ABSTRACT
We present the results of a new study investigating the relationship between observed Ly α equivalent width (Wλ(Ly α)) and the metallicity of the ionizing stellar population ( Z⋆) for a ...sample of 768 star-forming galaxies at 3 ≤ z ≤ 5 drawn from the VANDELS survey. Dividing our sample into quartiles of rest-frame Wλ(Ly α) across the range $-58 \,\rm {\mathring{\rm A}} \lesssim$Wλ(Ly α) $\lesssim 110 \,\rm {\mathring{\rm A}}$, we determine Z⋆ from full spectral fitting of composite far-ultraviolet spectra and find a clear anticorrelation between Wλ(Ly α) and Z⋆. Our results indicate that Z⋆ decreases by a factor ≳ 3 between the lowest Wλ(Ly α) quartile (〈Wλ(Ly α)$\rangle =-18\,\rm {\mathring{\rm A}}$) and the highest Wλ(Ly α) quartile (〈Wλ(Ly α)$\rangle =24\,\rm {\mathring{\rm A}}$). Similarly, galaxies typically defined as Lyman alpha emitters (LAEs; Wλ(Ly α) $\gt 20\,\rm {\mathring{\rm A}}$) are, on average, metal poor with respect to the non-LAE galaxy population (Wλ(Ly α) $\le 20\,\rm {\mathring{\rm A}}$) with Z⋆non-LAE ≳ 2 × Z⋆LAE. Finally, based on the best-fitting stellar models, we estimate that the increasing strength of the stellar ionizing spectrum towards lower Z⋆ is responsible for ${\simeq}15{-}25{{\ \rm per\ cent}}$ of the observed variation in Wλ(Ly α) across our sample, with the remaining contribution (${\simeq}75{-}85{{\ \rm per\ cent}}$) being due to a decrease in the H i/dust covering fractions in low- Z⋆ galaxies.
Abstract
We present the results of a new study of dust attenuation at redshifts 3 < z < 4 based on a sample of 236 star-forming galaxies from the VANDELS spectroscopic survey. Motivated by results ...from the First Billion Years (FiBY) simulation project, we argue that the intrinsic spectral energy distributions (SEDs) of star-forming galaxies at these redshifts have a self-similar shape across the mass range 8.2 ≤ log (M⋆/M⊙) ≤ 10.6 probed by our sample. Using FiBY data, we construct a set of intrinsic SED templates which incorporate both detailed star formation and chemical abundance histories, and a variety of stellar population synthesis (SPS) model assumptions. With this set of intrinsic SEDs, we present a novel approach for directly recovering the shape and normalization of the dust attenuation curve. We find, across all of the intrinsic templates considered, that the average attenuation curve for star-forming galaxies at z ≃ 3.5 is similar in shape to the commonly adopted Calzetti starburst law, with an average total-to-selective attenuation ratio of RV = 4.18 ± 0.29. In contrast, we find that an average attenuation curve as steep as the SMC extinction law is strongly disfavoured. We show that the optical attenuation (AV) versus stellar mass (M⋆) relation predicted using our method is consistent with recent ALMA observations of galaxies at 2 < z < 3 in the Hubble Ultra Deep Field (HUDF), as well as empirical AV − M⋆ relations predicted by a Calzetti-like law. In fact, our results, combined with other literature data, suggest that the AV–M⋆ relation does not evolve over the redshift range 0 < z < 5, at least for galaxies with log(M⋆/M⊙) ≳ 9.5. Finally, we present tentative evidence which suggests that the attenuation curve may become steeper at lower masses log(M⋆/M⊙) ≲ 9.0.
ABSTRACT
We present a comparative analysis of the properties of active galactic nuclei (AGNs) emitting at radio and X-ray wavelengths. The study is performed on 907 X-ray AGNs and 100 radio AGNs ...selected on the CDFS and UDS fields and makes use of new and ancillary data available to the VANDELS collaboration. Our results indicate that the mass of the host galaxy is a fundamental quantity that determines the level of AGN activity at the various wavelengths. Indeed, large stellar masses are found to be connected with AGN radio emission, as virtually all radio-active AGNs reside within galaxies of M* > 1010 M⊙. Large stellar masses also seem to favour AGN activity in the X-ray, even though X-ray AGNs present a mass distribution that is more spread out and with a non-negligible tail at M* ≲ 109 M⊙. Stellar mass alone is also observed to play a fundamental role in simultaneous radio and X-ray emission: the percentage of AGNs active at both wavelengths increases from around 1 per cent of all X-ray AGNs residing within hosts of M* < 1011 M⊙ to ∼13 per cent in more massive galaxies. In the case of radio-selected AGNs, such a percentage moves from ∼15 per cent to ∼45 per cent (but up to ∼80 per cent in the deepest fields). Neither cosmic epoch, nor radio luminosity, X-ray luminosity, Eddington ratio or star formation rate of the hosts are found to be connected to an enhanced probability for joint radio + X-ray emission of AGN origin. Furthermore, only a loose relation is observed between X-ray and radio luminosity in those AGNs that are simultaneously active at both frequencies.
We present the Galaxy Stellar Mass Function (GSMF) up to z~1 from the zCOSMOS-bright 10k spectroscopic sample. We investigate the total MF and the contribution of ETGs and LTGs, defined by broad-band ...SED, morphology, spectral properties or star formation activities. We unveil a galaxy bimodality in the global MF, at least up to the z~0.55, better represented by 2 Schechter functions dominated by ETGs and LTGs, respectively. For the global population we confirm that low-mass galaxies number density increases later and faster than for massive galaxies. We find that the MF evolution at intermediate-low Mstar (logM<10.6) is mostly explained by a growth in stellar mass driven by smooth and decreasing SFHs. The low/negligible evolution at higher Mstar sets a limit of 30-15%, decreasing with redshift, to the fraction of major merging. We find that ETGs increase in number density with cosmic time faster for decreasing Mstar, with a median "building redshift" increasing with mass, in contrast with hierarchical model predictions. For LTGs we find that the number density of blue or spiral galaxies remains almost constant with cosmic time from z~1. Instead, the most extreme population of active star forming galaxies is rapidly decreasing in number density. We suggest, firstly, a transformation from blue active spiral galaxies of intermediate mass into blue quiescent and successively (1-2 Gyr after) into red passive types. The complete morphological transformation into red spheroidal galaxies, required longer time-scales or follows after 1-2 Gyr. A continuous replacement of blue galaxies is expected by low-mass active spirals growing in stellar mass. We estimate that on average ~25% of blue galaxies is transforming into red per Gyr for logM<11. We expect a negligible evolution of the global Galaxy Baryonic MF. ABRIDGED
Context. To identify the galaxies responsible for the reionization of the Universe, we must rely on the investigation of the Lyman continuum (LyC) properties of z ≲ 5 star-forming galaxies, where we ...can still directly observe their ionizing radiation. Aims. The aim of this work is to explore the correlation between the LyC emission and some of the proposed indirect indicators of LyC radiation at z ~ 4 such as a bright Lyα emission and a compact UV continuum size. Methods. We selected a sample of 201 star-forming galaxies from the Vimos Ultra Deep Survey (VUDS) at 3.5 ≤ z ≤ 4.3 in the COSMOS, ECDFS, and VVDS-2h fields, including only those with reliable spectroscopic redshifts, a clean spectrum in the LyC range and clearly not contaminated by bright nearby sources in the same slit. For all galaxies we measured the Lyα EW, the Lyα velocity shift with respect to the systemic redshift, the Lyα spatial extension and the UV continuum effective radius. We then selected different sub-samples according to the properties predicted to be good LyC emission indicators: in particular we created sub-samples of galaxies with EW(Lyα) ≥ 70 Å, Lyαext ≤ 5.7 kpc, rUV ≤ 0.30 kpc and |ΔvLyα|≤ 200 km s−1. We stacked all the galaxies in each sub-sample and measured the flux density ratio (fλ(895)∕fλ(1470)), that we considered to be a proxy for LyC emission. We then compared these ratios to those obtained for the complementary samples. Finally, to estimate the statistical contamination from lower redshift inter-lopers in our samples, we performed dedicated Monte Carlo simulations using an ultradeep U-band image of the ECDFS field. Results. We find that the stacks of galaxies which are UV compact (rUV ≤ 0.30 kpc) and have bright Lyα emission (EW(Lyα) ≥ 70 Å), have much higher LyC fluxes compared to the rest of the galaxy population. These parameters appear to be good indicators of LyC radiation in agreement with theoretical studies and previous observational works. In addition we find that galaxies with a low Lyα spatial extent (Lyαext ≤ 5.7 kpc) have higher LyC flux compared to the rest of the population. Such a correlation had never been analysed before and seems even stronger than the correlation with high EW(Lyα) and small rUV. These results assume that the stacks from all sub-samples present the same statistical contamination from lower redshift interlopers. If we subtract a statistical contamination from low redshift interlopers obtained with the simulations from the flux density ratios (fλ(895)∕fλ(1470)) of the significant sub-samples we find that these samples contain real LyC leaking flux with a very high probability, although the true average escape fractions are very uncertain. Conclusions. Our work indicates that galaxies with very high EW(Lyα), small Ly αext and small rUV are very likely the best candidates to show Lyman continuum radiation at z ~ 4 and could therefore be the galaxies that have contributed most to reionisation.
We exploit the first public data release of VIPERS to investigate environmental effects in the evolution of galaxies between z ~ 0.5 and 0.9. The large number of spectroscopic redshifts (more than 50 ...000) over an area of about 10 deg2 provides a galaxy sample with high statistical power. We estimate the stellar mass function of galaxies residing in these two environments and constrain the high-mass end with unprecedented precision. We observe an evolution of the stellar mass function of VIPERS galaxies in high densities, while the low-density one is nearly constant. We compare these results to semi-analytical models and find consistent environmental signatures in the simulated stellar mass functions. We discuss how the halo mass function and fraction of central/satellite galaxies depend on the environments considered, making intrinsic and environmental properties of galaxies physically coupled, hence difficult to disentangle. The same formalism could also describe the evolution of the mass function in the high density regions, but only if a significant contribution from dry mergers is considered.
Aims. We trace the evolution and the star formation history of passive red galaxies, using a subset of the VIMOS Public Extragalactic Redshift Survey (VIPERS). The detailed spectral analysis of ...stellar populations of intermediate-redshift passive red galaxies allows the build up of their stellar content to be followed over the last 8 billion years. Methods. We extracted a sample of passive red galaxies in the redshift range 0.4 <z< 1.0 and stellar mass range 10 < log(M sub(star)/M sub(middot in circle)) < 12 from the VIPERS survey. The sample was selected using an evolving cut in the rest-frame U?V color distribution and additional cuts that ensured high quality. The spectra of passive red galaxies were stacked in narrow bins of stellar mass and redshift. We use the stacked spectra to measure the 4000 A break (D 4000) and the H delta Lick index (H delta sub(A)) with high precision. These spectral features are used as indicators of the star formation history of passive red galaxies. We compare the results with a grid of synthetic spectra to constrain the star formation epochs of these galaxies. We characterize the formation redshift-stellar mass relation for intermediate-redshift passive red galaxies. Results. We find that at z~ 1 stellar populations in low-mass passive red galaxies are younger than in high-mass passive red galaxies, similar to what is observed at the present epoch. Over the full analyzed redshift range 0.4 < z< 1.0 and stellar mass range 10 < log(M sub(star)/M sub(middot in circle)) < 12, the D 4000 index increases with redshift, while H delta sub(A) gets lower. This implies that the stellar populations are getting older with increasing stellar mass. Comparison to the spectra of passive red galaxies in the SDSS survey (z~ 0.2) shows that the shape of the relations of D 4000 and H delta sub(A) with stellar mass has not changed significantly with redshift. Assuming a single burst formation, this implies that high-mass passive red galaxies formed their stars at z sub(form)~ 1.7, while low-mass galaxies formed their main stellar populations more recently, at z sub(form)~ 1. The consistency of these results, which were obtained using two independent estimators of the formation redshift (D 4000 and H delta sub(A)), further strengthens a scenario in which star formation proceeds from higher to lower mass systems as time passes, i.e., what has become known as the downsizing picture.
We present a sample of 34 spectroscopically confirmed BzK-selected ~1011M⊙ quiescent galaxies (pBzK) in the COSMOS field. The targets were initially observed with VIMOS on the VLT to facilitate the ...calibration of the photometric redshifts of massive galaxies at z ≳ 1.5. Here we describe the reduction and analysis of the data, and the spectrophotometric properties of these pBzK galaxies. In particular, using a spatially resolved median 2D spectrum, we find that the fraction of stellar populations with ages <1 Gyr is at least 3 times higher in the outer regions of the pBzK galaxies than in their cores. This results in a mild age gradient of Δage ≤ 0.4 Gyr over ~6 kpc and suggests either the occurrence of widespread rejuvenation episodes or that inside-out quenching played a role in the passivization of this galaxy population. We also report on low-level star formation rates derived from the OII3727 Å emission line, with SFROII ~ 3.7−4.5M⊙ yr-1. This estimate is confirmed by an independent measurement on a separate sample of similarly-selected quiescent galaxies in the COSMOS field, using stacked far-infrared data (SFRFIR ~ 2−4M⊙ yr-1). This second, photometric sample also displays significant excess at 1.4 GHz, suggestive of the presence of radio-mode AGN activity.
The estimate of radial abundance gradients in high-redshift galaxies allows to constrain their star formation history and their interplay with the surrounding intergalactic medium. The authors ...present VLT/SINFONI integral-field spectroscopy of a first sample of 50 galaxies at z similar to 1.2 in the MASSIV survey. Using the N2 ratio between the NII 6584 and H alpha rest-frame optical emission lines as a proxy for oxygen abundance in the interstellar medium, they measured the metallicity of the sample galaxies. Seven galaxies display a positive gradient at a high confidence level. Four out of these are interacting, and one is a chain galaxy. They suggest that interactions might be responsible for shallowing and even inverting the abundance gradient. They also identify two interesting correlations in their sample: galaxies with higher gas velocity dispersion have shallower/positive gradients; and metal-poor galaxies tend to show a positive gradient, whereas metal-rich ones tend to show a negative one.
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