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Register a new userHeII emitters in the VIMOS VLT Deep Survey: PopIII star formation or peculiar stellar populations in galaxies at 2<z<4.6?
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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).
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Aims. The aim of this work is to study the contribution of the Ly-a emitters (LAE) to the star formation rate density (SFRD) of the Universe in the interval 2<z<6.6. Methods. We assembled a sample of 217 LAE from the Vimos-VLT Deep Survey (VVDS) with secure spectroscopic redshifts in the redshift range 2 < z < 6.62 and fluxes down to F=1.5x10^18 erg/s/cm^2. 133 LAE are serendipitous identifications in the 22 arcmin^2 total slit area surveyed with the VVDS-Deep and the 3.3 arcmin^2 from the VVDS Ultra-Deep survey, and 84 are targeted identifications in the 0.62 deg^2 surveyed with the VVDS-DEEP and 0.16 deg^2 from the Ultra-Deep survey. Among the serendipitous targets we estimate that 90% of the emission lines are most probably Ly-a, while the remaining 10% could be either [OII]3727 or Ly-a. We computed the LF and derived the SFRD from LAE at these redshifts. Results. The VVDS-LAE sample reaches faint line fluxes F(Lya) = 1.5x1^18 erg/s/cm^2 (corresponding to L(Lya)=10^41 erg/s at z~3) enabling the faint end slope of the luminosity function to be constrained to a=-1.6+-0.12 at redshift z~2.5 and to a=-1.78+0.1-0.12 at z=4, placing on firm statistical grounds trends found in previous LAE studies, and indicating that sub-L* LAE contribute significantly to the SFRD. The projected number density and volume density of faint LAE in 2<z<6.6 with F>1.5x10^18 erg/s/cm^2 are 33 galaxies/arcmin^2 and 4x10^-2 Mpc^-3, respectively. We find that the the observed luminosity function of LAE does not evolve from z=2 to z=6. This implies that, after correction for the redshift-dependent IGM absorption, the intrinsic LF must have evolved significantly over 3 Gyr. The SFRD from LAE contributes to about 20% of the SFRD at z =2-3, while the LAE appear to be the dominant source of star formation producing ionizing photons in the early universe z>5-6, becoming equivalent to that of Lyman Break galaxies.
The aim of this paper is to investigate spectral and photometric properties of 854 faint ($i_{AB}$<~25 mag) star-forming galaxies (SFGs) at 2<z<2.5 using the VIMOS Ultra-Deep Survey (VUDS) spectroscopic data and deep multi-wavelength photometric data in three extensively studied extragalactic fields (ECDFS, VVDS, COSMOS). These SFGs were targeted for spectroscopy based on their photometric redshifts. The VUDS spectra are used to measure the UV spectral slopes ($beta$) as well as Ly$alpha$ equivalent widths (EW). On average, the spectroscopically measured $beta$ (-1.36$pm$0.02), is comparable to the photometrically measured $beta$ (-1.32$pm$0.02), and has smaller measurement uncertainties. The positive correlation of $beta$ with the Spectral Energy Distribution (SED)-based measurement of dust extinction, E$_{rm s}$(B-V), emphasizes the importance of $beta$ as an alternative dust indicator at high redshifts. To make a proper comparison, we divide these SFGs into three subgroups based on their rest-frame Ly$alpha$ EW: SFGs with no Ly$alpha$ emission (SFG$_{rm N}$; EW$le$0AA), SFGs with Ly$alpha$ emission (SFG$_{rm L}$; EW$>$0AA), and Ly$alpha$ emitters (LAEs; EW$ge$20AA). The fraction of LAEs at these redshifts is $sim$10%, which is consistent with previous observations. We compared best-fit SED-estimated stellar parameters of the SFG$_{rm N}$, SFG$_{rm L}$ and LAE samples. For the luminosities probed here ($sim$L$^*$), we find that galaxies with and without Ly$alpha$ in emission have small but significant differences in their SED-based properties. We find that LAEs have less dust, and lower star-formation rates (SFR) compared to non-LAEs. We also find that LAEs are less massive compared to non-LAEs, though the difference is smaller and less significant compared to the SFR and E$_{rm s}$(B-V). [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.
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.
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.
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