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Register a new userThe VANDELS survey: the stellar metallicities of star-forming galaxies at 2.5 < z < 5.0
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We present the results of a study utilising ultra-deep, rest-frame UV, spectroscopy to quantify the relationship between stellar mass and stellar metallicity for 681 star-forming galaxies at $2.5<z<5.0$ ($langle z rangle = 3.5 pm 0.6$) drawn from the VANDELS survey. Via a comparison with high-resolution stellar population models, we determine stellar metallicities for a set of composite spectra formed from subsamples selected by mass and redshift. Across the stellar mass range $8.5 < mathrm{log}(langle M_{ast} rangle/rm{M}_{odot}) < 10.2$ we find a strong correlation between stellar metallicity and stellar mass, with stellar metallicity monotonically increasing from $Z_{ast}/mathrm{Z}_{odot} < 0.09$ at $langle M_{ast} rangle = 3.2 times 10^{8} rm{M}_{odot}$ to $Z_{ast}/Z_{odot} = 0.27$ at $langle M_{ast} rangle = 1.7 times 10^{10} rm{M}_{odot}$. In contrast, at a given stellar mass, we find no evidence for significant metallicity evolution across the redshift range of our sample. However, comparing our results to the $z=0$ stellar mass-metallicity relation, we find that the $langle z rangle = 3.5$ relation is consistent with being shifted to lower metallicities by $simeq 0.6$ dex. Contrasting our derived stellar metallicities with estimates of gas-phase metallicities at similar redshifts, we find evidence for enhanced $rm{O}/rm{Fe}$ ratios of the order (O/Fe) $gtrsim 1.8$ $times$ (O/Fe)$_{odot}$. Finally, by comparing our results to simulation predictions, we find that the $langle z rangle = 3.5$ stellar mass-metallicity relation is consistent with current predictions for how outflow strength scales with galaxy mass. This conclusion is supported by an analysis of analytic models, and suggests that the mass loading parameter ($eta=dot{M}_{mathrm{outflow}}/M_{ast}$) scales as $eta propto M_{ast}^{beta}$ with $beta simeq -0.4$.
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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 $8.2 leq$ log$(M_{star}/M_{odot}) leq 10.6$ 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 $zsimeq3.5$ is similar in shape to the commonly-adopted Calzetti starburst law, with an average total-to-selective attenuation ratio of $R_{V}=4.18pm0.29$. We show that the optical attenuation ($A_V$) versus stellar mass ($M_{star}$) relation predicted using our method is consistent with recent ALMA observations of galaxies at $2<z<3$ in the emph{Hubble} emph{Ultra} emph{Deep} emph{Field} (HUDF), as well as empirical $A_V - M_{star}$ relations predicted by a Calzetti-like law. Our results, combined with other literature data, suggest that the $A_V - M_{star}$ relation does not evolve over the redshift range $0<z<5$, at least for galaxies with log$(M_{star}/M_{odot}) gtrsim 9.5$. Finally, we present tentative evidence which suggests that the attenuation curve may become steeper at log$(M_{star}/M_{odot}) lesssim 9.0$.
We present a rest-frame UV-optical stacked spectrum representative of quiescent galaxies at $1.0 < z < 1.3$ with log$(M_*/rm{M_odot}) > 10.8$. The stack is constructed using VANDELS survey data, combined with new KMOS observations. We apply two independent full-spectral-fitting approaches, obtaining consistent stellar ages and metallicities. We measure a total metallicity, [Z/H] = $-0.13pm0.08$, and an iron abundance, [Fe/H] = $-0.18pm0.08$, representing falls of $sim0.3$ dex and $sim0.15$ dex respectively compared with the local Universe. We also measure the alpha enhancement via the magnesium abundance, obtaining [Mg/Fe] = 0.23$pm$0.12, consistent with similar-mass galaxies in the local Universe, indicating no evolution in the average alpha enhancement of log$(M_*/rm{M_odot}) sim 11$ quiescent galaxies over the last 8 Gyr. This suggests the very high alpha enhancements recently reported for several very bright $zsim1-2$ quiescent galaxies are due to their extreme masses, in accordance with the well-known downsizing trend, rather than being typical of the $zgtrsim1$ population. The metallicity evolution we observe with redshift (falling [Z/H], [Fe/H], but constant [Mg/Fe]) is consistent with recent studies. We recover a mean stellar age of $2.5^{+0.6}_{-0.4}$ Gyr, corresponding to a formation redshift, $z_rm{form} = 2.4^{+0.6}_{-0.3}$. Recent studies have obtained varying average formation redshifts for $zgtrsim1$ massive quiescent galaxies, and, as these studies report consistent metallicities, we identify different star-formation-history models as the most likely cause. Larger spectroscopic samples from upcoming ground-based instruments will provide precise constraints on ages and metallicities at $zgtrsim1$. Combining these with precise $z>2$ quiescent-galaxy stellar-mass functions from JWST will provide an independent test of formation redshifts from spectral fitting.
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]
We study the mean properties of a large representative sample of 217 galaxies showing CIII] emission at $2<z<4$, selected from a parent sample of $sim$750 main-sequence star-forming galaxies in the VANDELS survey. These CIII] emitters have a broad range of UV luminosities, thus allowing a detailed stacking analysis to characterize their stellar mass, star formation rate (SFR) and stellar metallicity, as a function of the UV emission line ratios, EWs, and the carbon-to-oxygen (C/O) abundance ratio. Reliable CIII] detections represent $sim$30% of the parent sample. Extreme CIII] emitters (EW(CIII])$gtrsim$8r{A}) are exceedingly rare ($sim$3%) in VANDELS. The UV line ratios of the sample suggest no ionization source other than massive stars. Stacks with larger EW(CIII]) show larger EW(Ly$alpha$) and lower metallicity, but not all CIII] emitters are Ly$alpha$ emitters. The stellar metallicities of CIII] emitters are not significantly different from that of the parent sample, increasing from $sim$10% to $sim$40% solar for stellar masses $log$(M$_{star}$/M$_{odot})sim$9-10.5. The stellar mass-metallicity relation of the CIII] emitters is consistent with previous works showing strong evolution from $z=0$ to $zsim3$. The C/O abundances of the sample range 35%-150% solar, with a noticeable increase with FUV luminosity and a smooth decrease with the CIII] and Ly$alpha$ EWs. We discuss the CIII] emitters in the C/O-Fe/H and the C/O-O/H planes and find they follow stellar and nebular abundance trends consistent with those of Milky Way halo and thick disc stars and local HII galaxies, respectively. A qualitative agreement is also found with chemical evolution models, which suggests that CIII] emitters at $zsim$3 are experiencing an active phase of chemical enrichment.
Strong He II emission is produced by low-metallicity stellar populations. Here, we aim to identify and study a sample of He II $lambda 1640$-emitting galaxies at redshifts of $z sim 2.5-5$ in the deep VANDELS spectroscopic survey.. We identified a total of 33 Bright He II emitters (S/N > 2.5) and 17 Faint emitters (S/N < 2.5) in the VANDELS survey and used the available deep multi-wavelength data to study their physical properties. After identifying seven potential AGNs in our sample and discarding them from further analysis, we divided the sample of emph{Bright} emitters into 20 emph{Narrow} (FWHM < 1000 km s$^{-1}$) and 6 emph{Broad} (FWHM > 1000 km s$^{-1}$) He II emitters. We created stacks of Faint, Narrow, and Broad emitters and measured other rest-frame UV lines such as O III] and C III] in both individual galaxies and stacks. We then compared the UV line ratios with the output of stellar population-synthesis models to study the ionising properties of He II emitters. We do not see a significant difference between the stellar masses, star-formation rates, and rest-frame UV magnitudes of galaxies with He II and no He II emission. The stellar population models reproduce the observed UV line ratios from metals in a consistent manner, however they under-predict the total number of heii ionising photons, confirming earlier studies and suggesting that additional mechanisms capable of producing He II are needed, such as X-ray binaries or stripped stars. The models favour subsolar metallicities ($sim0.1Z_odot$) and young stellar ages ($10^6 - 10^7$ years) for the He II emitters. However, the metallicity measured for He II emitters is comparable to that of non-He II emitters at similar redshifts. We argue that galaxies with He II emission may have undergone a recent star-formation event, or may be powered by additional sources of He II ionisation.
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