R. Amorin

The VIMOS Ultra-Deep Survey: ~10 000 galaxies with spectroscopic redshifts to study galaxy assembly at early epochs 2 < z ≃ 6

We present the VIMOS Ultra Deep Survey (VUDS), a spectroscopic redshift survey of ∼10 000 very faint galaxies to study the major phase of galaxy assembly 2 < z ≃ 6. The survey covers 1 deg^2 in 3 separate fields: COSMOS, ECDFS and VVDS-02h, with targets selection based on an inclusive combination of photometric redshifts and color properties. Spectra covering 3650 < λ < 9350A are obtained with VIMOS on the ESO-VLT with integration times of 14h. Here we present the survey strategy, the target selection, the data processing, as well as the redshift measurement process, emphasizing the specific methods adapted to this high redshift range. The spectra quality and redshift reliability are discussed, and we derive a completeness in redshift measurement of 91%, or 74% for the most reliable measurements, down to i_(AB) = 25, and measurements are performed all the way down to i_(AB) = 27. The redshift distribution of the main sample peaks at z = 3 − 4 and extends over a large redshift range mainly in 2 < z < 6. At 3 < z < 5, the galaxies cover a large range of luminosities −23 < M_(NUV) < −20.5, stellar mass 10^9M_⊙ < M_∗ < 10^(11)M_⊙, and star formation rates 1 M_⊙/yr< S FR < 10^3M_⊙/yr. We discuss the spectral properties of galaxies using individual as well as stacked spectra. The comparison between spectroscopic and photometric redshifts as well as color selection demonstrate the effectiveness of our selection scheme. With ∼ 6000 galaxies with reliable spectroscopic redshifts in 2 < z < 6 expected when complete, this survey is the largest at these redshifts and offers the opportunity for unprecedented studies of the star-forming galaxy population and its distribution in large scale structures during the major phase of galaxy assembly.

Spectroscopy of superluminous supernova host galaxies: A preference of hydrogen-poor events for extreme emission line galaxies

Superluminous supernovae (SLSNe) were only discovered recently due to their preference for occurring in faint dwarf galaxies. Understanding why stellar evolution yields dierent types of stellar explosions in these environments is fundamental in order to both uncover the elusive progenitors of SLSNe and to study star formation in dwarf galaxies. In this paper, we present the rst results of our project to study SUperluminous Supernova Host galaxIES (SUSHIES), focusing on the sample for which we have obtained spectroscopy. We show that SLSNe-I and SLSNe-R (hydrogen-poor) often ( 50% in our sample) occur in a class of galaxies that is known as Extreme Emission Line Galaxies (EELGs). The probability of this happening by chance is negligible and we therefore conclude that the extreme environmental conditions and the SLSN phenomenon are related. In contrast, SLSNe-II (hydrogen-rich) occur in more massive, more metal-rich galaxies with softer radiation elds. Therefore, if SLSNe-II constitute a uniform class, their progenitor systems must be dierent from those of H-poor SLSNe. Gamma-ray bursts (GRBs) are, on average, not found in as extreme environments as H-poor SLSNe. We propose that H-poor SLSNe result from the very rst stars exploding in a starburst, even earlier than GRBs. This might indicate a bottom-light initial mass function in these systems. SLSNe present a novel method of selecting candidate EELGs independent of their luminosity.

The galaxy stellar mass function at 3.5 ≤ z ≤ 7.5 in the CANDELS/UDS, GOODS-South, and HUDF fields

Context. The form and evolution of the galaxy stellar mass function (GSMF) at high redshifts provide crucial information on star formation history and mass assembly in the young Universe, close or even prior to the epoch of reionization. Aims. We used the unique combination of deep optical/near-infrared/mid-infrared imaging provided by HST, Spitzer, and the VLT in the CANDELS-UDS, GOODS-South, and HUDF fields to determine the GSMF over the redshift range 3.5 ≤ z ≤ 7.5. Methods. We used the HST WFC3/IR near-infrared imaging from CANDELS and HUDF09, reaching H ≃ 27 − 28.5 over a total area of 369 arcmin^2, in combination with associated deep HST ACS optical data, deep Spitzer IRAC imaging from the SEDS programme, and deep Y and K-band VLT Hawk-I images from the HUGS programme, to select a galaxy sample with high-quality photometric redshifts. These have been calibrated with more than 150 spectroscopic redshifts in the range 3.5 ≤ z ≤ 7.5, resulting in an overall precision of σ_z/ (1 + z) ~ 0.037. With this database we have determined the low-mass end of the high-redshift GSMF with unprecedented precision, reaching down to masses as low as M^∗ ~ 10^9 M_⊙ at z = 4 and ~6 × 10^9 M_⊙ at z = 7. Results. We find that the GSMF at 3.5 ≤ z ≤ 7.5 depends only slightly on the recipes adopted to measure the stellar masses, namely the photometric redshifts, the star formation histories, the nebular contribution, or the presence of AGN in the parent sample. The low-mass end of the GSMF is steeper than has been found at lower redshifts, but appears to be unchanged over the redshift range probed here. Meanwhile the high-mass end of the GSMF appears to evolve primarily in density, although there is also some evidence of evolution in characteristic mass. Our results are very different from previous mass function estimates based on converting UV galaxy luminosity functions into mass functions via tight mass-to-light relations. Integrating our evolving GSMF over mass, we find that the growth of stellar mass density is barely consistent with the time-integral of the star formation rate density over cosmic time at z> 4. Conclusions. These results confirm the unique synergy of the CANDELS+HUDF, HUGS, and SEDS surveys for the discovery and study of moderate/low-mass galaxies at high redshifts, and reaffirm the importance of space-based infrared selection for the unbiased measurement of the evolving GSMF in the young Universe.The galaxy stellar mass function (GSMF) at high-z provides key information on star-formation history and mass assembly in the young Universe. We aimed to use the unique combination of deep optical/NIR/MIR imaging provided by HST, Spitzer and the VLT in the CANDELS-UDS, GOODS-South, and HUDF fields to determine the GSMF over the redshift range 3.5 4. These results confirm the unique synergy of the CANDELS+HUDF, HUGS, and SEDS surveys for the discovery and study of moderate/low-mass galaxies at high redshifts.

STELLAR MASSES FROM THE CANDELS SURVEY: THE GOODS-SOUTH AND UDS FIELDS

We present the public release of the stellar mass catalogs for the GOODS-S and UDS fields obtained using some of the deepest near-IR images available, achieved as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey project. We combine the effort from 10 different teams, who computed the stellar masses using the same photometry and the same redshifts. Each team adopted their preferred fitting code, assumptions, priors, and parameter grid. The combination of results using the same underlying stellar isochrones reduces the systematics associated with the fitting code and other choices. Thanks to the availability of different estimates, we can test the effect of some specific parameters and assumptions on the stellar mass estimate. The choice of the stellar isochrone library turns out to have the largest effect on the galaxy stellar mass estimates, resulting in the largest distributions around the median value (with a semi interquartile range larger than 0.1 dex). On the other hand, for most galaxies, the stellar mass estimates are relatively insensitive to the different parameterizations of the star formation history. The inclusion of nebular emission in the model spectra does not have a significant impact for the majority of galaxies (less than a factor of 2 for ~80% of the sample). Nevertheless, the stellar mass for the subsample of young galaxies (age <100 Myr), especially in particular redshift ranges (e.g., 2.2 < z < 2.4, 3.2 < z < 3.6, and 5.5 < z < 6.5), can be seriously overestimated (by up to a factor of 10 for <20 Myr sources) if nebular contribution is ignored.

An extreme [O III] emitter at z = 3.2: a low metallicity Lyman continuum source

Aims. Cosmic reionization is an important process occurring in the early epochs of the Universe. However, because of observational limitations due to the opacity of the intergalactic medium to Lyman continuum photons, the nature of ionizing sources is still not well constrained. While high-redshift star-forming galaxies are thought to be the main contributors to the ionizing background at z > 6, it is impossible to directly detect their ionizing emission. Therefore, looking at intermediate redshift analogues (z ∼ 2−4) can provide useful hints about cosmic reionization. Methods. We investigate the physical properties of one of the best Lyman continuum emitter candidate at z = 3.212 found in the GOODS-S/CANDELS field with photometric coverage from the U to the MIPS 24 μ mb and and VIMOS/VLT and MOSFIRE/Keck spectroscopy. These observations allow us to derive physical properties such as stellar mass, star formation rate, age of the stellar population, dust attenuation, metallicity, and ionization parameter, and to determine how these parameters are related to the Lyman continuum emission. Results. Investigation of the UV spectrum confirms a direct spectroscopic detection of the Lyman continuum emission with S /N > 5. Non-zero Lyα flux at the systemic redshift and high Lyman-α escape fraction (fesc(Lyα) ≥ 0.78) suggest a low H i column density. The weak C and Si low-ionization absorption lines are also consistent with a low covering fraction along the line of sight. The subsolar abundances are consistent with a young and extreme starburst. The [O iii]λλ4959, 5007+Hβ equivalent width (EW) is one of the largest reported for a galaxy at z > 3( EW([O iii]λλ4959, 5007 + Hβ) � 1600 A, rest-frame; 6700 A observed-frame) and the near-infrared spectrum shows that this is mainly due to an extremely strong [O iii] emission. The large observed [O iii]/[O ii] ratio (>10) and high ionization parameter are consistent with prediction from photoionization models in the case of a density-bounded nebula scenario. Furthermore, the EW([O iii]λλ4959, 5007+Hβ) is comparable to recent measurements reported at z ∼ 7−9, in the reionization epoch. We also investigate the possibility of an AGN contribution to explain the ionizing emission but most of the AGN identification diagnostics suggest that stellar emission dominates instead. Conclusions. This source is currently the first high-z example of a Lyman continuum emitter exhibiting indirect and direct evidences of a Lyman continuum leakage and having physical properties consistent with theoretical expectation from Lyman continuum emission from a density-bounded nebula. A low H i column density, low covering fraction, compact star formation activity, and a possible interaction/merging of two systems may contribute to the Lyman continuum photon leakage.

The Star Formation History and Metal Content of the Green Peas. New Detailed GTC-OSIRIS Spectrophotometry of Three Galaxies

We present deep broadband imaging and long-slit spectroscopy of three compact, low-mass starburst galaxies at redshift z ~ 0.2-0.3, also referred to as Green Peas (GP). We measure physical properties of the ionized gas and derive abundances for several species with high precision. We find that the three GPs display relatively low extinction, low oxygen abundances, and remarkably high nitrogen-to-oxygen ratios. We also report on the detection of clear signatures of Wolf-Rayet (W-R) stars in these galaxies. We carry out a pilot spectral synthesis study using a combination of both population and evolutionary synthesis models. Their outputs are in qualitative agreement, strongly suggesting a formation history dominated by starbursts. In agreement with the presence of W-R stars, these models show that these GPs currently undergo a major starburst producing between ~4% and ~20% of their stellar mass. However, as models imply, they are old galaxies that formed most of their stellar mass several Gyr ago. The presence of old stars has been spectroscopically verified in one of the galaxies by the detection of Mg I λλ5167, 5173 absorption lines. Additionally, we perform a surface photometry study based on Hubble Space Telescope data, which indicates that the three galaxies possess an exponential low surface brightness envelope. If due to stellar emission, the latter is structurally compatible with the evolved hosts of luminous blue compact dwarf (BCD)/H II galaxies, suggesting that GPs are identifiable with major episodes in the assembly history of local BCDs. These conclusions highlight the importance of these objects as laboratories for studying galaxy evolution at late cosmic epochs.

Constraints on the star-formation rate of z ~ 3 LBGs with measured metallicity in the CANDELS GOODS-South field

We analyse 14 LBGs at z~2.8-3.8 constituting the only sample where both a spectroscopic measurement of their metallicity and deep IR observations (CANDELS+HUGS survey) are available. Fixing the metallicity of population synthesis models to the observed values, we determine best-fit physical parameters under different assumptions about the star-formation history and also consider the effect of nebular emission. For comparison we determine the UV slope of the objects, and use it to estimate their SFR_UV99 by correcting the UV luminosity following Meurer et al. (1999). A comparison between SFR obtained through SED-fitting (SFR_fit) and the SFR_UV99 shows that the latter are underestimated by a factor 2-10, regardless of the assumed SFH. Other SFR indicators (radio, far-IR, X-ray, recombination lines) coherently indicate SFRs a factor of 2-4 larger than SFR_UV99 and in closer agreement with SFR_fit. This discrepancy is due to the solar metallicity implied by the usual beta-A1600 conversion factor. We propose a refined relation, appropriate for sub-solar metallicity LBGs: A1600 = 5.32+1.99beta. This relation reconciles the dust-corrected UV with the SED-fitting and the other SFR indicators. We show that the fact that z~3 galaxies have sub-solar metallicity implies an upward revision by a factor of ~1.5-2 of the global SFRD, depending on the assumptions about the age of the stellar populations. We find very young best-fit ages (10-500 Myrs) for all our objects. From a careful examination of the uncertainties in the fit and the amplitude of the Balmer break we conclude that there is little evidence of the presence of old stellar population in at least half of the LBGs in our sample, suggesting that these objects are probably caught during a huge star-formation burst, rather than being the result of a smooth evolution.

HUBBLE IMAGING OF THE IONIZING RADIATION FROM A STAR-FORMING GALAXY AT Z = 3.2 WITH *

Star-forming galaxies are considered to be the leading candidate sources that dominate the cosmic reionization at z>7, and the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently a active line of research. We have observed a star-forming galaxy at z=3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730-890A rest-frame, and detect LyC emission. This galaxy is very compact and also has large Oxygen ratio [OIII]5007/[OII]3727 (>=10). No nuclear activity is revealed from optical/near-infrared spectroscopy and deep multi-band photometry (including the 6Ms X-ray, Chandra). The measured escape fraction of ionizing radiation spans the range 50-100\%, depending on the IGM attenuation. The LyC emission is detected at S/N=10 with m(F336W)=27.57+/-0.11 and it is spatially unresolved, with effective radius R_e 7 allowing a direct comparison with lower redshift LyC emitters, as reported here.

The VIMOS Ultra-Deep Survey (VUDS): fast increase in the fraction of strong Lyman-α emitters from z = 2 to z = 6

Aims. The aim of this work is to constrain the evolution of the fraction of strong Ly alpha emitters among UV selected star-forming galaxies at 2 \textless z \textless 6, and to measure the stellar escape fraction of Ly alpha photons over the same redshift range. Methods. We exploit the ultradeep spectroscopic observations with VIMOS on the VLT collected by the VIMOS Ultra-Deep Survey (VUDS) to build an unique, complete, and unbiased sample of similar to 4000 spectroscopically confirmed star-forming galaxies at 2 \textless z \textless 6. Our galaxy sample includes UV luminosities brighter than M-FUV* at 2 \textless z \textless 6, and luminosities down to one magnitude fainter than M-FUV* at 2 \textless z \textless 3.5. Results. We find that 80% of the star-forming galaxies in our sample have EW0(Ly alpha) \textless 10 angstrom, and correspondingly f(esc)(Ly alpha) \textless 1%. By comparing these results with the literature, we conclude that the bulk of the Ly alpha luminosity at 2 \textless z \textless 6 comes from galaxies that are fainter in the UV than those we sample in this work. The strong Ly alpha emitters constitute, at each redshift, the tail of the distribution of the galaxies with extreme EW0(Ly alpha) and f(esc)(Ly alpha). This tail of large EW0(Ly alpha) and f(esc)(Ly alpha) becomes more important as the redshift increases, and causes the fraction of strong Ly alpha with EW0(Ly alpha) \textgreater 25 angstrom to increase from similar to 5% at z similar to 2 to similar to 30% at z similar to 6, with the increase being stronger beyond z similar to 4. We observe no difference, for the narrow range of UV luninosities explored in this work, between the fraction of strong Ly alpha emitters among galaxies fainter or brighter than M-FUV*, although the fraction for the faint galaxies evolves faster, at 2 \textless z \textless 3.5, than for the bright ones. We do observe an anticorrelation between E(B - V) and f(esc)(Ly alpha): generally galaxies with high f(esc)(Ly alpha) also have small amounts of dust (and vice versa). However, when the dust content is low (E(B - V) \textless 0.05) We observe a very broad range of f(esc)(Ly alpha), ranging from 10(-3) to 1. This implies that the dust alone is not the only regulator of the amount of escaping Ly alpha photons.

Hubble imaging of the ionizing radiation from a star-forming galaxy at Z = 3.2 with f(esc) > 50%

Star-forming galaxies are considered to be the leading candidate sources that dominate the cosmic reionization at z>7, and the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently a active line of research. We have observed a star-forming galaxy at z=3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730-890A rest-frame, and detect LyC emission. This galaxy is very compact and also has large Oxygen ratio [OIII]5007/[OII]3727 (>=10). No nuclear activity is revealed from optical/near-infrared spectroscopy and deep multi-band photometry (including the 6Ms X-ray, Chandra). The measured escape fraction of ionizing radiation spans the range 50-100\%, depending on the IGM attenuation. The LyC emission is detected at S/N=10 with m(F336W)=27.57+/-0.11 and it is spatially unresolved, with effective radius R_e 7 allowing a direct comparison with lower redshift LyC emitters, as reported here.