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Register a new userThe VIMOS VLT Deep Survey: the redshift distribution N(z) of magnitude-limited samples down to iAB=24.75 and KsAB=22
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O. Le Fevre
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We measure and analyse the redshift distribution N(z) of magnitude-selected samples using spectroscopic redshift measurement from the magnitude-selected VIMOS VLT Deep Survey (VVDS) with 17<iAB<24.75. We compute the N(z) and provide reference parametric fits for i band, J, H and Ks band magnitud limited samples. The N(z) of a sample with iAB<24 has a mean redshift z=0.92, with 8.2% of the galaxies with z>2. Down to iAB<24.75 the sample has a mean redshift z=1.15 and 17.1% of the galaxies are beyond z=2. The projected sky density is 2.07+/-0.12 gal/arcmin2 at 1.4<z<2.5 and KsAB<22.5, 1.72+/-0.15 gal/arcmin2 at 2.7<z<3.4 and 0.59+/-0.09 gal/arcmin2 at 3.4<z<4.5 brighter than iAB=24.75. Galaxies at z~3 identified from magnitude-selected samples are 1.5 to 3 times more numerous than when they are colour-colour selected. We demonstrate that colour-colour selected samples over 1.4<z<4.5 are strongly contaminated by galaxies at other redshifts. Semi-analytic models on the Millennium simulations under-predict the number of luminous star-forming galaxies at zsim1.8-2, as well as over-predict the number of low-luminosity galaxies at z<0.8. Our study provides comprehensive galaxy number counts N(z) from galaxies with spectroscopic redshifts over a large redshift domain 0<z<5, a solid basis for the measurement of volume-complete quantities. Magnitude-selected surveys identify a higher number of galaxies at z>2 than in colour-colour selected samples, and we use the magnitude-selected VVDS to emphasize the large uncertainties associated to other surveys using colour or colour-colour selected samples. Our results further demonstrate that semi-analytical models on dark matter simulations have yet to find the right balance of physical processes and time-scales to properly reproduce a fundamental galaxy population property like the observed N(z).
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This paper presents a detailed comparison between high-redshift observations from the VIMOS-VLT Deep Survey (VVDS) and predictions from the Munich semi-analytical model of galaxy formation. In particular, we focus this analysis on the magnitude, redshift, and colour distributions of galaxies, as well as their clustering properties. We constructed 100 quasi-independent mock catalogues, using the output of the semi-analytical model presented in De Lucia & Blaizot (2007).We then applied the same observational selection function of the VVDS-Deep survey, so as to carry out a fair comparison between models and observations. We find that the semi-analytical model reproduces well the magnitude counts in the optical bands. It tends, however, to overpredict the abundance of faint red galaxies, in particular in the i and z bands. Model galaxies exhibit a colour bimodality that is only in qualitative agreement with the data. In particular, we find that the model tends to overpredict the number of red galaxies at low redshift and of blue galaxies at all redshifts probed by VVDS-Deep observations, although a large fraction of the bluest observed galaxies is absent from the model. In addition, the model overpredicts by about 14 per cent the number of galaxies observed at 0.2<z<1 with I_AB<24. When comparing the galaxy clustering properties, we find that model galaxies are more strongly clustered than observed ones at all redshift from z=0.2 to z=2, with the difference being less significant above z~1. When splitting the samples into red and blue galaxies, we find that the observed clustering of blue galaxies is well reproduced by the model, while red model galaxies are much more clustered than observed ones, being principally responsible for the strong global clustering found in the model. [abridged]
[Abridged] We present a homogeneous and complete catalogue of optical groups identified in the purely flux limited (17.5<=I<=24.0) VIMOS-VLT Deep Survey (VVDS). We use mock catalogues extracted from the MILLENNIUM simulation, to correct for potential systematics that might affect the overall distribution as well as the individual properties of the identified systems. Simulated samples allow us to forecast the number and properties of groups that can be potentially found in a survey with VVDS-like selection functions. We use them to correct for the expected incompleteness and also to asses how well galaxy redshifts trace the line-of-sight velocity dispersion of the underlying mass overdensity. In particular, we train on these mock catalogues the adopted group-finding technique (the Voronoi-Delaunay Method, VDM). The goal is to fine-tune its free parameters, recover in a robust and unbiased way the redshift and velocity dispersion distributions of groups and maximize the level of completeness (C) and purity (P) of the group catalogue. We identify 318 VVDS groups with at least 2 members within 0.2<=z<=1.0, among which 144 (/30) with at least 3 (/5) members. The sample has globally C=60% and P=50%. Nearly 45% of the groups with at least 3 members are still recovered if we run the algorithm with a parameter set which maximizes P (75%). We exploit the group sample to study the redshift evolution of the fraction f_b of blue galaxies (U-B<=1) within 0.2<=z<=1. We find that f_b is significantly lower in groups than in the whole ensemble of galaxies irrespectively of their environment. These quantities increase with redshift, with f_b in groups showing a marginally significant steeper increase. We also confirm that, at any explored redshift, f_b decreases for increasing group richness, and we extend towards fainter luminosities the magnitude range over which this result holds.
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<z<1.4. We observe a significant mass and optical color segregation over the whole redshift interval covered by our sample, such that the median value of the mass distribution is larger and the rest-frame optical color is redder in regions of high galaxy density. The amplitude of the mass segregation changes little with redshift, at least in the high stellar mass regime that we can uniformely sample over the 0.2<z<1.4 redshift interval. The color segregation, instead, decreases significantly for z>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.
We model the evolution of the mean galaxy occupation of dark-matter halos over the range $0.1<z<1.3$, using the data from the VIMOS-VLT Deep Survey (VVDS). The galaxy projected correlation function $w_p(r_p)$ was computed for a set of luminosity-limited subsamples and fits to its shape were obtained using two variants of Halo Occupation Distribution models. These provide us with a set of best-fitting parameters, from which we obtain the average mass of a halo and average number of galaxies per halo. We find that after accounting for the evolution in luminosity and assuming that we are largely following the same population, the underlying dark matter halo shows a growth in mass with decreasing redshift as expected in a hierarchical structure formation scenario. Using two different HOD models, we see that the halo mass grows by 90% over the redshift interval z=[0.5,1.0]. This is the first time the evolution in halo mass at high redshifts has been obtained from a single data survey and it follows the simple form seen in N-body simulations with $M(z) = M_0 e^{-beta z}$, and $beta = 1.3 pm 0.30$. This provides evidence for a rapid accretion phase of massive halos having a present-day mass $M_0 sim 10^{13.5} h^{-1} M_odot$, with a $m > 0.1 M_0$ merger event occuring between redshifts of 0.5 and 1.0. Futhermore, we find that more luminous galaxies are found to occupy more massive halos irrespectively of the redshift. Finally, the average number of galaxies per halo shows little increase from redshift z$sim$ 1.0 to z$sim$ 0.5, with a sharp increase by a factor $sim$3 from z$sim$ 0.5 to z$sim$ 0.1, likely due to the dynamical friction of subhalos within their host halos.
[Abridged] We perform on galaxy mock catalogues the same colour-density analysis made by Cucciati et al. (2006) on a 5 Mpc/h scale using the VVDS-Deep survey, and compare the results from mocks with observed data. We use mocks with the same flux limits (I=24) as the VVDS (CMOCKS), built using the semi- analytic model by De Lucia & Blaizot (2007) applied to the Millennium Simulation. From CMOCKS, we extracted samples of galaxies mimicking the VVDS observational strategy (OMOCKS). We computed the B-band Luminosity Function LF and the colour-density relation (CDR) in the mocks. We find that the LF in mocks roughly agrees with the observed LF, but at z<0.8 the faint-end slope of the model LF is steeper than the VVDS one. Computing the LF for early and late type galaxies, we show that mocks have an excess of faint early-type and of bright late-type galaxies with respect to data. We find that the CDR in OMOCKS is in excellent agreement with the one in CMOCKS. At z~0.7, the CDR in mocks agrees with the VVDS one (red galaxies reside mainly in high densities). Yet, the strength of the CDR in mocks does not vary within 0.2<z<1.5, while the observed relation flattens with increasing z and possibly inverts at z=1.3. We argue that the lack of evolution in the CDR in mocks is not due only to inaccurate prescriptions for satellite galaxies, but that also the treatment of central galaxies has to be revised. The reversal of the CDR can be explained by wet mergers between young galaxies, producing a starburst event. This should be seen on group scales. A residual of this is found in observations at z=1.5 on larger scales, but not in the mocks, suggesting that the treatment of physical processes affecting satellites and central galaxies in models should be revised.
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