The zCOSMOS 10k-sample: the role of galaxy stellar mass in the colour-density relation up to z ~ 1

Aims. With the first ~10 000 spectra of the flux limited zCOSMOS sample (I_(AB) ≤ 22.5) we want to study the evolution of environmental effects on galaxy properties since z ~ 1.0, and to disentangle the dependence among galaxy colour, stellar mass and local density. Methods. We use our previously derived 3D local density contrast δ, computed with the 5th nearest neighbour approach, to study the evolution with z of the environmental effects on galaxy U-B colour, D4000 Å break and [OII]λ3727 equivalent width (EW[OII]). We also analyze the implications due to the use of different galaxy selections, using luminosity or stellar mass, and we disentangle the relations among colour, stellar mass and δ studying the colour-density relation in narrow mass bins. Results. We confirm that within a luminosity-limited sample (M_B ≤ −20.5 − z) the fraction of red (U − B ≥ 1) galaxies depends on δ at least up to z ~ 1, with red galaxies residing mainly in high densities. This trend becomes weaker for increasing redshifts, and it is mirrored by the behaviour of the fraction of galaxies with D4000 Å break ≥1.4. We also find that up to z ~ 1 the fraction of galaxies with log(EW[OII]) ≥ 1.15 is higher for lower δ, and also this dependence weakens for increasing z. Given the triple dependence among galaxy colours, stellar mass and δ, the colour-δ relation that we find in the luminosity-selected sample can be due to the broad range of stellar masses embedded in the sample. Thus, we study the colour-δ relation in narrow mass bins within mass complete subsamples, defining red galaxies with a colour threshold roughly parallel to the red sequence in the colour-mass plane. We find that once mass is fixed the colour-δ relation is globally flat up to z ~ 1 for galaxies with log(M/M_⊙) ≳ 10.7. This means that for these masses any colour-δ relation found within a luminosity-selected sample is the result of the combined colour-mass and mass-δ relations. On the contrary, even at fixed mass we observe that within 0.1 ≤ z ≤ 0.5 the fraction of red galaxies with log(M/M_⊙) ≲ 10.7 depends on δ. For these mass and redshift ranges, environment affects directly also galaxy colours. Conclusions. We suggest a scenario in which the colour depends primarily on stellar mass, but for an intermediate mass regime (10.2 ≲ log(M/M_⊙) ≲ 10.7) the local density modulates this dependence. These relatively low mass galaxies formed more recently, in an epoch when more evolved structures were already in place, and their longer SFH allowed environment-driven physical processes to operate during longer periods of time

The VIMOS VLT Deep Survey. The different assembly history of passive and star-forming L_B >= L*_B galaxies in the group environment at z < 1

We use the VIMOS VLT Deep Survey to study the close environment of galaxies in groups at 0.2 = L*_B galaxies (Me_B = M_B + 1.1z <= -20) are identified with Me_B <= -18.25 and within a relative distance 5h^-1 kpc <= rp <= 100h^-1 kpc and relative velocity Delta v <= 500 km/s . The richness N of a group is defined as the number of Me_B <= -18.25 galaxies belonging to that group. We split our principal sample into red, passive galaxies with NUV - r >= 4.25 and blue, star-forming galaxies with NUV - r < 4.25. We find that blue galaxies with a close companion are primarily located in poor groups, while the red ones are in rich groups. The number of close neighbours per red galaxy increases with N, with n_red being proportional to 0.11N, while that of blue galaxies does not depend on N and is roughly constant. In addition, these trends are found to be independent of redshift, and only the average n_blue evolves, decreasing with cosmic time. Our results support the following assembly history of L_B >= L*_B galaxies in the group environment: red, massive galaxies were formed in or accreted by the dark matter halo of the group at early times (z >= 1), therefore their number of neighbours provides a fossil record of the stellar mass assembly of groups, traced by their richness N. On the other hand, blue, less massive galaxies have recently been accreted by the group potential and are still in their parent dark matter halo, having the same number of neighbours irrespective of N. As time goes by, these blue galaxies settle in the group potential and turn red and/or fainter, thus becoming satellite galaxies in the group. With a toy quenching model, we estimate an infall rate of field galaxies into the group environment of R_infall = 0.9 - 1.5 x 10^-4 Mpc^-3 Gyr^-1 at z ~ 0.7.Comment: Astronomy and Astrophysics, in press. 11 pages, 11 figures, 4 tables. Minor changes with respect to the first versio

Discovery of a rich proto-cluster at z = 2.9 and associated diffuse cold gas in the VIMOS Ultra-Deep Survey (VUDS)

High-density environments are crucial places for studying the link between hierarchical structure formation and stellar mass growth in galaxies. In this work, we characterise a massive proto-cluster at z = 2.895 that we found in the COSMOS field using the spectroscopic sample of the VIMOS Ultra-Deep Survey (VUDS). This is one of the rare structures at z ~ 3 not identified around an active galactic nucleus (AGN) or a radio galaxy, thus it represents an ideal laboratory for investigating the formation of galaxies in dense environments. The structure comprises 12 galaxies with secure spectroscopic redshift in an area of ~ 7′ × 8′, in a total z range of Δz = 0.016. The measured galaxy number overdensity is δ_g = 12 ± 2. This overdensity has a total mass of M ~ 8.1 × 10^(14) M_⊙ in a volume of 13 × 15 × 17 Mpc^3. Simulations indicate that such an overdensity at z ~ 2.9 is a proto-cluster, which will collapse in a cluster of total mass M_(z = 0) ~ 2.5 × 10^(15) M_⊙ at z = 0, i.e. a massive cluster in the local Universe. We analysed the properties of the galaxies within the overdensity, and we compared them with acontrol sample at the same redshift but outside the overdensity. We could not find any statistically significant difference between the properties (stellar mass, star formation rate, specific star formation rate, NUV-r and r − K colours) of the galaxies inside and outside the overdensity, but this result might be due to the lack of statistics or possibly to the specific galaxy population sampled by VUDS, which could be less affected by environment than the other populations not probed by the survey. The stacked spectrum of galaxies in the background of the overdensity shows a significant absorption feature at the wavelength of Lyα redshifted at z = 2.895 (λ = 4736 Å), with a rest frame equivalent width (EW) of 4 ± 1.4 Å. Stacking only background galaxies without intervening sources at z ~ 2.9 along their line of sight, we find that this absorption feature has a rest frame EW of 10.8 ± 3.7 Å, with a detection S/N of ~4. We verify that this measurement is not likely to be due to noise fluctuations. These EW values imply a high column density (N(HI) ~ 3−20 × 10^(19) cm^(-2)), consistent with a scenario where such absorption is due to intervening cold streams of gas that are falling into the halo potential wells of the proto-cluster galaxies. Nevertheless, we cannot rule out the hypothesis that this absorption line is related to the diffuse gas within the overdensity

Comparison of the VIMOS-VLT Deep Survey with the Munich semi-analytical model. II. The colour-density relation up to z=1.5

[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.Comment: 15 pages, 12 figures, accepted for publication in A&

HeII emitters in the VIMOS VLT Deep Survey: PopIII star formation or peculiar stellar populations in galaxies at 2<z<4.6?

The aim of this work is to identify HeII emitters at 2<z<4.6 and to constrain the source of the hard ionizing continuum that powers the HeII emission. We have assembled a sample of 277 galaxies with a high quality spectroscopic redshift at 2<z<4.6 from the VVDS survey, and we have identified 39 HeII1640A emitters. We study their spectral properties, measuring the fluxes, equivalent widths (EW) and FWHM for most relevant lines. About 10% of galaxies at z~3 show HeII in emission, with rest frame equivalent widths EW0~1-7A, equally distributed between galaxies with Lya in emission or in absorption. We find 11 high-quality HeII emitters with unresolved HeII line (FWHM_0<1200km/s), 13 high-quality emitters with broad He II emission (FWHM_0>1200km/s), 3 AGN, and an additional 12 possible HeII emitters. The properties of the individual broad emitters are in agreement with expectations from a W-R model. On the contrary, the properties of the narrow emitters are not compatible with such model, neither with predictions of gravitational cooling radiation produced by gas accretion. Rather, we find that the EW of the narrow HeII line emitters are in agreement with expectations for a PopIII star formation, if the episode of star formation is continuous, and we calculate that a PopIII SFR of 0.1-10 Mo yr-1 only is enough to sustain the observed HeII flux. We conclude that narrow HeII emitters are either powered by the ionizing flux from a stellar population rare at z~0 but much more common at z~3, or by PopIII star formation. As proposed by Tornatore et al. (2007), incomplete ISM mixing may leave some small pockets of pristine gas at the periphery of galaxies from which PopIII may form, even down to z~2 or lower. If this interpretation is correct, we measure at z~3 a SFRD in PopIII stars of 10^6Mo yr^-1 Mpc^-3 qualitatively comparable to the value predicted by Tornatore et al. (2007).Comment: accepted for publication in A&

Studying the evolution of large-scale structure with the VIMOS-VLT Deep Survey

The VIMOS-VLT Deep Survey (VVDS) currently offers a unique combination of depth, angular size and number of measured galaxies among surveys of the distant Universe: ~ 11,000 spectra over 0.5 deg2 to I_{AB}=24 (VVDS-Deep), 35,000 spectra over ~ 7 deg2 to I_{AB}=22.5 (VVDS-Wide). The current ``First Epoch'' data from VVDS-Deep already allow investigations of galaxy clustering and its dependence on galaxy properties to be extended to redshifts ~1.2-1.5, in addition to measuring accurately evolution in the properties of galaxies up to z~4. This paper concentrates on the main results obtained so far on galaxy clustering. Overall, L* galaxies at z~ 1.5 show a correlation length r_0=3.6\pm 0.7. As a consequence, the linear galaxy bias at fixed luminosity rises over the same range from the value b~1 measured locally, to b=1.5 +/- 0.1. The interplay of galaxy and structure evolution in producing this observation is discussed in some detail. Galaxy clustering is found to depend on galaxy luminosity also at z~ 1, but luminous galaxies at this redshift show a significantly steeper small-scale correlation function than their z=0 counterparts. Finally, red galaxies remain more clustered than blue galaxies out to similar redshifts, with a nearly constant relative bias among the two classes, b_{rel}~1.4, despite the rather dramatic evolution of the color-density relation over the same redshift range.Comment: 14 pages. Extended, combined version of two invited review papers presented at: 1) XXVIth Astrophysics Moriond Meeting: "From Dark Halos to Light", March 2006, proc. edited by L.Tresse, S. Maurogordato and J. Tran Thanh Van (Editions Frontieres); 2) Vulcano Workshop 2006 "Frontier Objects in Astrophysics and Particle Physics", May 2006, proc. edited by F. Giovannelli & G. Mannocchi, Italian Physical Society (Editrice Compositori, Bologna

Measuring galaxy environment with the synergy of future photometric and spectroscopic surveys

We exploit the synergy between low-resolution spectroscopy and photometric redshifts to study environmental effects on galaxy evolution in slitless spectroscopic surveys from space. As a test case, we consider the future Euclid Deep survey (∼40 deg2), which combines a slitless spectroscopic survey limited at Hα flux ≥5 × 10−17 erg cm−2 s−1 and a photometric survey limited in H band (H ≤ 26). We use Euclid-like galaxy mock catalogues, in which we anchor the photometric redshifts to the 3D galaxy distribution of the available spectroscopic redshifts. We then estimate the local density contrast by counting objects in cylindrical cells with radius from 1 to 10 h−1Mpc, over the redshift range 0.9 < z < 1.8. We compare this density field with the one computed in a mock catalogue with the same depth as the Euclid Deep survey (H = 26) but without redshift measurement errors. We find that our method successfully separates high- from low-density environments (the last from the first quintile of the density distribution), with higher efficiency at low redshift and large cells: the fraction of low-density regions mistaken by high-density peaks is <1 per cent for all scales and redshifts explored, but for scales of 1 h−1Mpc for which is a few per cent. These results show that we can efficiently study environment in photometric samples if spectroscopic information is available for a smaller sample of objects that sparsely samples the same volume. We demonstrate that these studies are possible in the Euclid Deep survey, i.e. in a redshift range in which environmental effects are different from those observed in the local Universe, hence providing new constraints for galaxy evolution models

Structure detection in the D1 CFHTLS deep field using accurate photometric redshifts: a benchmark

We investigate structures in the D1 CFHTLS deep field in order to test the method that will be applied to generate homogeneous samples of clusters and groups of galaxies in order to constrain cosmology and detailed physics of groups and clusters. Adaptive kernel technique is applied on galaxy catalogues. This technique needs none of the usual a-priori assumptions (luminosity function, density profile, colour of galaxies) made with other methods. Its main drawback (decrease of efficiency with increasing background) is overcame by the use of narrow slices in photometric redshift space. There are two main concerns in structure detection. One is false detection and the second, the evaluation of the selection function in particular if one wants "complete" samples. We deal here with the first concern using random distributions. For the second, comparison with detailed simulations is foreseen but we use here a pragmatic approach with comparing our results to GalICS simulations to check that our detection number is not totally at odds compared to cosmological simulations. We use XMM-LSS survey and secured VVDS redshifts up to z~1 to check individual detections. We show that our detection method is basically capable to recover (in the regions in common) 100% of the C1 XMM-LSS X-ray detections in the correct redshift range plus several other candidates. Moreover when spectroscopic data are available, we confirm our detections, even those without X-ray data.Comment: 14 pages, 22 additionnal jpeg figures, accepted in A&

The VIMOS Public Extragalactic Redshift Survey (VIPERS). Exploring the dependence of the three-point correlation function on stellar mass and luminosity at 0.5<z<1.1

The three-point correlation function (3PCF) is a powerful probe to investigate the clustering of matter in the Universe in a complementary way with respect to lower-order statistics, providing additional information with respect to the two-point correlation function and allowing us to shed light on biasing, nonlinear processes, and deviations from Gaussian statistics. In this paper, we analyse the first data release of the VIMOS Public Extragalactic Redshift Survey (VIPERS), determining the dependence of the three-point correlation function on luminosity and stellar mass at $z=[0.5,1.1]$. We exploit the VIPERS Public Data Release 1, consisting of more than 50,000 galaxies with B-band magnitudes in the range $-21.6\lesssim M_{\rm B}-5\log(h)\lesssim-19.9$ and stellar masses in the range $9.8\lesssim\log(M_\star[h^{-2}\,M_\odot])\lesssim 10.7$. We measure both the connected 3PCF and the reduced 3PCF in redshift space, probing different configurations and scales, in the range $2.5<r\,$[Mpc/h]$<20$. We find a significant dependence of the reduced 3PCF on scales and triangle shapes, with stronger anisotropy at larger scales ($r\sim10$ Mpc/h) and an almost flat trend at smaller scales, $r\sim2.5$ Mpc/h. Massive and luminous galaxies present a larger connected 3PCF, while the reduced 3PCF is remarkably insensitive to magnitude and stellar masses in the range we explored. These trends, already observed at low redshifts, are confirmed for the first time to be still valid up to $z=1.1$, providing support to the hierarchical scenario for which massive and bright systems are expected to be more clustered. The possibility of using the measured 3PCF to provide independent constraints on the linear galaxy bias $b$ has also been explored, showing promising results in agreement with other probes.Comment: 12 pages, 11 figures, 2 tables, accepted for publication in A&

The VIMOS Public Extragalactic Redshift Survey (VIPERS): PCA-based automatic cleaning and reconstruction of survey spectra

Identifying spurious reduction artefacts in galaxy spectra is a challenge for large surveys. We present an algorithm for identifying and repairing residual spurious features in sky-subtracted galaxy spectra with application to the VIPERS survey. The algorithm uses principal component analysis (PCA) applied to the galaxy spectra in the observed frame to identify sky line residuals imprinted at characteristic wavelengths. We further model the galaxy spectra in the rest-frame using PCA to estimate the most probable continuum in the corrupted spectral regions, which are then repaired. We apply the method to 90,000 spectra from the VIPERS survey and compare the results with a subset where careful editing was performed by hand. We find that the automatic technique does an extremely good job in reproducing the time-consuming manual cleaning and does it in a uniform and objective manner across a large data sample. The mask data products produced in this work are released together with the VIPERS second public data release (PDR-2).Comment: Find the VIPERS data release at http://vipers.inaf.i