The VIRMOS/VLT Deep Survey (VVDS)

People involved: Bardelli, Cappi, Ciliegi, Merighi, McCracken, Pozzetti, Zamorani, Zucca.

The Bologna Astronomical Observatory is part of the Consortium for the ESO-VLT Instrument VIRMOS (Visual Infra-Red Multi-Object Spectrographs).

The visual spectrograph VIMOS is mounted at VLT-Melipal and saw its first light successfully on February 26, 2002. Commissioning and testing were completed in 2002 and the guaranteed time observations started in Autumn 2002.

The huge multiplex capabilities of VIMOS will allow to assemble redshift databases of large samples of faint galaxies. The Consortium's guaranteed time will be used to perform a deep redshift survey of about $10^5$ galaxies selected from both visual ($B$ and $I$) and infrared ($K$) defined samples. Given the large number of expected redshift measurements and the expected redshift coverage (up to $z \lower.5ex\hbox{$\; \buildrel > \over \sim \;$}2$, with a median redshift in the interval 0.6-0.9), this survey will allow to determine with excellent statistics the evolution with redshift of the luminosity functions in different bands for each galaxy type.
Just a few of the crucial issues which these data will allow to address are:
$\bullet$ detailed tests of the predictions of various models of galaxy evolution (e.g. hierarchical versus monolithic models);
$\bullet$ precise estimates, on the basis of a single sample with well understood selection criteria, of the star formation history up to at least $z\sim2$;
$\bullet$ studies of the still uncertain nature of the extremely red galaxies (EROs), determining which fraction of them is actually associated with old elliptical galaxies at high redshift and which fraction is associated with dusty starburst galaxies.

As a by-product, since no morphological selection will be applied to the objects to be observed, this survey, with its expected $\sim 1000$ AGNs down to $I\sim 24.0$, will allow the study of the optical luminosity function and evolution of the faint (e.g. Seyfert-like) AGNs in a magnitude range where the selection of AGN candidates with the standard color and morphological criteria is very difficult and, possibly, largely incomplete.

In particular, during the year 2003 the researchers at the Observatory involved in the VVDS have worked and contributed to the following topics:

$\bullet$ Observations and data reduction which have led to the public release (in 2004) of about 1600 redshifts to $I_{AB} \le 24$ across the Chandra Deep Field South.
$\bullet$ Production of the photometric multiband ($BVRI + K$) catalogues which are the starting point of the spectroscopic survey. Various tests have been applied to these catalogues in order to verify the quality of the data.
$\bullet$ Development of tools for the scientific analysis of the survey: in particular for the luminosity function analysis, with the implementation of different estimators and testing their statistical robustness on simulated samples with different completeness (Ilbert et al. 2004).
$\bullet$ Comparison of different cluster-finding methods, applying the algorithms on simulated samples (Rizzo et al. 2004).
$\bullet$ Preparation of the tools which will be used in the statistical analysis of the galaxy distribution, in particular the correlation function and high-order statistics.
$\bullet$ Preparation of different models of galaxy evolution in the framework of a Pure Luminosity Evolution (PLE) scenario to be used for the comparison with the observed galaxy redshift distribution.

Figure 8: Luminosity function in the B band (rest frame) for galaxies from the first epoch VVDS, derived with various estimators: $1/V_{max}$ (open circles), $C^+$ (open squares), SWML (open triangles) and STY (solid line). The grey area represents the $1\sigma$ uncertainties for the STY estimate. The dashed line represents the local estimate from the Sloan Digitized Sky Survey.

The first epoch data (to $I_{AB} = 24$) have been used to derive the luminosity function in different bands up to $z\sim 1.5$. The global luminosity function has been estimated in different redshift bins, using various estimators (see Fig. 8), revealing an overall brightening of this function by $\sim 1.5$ magnitudes at $z\sim1$, with a significant strong steepening of the faint end slope.
Given the large number of galaxies, it is possible to follow the evolution of the luminosity function for the different galaxy spectral types. All galaxies have been classified in four types, from early-type to blue star forming galaxies: while the luminosity function of the early-type population does not seem to evolve by more than $\sim 0.5$ magnitudes, the function of the blue star forming galaxies is strongly evolving with redshift.