterized by mag-1 Mpc-3 and , assuming a slope consistent with most previous studies. While is comparable to previously found values, M* is significantly brighter by about 0.5 mag at least. Using the conservative slope , we find a luminosity density at 1700 Å W Mpc-3 and W Mpc-3, comparable to that estimated in other studies.Conclusions.The unexpectedly large number of very bright galaxies found in the VVDS indicates that the color-selection and photometric-redshift techniques that are generally used to build high-redshift 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-redshift 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 luminosity density 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.

The Multi Unit Spectroscopic Explorer (MUSE) is a second-generation VLT panoramic integral-field spectrograph under preliminary design study. MUSE has a field of 1x1 arcmin² sampled at 0.2x0.2 arcsec² and is assisted by the VLT ground layer adaptive optics ESO facility using four laser guide stars. The simultaneous spectral range is 0.465-0.93 μm, at a resolution of R~3000. MUSE couples the discovery potential of a large imaging device to the measuring capabilities of a high-quality spectrograph, while taking advantage of the increased spatial resolution provided by adaptive optics. This makes MUSE a unique and tremendously powerful instrument for discovering and characterizing objects that lie beyond the reach of even the deepest imaging surveys. MUSE has also a high spatial resolution mode with 7.5x7.5 arcsec² field of view sampled at 25 milli-arcsec. In this mode MUSE should be able to obtain diffraction limited data-cubes in the 0.6-0.93 μm wavelength range. Although the MUSE design has been optimized for the study of galaxy formation and evolution, it has a wide range of possible applications; e.g. monitoring of outer planets atmosphere, environment of young stellar objects, super massive black holes and active nuclei in nearby galaxies or massive spectroscopic surveys of stellar fields in the Milky Way and nearby galaxies.

We used the Vimos VLT Deep Survey in combination with other spectroscopic, photometric, and X-ray surveys from the literature to detect several galaxy structures in the Chandra Deep Field South (CDFS). Both a friend-of-friend-based algorithm applied to the spectroscopic redshift catalog and an adaptative kernel galaxy density and color maps correlated with photometric redshift estimates were used. We mainly detected a chain-like structure at and two massive groups at and 1.098 showing signs of ongoing collapse. We also detected two galaxy walls at and at (extremely compact in redshift space). The first one contains the chain-like structure and the last one contains one of the two massive groups in its center. Finally, other galaxy structures that are probably loose low mass groups were detected. We compared the group galaxy population with simulations in order to assess the richness of these structures and studied their galaxy morphological contents. The higher redshift structures appear to probably have lower velocity dispersion than the nearby ones. The number of moderately massive structures we detected is consistent with what is expected for an LCDM model, but a larger sample is required to put significant cosmological constraints.

With the new generation of spectrographs, integral field spectroscopy is becoming a widely used observational technique. The Integral Field Unit (IFU) of the Visible Multi–Object Spectrograph (VIMOS) on the ESO VLT allows sampling of a field as large as 54^'' × 54^'', covered by 6400 fibers coupled with microlenses. We present here the methods of the data‐processing software that has been developed to extract the astrophysical signal of faint sources from the VIMOS IFU observations. We focus on the treatment of the fiber‐to‐fiber relative transmission and the sky subtraction, and the dedicated tasks we have built to address the peculiarities and unprecedented complexity of the data set. We review the automated process we have developed under the VIPGI data organization and reduction environment (Scodeggio et al. 2005), along with the quality control performed to validate the process. The VIPGI IFU data‐processing environment has been available to the scientific community to process VIMOS IFU data since 2003 November.

We present the first measurements of the Probability Distribution Function (PDF) of galaxy fluctuations in the four-passes, first-epoch VIMOS-VLT Deep Survey (VVDS) cone, covering deg between . We show that the PDF of density contrasts of the VVDS galaxies is an unbiased tracer of the underlying parent distribution up to redshift z=1.5, on scales R=8 and 10 h -1Mpc. The second moment of the PDF, i.e. the rms fluctuations of the galaxy density field, is to a good approximation constant over the full redshift baseline investigated: we find that, in redshift space, for galaxies brighter than has a mean value of in the redshift interval . The third moment, i.e. the skewness, increases with cosmic time: we find that the probability of having underdense regions is greater at than it was at . By comparing the PDF of galaxy density contrasts with the theoretically predicted PDF of mass fluctuations we infer the redshift-, density- and scale-dependence of the biasing function between galaxy and matter overdensities up to redshift z=1.5. Our results can be summarized as follows: i) the galaxy bias is an increasing function of redshift: evolution is marginal up to and more pronounced for ; ii) the formation of bright galaxies is inhibited below a characteristic mass-overdensity threshold whose amplitude increases with redshift and luminosity; iii) the biasing function is non linear in all the redshift bins investigated with non-linear effects of the order of a few to ~10 % on scales >5 h -1Mpc. By subdividing the sample according to galaxy luminosity and colors, we also show that: iv) brighter galaxies are more strongly biased than less luminous ones at every redshift and the dependence of biasing on luminosity at is in good agreement with what is observed in the local Universe; v) red objects are systematically more biased than blue objects at all cosmic epochs investigated, but the relative bias between red and blue objects is constant as a function of redshift in the interval , and its value () is similar to what is found at .