In this paper we aim at improving constraints on the epoch of galaxy formation by measuring the ages of 3597 galaxies with reliable spectroscopic redshifts 2 ≤ z ≤ 6.5 in the VIMOS Ultra Deep Survey ...(VUDS). We derive ages and other physical parameters from the simultaneous fitting with the GOSSIP+ software of observed UV rest-frame spectra and photometric data from the u band up to 4.5 μm using model spectra from composite stellar populations. We perform extensive simulations and conclude that at z ≥ 2 the joint analysis of spectroscopy and photometry, combined with restricted age possibilities when taking the age of the Universe into account, substantially reduces systematic uncertainties and degeneracies in the age derivation; we find that age measurements from this process are reliable. We find that galaxy ages range from very young with a few tens of million years to substantially evolved with ages up to 1.5 Gyr or more. This large age spread is similar for different age definitions including ages corresponding to the last major star formation event, stellar mass-weighted ages, and ages corresponding to the time since the formation of 25% of the stellar mass. We derive the formation redshift zf from the measured ages and find galaxies that may have started forming stars as early as zf ~ 15. We produce the formation redshift function (FzF), the number of galaxies per unit volume formed at a redshift zf, and compare the FzF in increasing observed redshift bins finding a remarkably constant FzF. The FzF is parametrized with (1 + z)ζ, where ζ ≃ 0.58 ± 0.06, indicating a smooth increase of about 2 dex from the earliest redshifts, z ~ 15, to the lowest redshifts of our sample at z ~ 2. Remarkably, this observed increase in the number of forming galaxies is of the same order as the observed rise in the star formation rate density (SFRD). The ratio of the comoving SFRD with the FzF gives an average SFR per galaxy of ~7−17M⊙/yr at z ~ 4−6, in agreement with the measured SFR for galaxies at these redshifts. From the smooth rise in the FzF we infer that the period of galaxy formation extends all the way from the highest possible formation redshifts that we can probe at z ~ 15 down to redshifts z ~ 2. This indicates that galaxy formation is a continuous process over cosmic time, with a higher number of galaxies forming at the peak in SFRD at z ~ 2 than at earlier epochs.
Using new spectroscopic observations obtained as part of the VIMOS Ultra-Deep Survey (VUDS), we performed a systematic search for overdense environments in the early universe (z> 2) and report here ...on the discovery of Cl J0227-0421, a massive protocluster at z = 3.29. This protocluster is characterized by both the large overdensity of spectroscopically confirmed members, δgal = 10.5 ± 2.8, and a significant overdensity in photometric redshift members. The halo mass of this protocluster is estimated by a variety of methods to be ~3 × 1014ℳ⊙ at z ~ 3.3, which, evolved to z = 0 results in a halo mass rivaling or exceeding that of the Coma cluster. The properties of 19 spectroscopically confirmed member galaxies are compared with a large sample of VUDS/VVDS galaxies in lower density field environments at similar redshifts. We find tentative evidence for an excess of redder, brighter, and more massive galaxies within the confines of the protocluster relative to the field population, which suggests that we may be observing the beginning ofenvironmentally induced quenching. The properties of these galaxies are investigated, including a discussion of the brightest protocluster galaxy, which appears to be undergoing vigorous coeval nuclear and starburst activity. The remaining member galaxies appear to have characteristics that are largely similar to the field population. Though we find weaker evidence of the suppression of the median star formation rates among and differences in the stacked spectra of member galaxies with respect to the field, we defer any conclusions about these trends to future work with the ensemble of protostructures that are found in the full VUDS sample.
Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly and size growth, remains an open question. In this paper we measure the merger fraction and rate, both ...minor and major, of massive early-type galaxies (M ⋆ ≥ 1011 M⊙) in the COSMOS field, and study their role in mass and size evolution. Methods. We used the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation on the sky plane 10 h-1 kpc ≤ rp ≤ 30 h-1 kpc and a relative velocity Δv ≤ 500 km s-1 in redshift space. We measured both major (stellar mass ratio μ ≡ M ⋆ ,2/M ⋆ ,1 ≥ 1/4) and minor (1/10 ≤ μ < 1/4) merger fractions of massive galaxies, and studied their dependence on redshift and on morphology (early types vs. late types). Results. The merger fraction and rate of massive galaxies evolves as a power-law (1 + z)n, with major mergers increasing with redshift, nMM = 1.4, and minor mergers showing little evolution, nmm ~ 0. When split by their morphology, the minor merger fraction for early-type galaxies (ETGs) is higher by a factor of three than that for late-type galaxies (LTGs), and both are nearly constant with redshift. The fraction of major mergers for massive LTGs evolves faster (nMMLT ~ 4 ) than for ETGs (nMMET= 1.8). Conclusions. Our results show that massive ETGs have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ~ 1, leading to a mass growth of ~30%. We find that μ ≥ 1/10 mergers can explain ~55% of the observed size evolution of these galaxies since z ~ 1. Another ~20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (μ < 1/10) could contribute with an extra ~20%. The remaining ~5% should come from other processes (e.g., adiabatic expansion or observational effects). This picture also reproduces the mass growth and the velocity dispersion evolution of these galaxies. We conclude from these results, and after exploring all the possible uncertainties in our picture, that merging is the main contributor to the size evolution of massive ETGs at z ≲ 1, accounting for ~50−75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (μ < 1/4) events.
The growth of brightest cluster galaxies (BCGs) is closely related to the properties of their host cluster. We present evidence for dry mergers as the dominant source of BCG mass growth at z ≲ 1 in ...the XXL 100 brightest cluster sample. We use the global red sequence, Hα emission and mean star formation history to show that BCGs in the sample possess star formation levels comparable to field ellipticals of similar stellar mass and redshift. XXL 100 brightest clusters are less massive on average than those in other X-ray selected samples such as LoCuSS or HIFLUGCS. Few clusters in the sample display high central gas concentration, rendering inefficient the growth of BCGs via star formation resulting from the accretion of cool gas. Using measures of the relaxation state of their host clusters, we show that BCGs grow as relaxation proceeds. We find that the BCG stellar mass corresponds to a relatively constant fraction 1 per cent of the total cluster mass in relaxed systems. We also show that, following a cluster scale merger event, the BCG stellar mass lags behind the expected value from the Mcluster–MBCG relation but subsequently accretes stellar mass via dry mergers as the BCG and cluster evolve towards a relaxed state.
We explore the role of environment in the evolution of galaxies over 0.1 < z < 0.7 using the final zCOSMOS-bright data set. Using the red fraction of galaxies as a proxy for the quenched population, ...we find that the fraction of red galaxies increases with the environmental overdensity δ and with the stellar mass M
*, consistent with previous works. As at lower redshift, the red fraction appears to be separable in mass and environment, suggesting the action of two processes: mass m(M
*) and environmental ρ(δ) quenching. The parameters describing these appear to be essentially the same at z ∼ 0.7 as locally. We explore the relation between red fraction, mass and environment also for the central and satellite galaxies separately, paying close attention to the effects of impurities in the central-satellite classification and using carefully constructed samples well matched in stellar mass. There is little evidence for a dependence of the red fraction of centrals on overdensity. Satellites are consistently redder at all overdensities, and the satellite quenching efficiency, sat(δ, M
*), increases with overdensity at 0.1 < z < 0.4. This is less marked at higher redshift, but both are nevertheless consistent with the equivalent local measurements. At a given stellar mass, the fraction of galaxies that are satellites, f
sat(δ, M
*), also increases with overdensity. The obtained ρ(δ)/f
sat(δ, M
*) agrees well with sat(δ, M
*), demonstrating that the environmental quenching in the overall population is consistent with being entirely produced by a satellite quenching process at least up to z = 0.7. However, despite the unprecedented size of our high-redshift samples, the associated statistical uncertainties are still significant and our statements should be understood as approximations to physical reality, rather than physically exact formulae.
We explore the accuracy of the clustering-based redshift estimation proposed by Ménard et al. when applied to VIMOS Public Extragalactic Redshift Survey (VIPERS) and Canada–France–Hawaii Telescope ...Legacy Survey (CFHTLS) real data. This method enables us to reconstruct redshift distributions from measurement of the angular clustering of objects using a set of secure spectroscopic redshifts. We use state-of-the-art spectroscopic measurements with i
AB < 22.5 from the VIPERS as reference population to infer the redshift distribution of galaxies from the CFHTLS T0007 release. VIPERS provides a nearly representative sample to a flux limit of i
AB < 22.5 at a redshift of >0.5 which allows us to test the accuracy of the clustering-based redshift distributions. We show that this method enables us to reproduce the true mean colour–redshift relation when both populations have the same magnitude limit. We also show that this technique allows the inference of redshift distributions for a population fainter than the reference and we give an estimate of the colour–redshift mapping in this case. This last point is of great interest for future large-redshift surveys which require a complete faint spectroscopic sample.
Context. The mass assembly of galaxies can proceed through different physical processes. Here we report on the spectroscopic identification of close physical pairs of galaxies at redshifts 2 ≲ z< 4 ...and discuss the impact of major mergers in building galaxies at these early cosmological times. Aims. We aim to identify and characterize close physical pairs of galaxies destined to merge and use their properties to infer the contribution of merging processes to the early mass assembly of galaxies. Methods. We searched for galaxy pairs with a transverse separation rp ≤ 25h-1 kpc and a velocity difference Δv ≤ 500 km s-1 using early data from the VIMOS Ultra Deep Survey (VUDS) that comprise a sample of 1111 galaxies with spectroscopic redshifts measurements at redshifts 1.8 ≤ z ≤ 4 in the COSMOS, ECDFS, and VVDS–02h fields, combined with VVDS data. We analysed their spectra and associated visible and near-infrared photometry to assess the main properties of merging galaxies that have an average stellar mass M⋆ = 2.3 × 1010 M⊙ at these redshifts. Results. Using the 12 physical pairs found in our sample we obtain a first robust measurement of the major merger fraction at these redshifts, fMM = 19.4-6+9%. These pairs are expected to merge within 1 Gyr on average each producing a more massive galaxy by the time the cosmic star formation peaks at z ~ 1 − 2. Using the pairs’ merging time scales, we derive a merging rate of RMM = 0.17-0.05+0.08 Gyr−1. From the average mass ratio between galaxies in the pairs, the stellar mass of the resulting galaxy after merging will be ~60% higher than the most massive galaxy in the pair before merging. We conclude that major merging of galaxy pairs is on-going at 2 ≲ z< 4 and is significantly contributing to the major mass assembly phase of galaxies at this early epoch.
The VIMOS Ultra Deep Survey Durkalec, A.; Le Fèvre, O.; Pollo, A. ...
Astronomy and astrophysics (Berlin),
04/2018, Volume:
612
Journal Article
Peer reviewed
Open access
We present a study of the dependence of galaxy clustering on luminosity and stellar mass in the redshift range 2 < z < 3.5 using 3236 galaxies with robust spectroscopic redshifts from the VIMOS Ultra ...Deep Survey (VUDS), covering a total area of 0.92 deg2. We measured the two-point real-space correlation function wp(rp) for four volume-limited subsamples selected by stellar mass and four volume-limited subsamples selected by MUV absolute magnitude. We find that the scale-dependent clustering amplitude r0 significantly increases with increasing luminosity and stellar mass. For the least luminous galaxies (MUV < −19.0), we measured a correlation length r0 = 2.87 ± 0.22 h−1 Mpc and slope γ = 1.59 ± 0.07, while for the most luminous (MUV < −20.2) r0 = 5.35 ± 0.50 h−1 Mpc and γ = 1.92 ± 0.25. These measurements correspond to a strong relative bias between these two subsamples of Δb∕b* = 0.43. Fitting a five-parameter halo occupation distribution (HOD) model, we find that the most luminous (MUV < −20.2) and massive (M⋆ > 1010 h−1 M⊙) galaxies occupy the most massive dark matter haloes with ⟨Mh⟩ = 1012.30 h−1 M⊙. Similar to the trends observed at lower redshift, the minimum halo mass Mmin depends on the luminosity and stellar mass of galaxies and grows from Mmin = 109.73 h−1 M⊙ to Mmin = 1011.58 h−1 M⊙ from the faintest to the brightest among our galaxy sample, respectively. We find the difference between these halo masses to be much more pronounced than is observed for local galaxies of similar properties. Moreover, at z ~ 3, we observe that the masses at which a halo hosts, on average, one satellite and one central galaxy is M1 ≈ 4Mmin over all luminosity ranges, which is significantly lower than observed at z ~ 0; this indicates that the halo satellite occupation increases with redshift. The luminosity and stellar mass dependence is also reflected in the measurements of the large-scale galaxy bias, which we model as bg,HOD (>L) = 1.92 + 25.36(L/L*)7.01. We conclude our study with measurements of the stellar-to-halo mass ratio (SHMR). We observe a significant model-observation discrepancy for low-mass galaxies, suggesting a higher than expected star formation efficiency of these galaxies.
Context.
Future weak lensing surveys, such as the
Euclid
mission, will attempt to measure the shapes of billions of galaxies in order to derive cosmological information. These surveys will attain ...very low levels of statistical error, and systematic errors must be extremely well controlled. In particular, the point spread function (PSF) must be estimated using stars in the field, and recovered with high accuracy.
Aims.
The aims of this paper are twofold. Firstly, we took steps toward a nonparametric method to address the issue of recovering the PSF field, namely that of finding the correct PSF at the position of any galaxy in the field, applicable to
Euclid
. Our approach relies solely on the data, as opposed to parametric methods that make use of our knowledge of the instrument. Secondly, we studied the impact of imperfect PSF models on the shape measurement of galaxies themselves, and whether common assumptions about this impact hold true in an
Euclid
scenario.
Methods.
We extended the recently proposed resolved components analysis approach, which performs super-resolution on a field of under-sampled observations of a spatially varying, image-valued function. We added a spatial interpolation component to the method, making it a true 2-dimensional PSF model. We compared our approach to
PSFEx
, then quantified the impact of PSF recovery errors on galaxy shape measurements through image simulations.
Results.
Our approach yields an improvement over
PSFEx
in terms of the PSF model and on observed galaxy shape errors, though it is at present far from reaching the required
Euclid
accuracy. We also find that the usual formalism used for the propagation of PSF model errors to weak lensing quantities no longer holds in the case of an
Euclid
-like PSF. In particular, different shape measurement approaches can react differently to the same PSF modeling errors.
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