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Register a new userThe VIMOS Ultra Deep Survey: Nature, ISM properties, and ionizing spectra of CIII]1909 emitters at z=2-4
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Context: Ultraviolet (UV) emission-line spectra are used to spectroscopically confirm high-z galaxies and increasingly also to determine their physical properties. Aims: To interpret the observed UV spectra of distant galaxies in terms of the dominant radiation field and the physical condition of the interstellar medium. Methods: We construct a large grid of photoionization models and derive new spectral UV line diagnostics using equivalent widths (EWs) of CIII]1909, CIV1549 and the line ratios of CIII], CIV, and HeII1640 recombination lines. We apply these diagnostics to a sample of 450 CIII]-emitting galaxies at z=2-4 previously identified in the VIMOS Ultra Deep Survey. Results: We show that the average star-forming galaxy (EW(CIII])~2A) is well described by stellar photoionization from single and binary stars. The inferred metallicity and ionization parameter is typically Z=0.3-0.5Zsun and logU=-2.7 to -3, in agreement with earlier works at similar redshifts. The models also indicate an average age of 50-200Myr since the beginning of the current star-formation, and an ionizing photon production rate, xi_ion, of log(xi_ion/[Hz/erg])~25.3-25.4. Among the sources with EW(CIII])=10-20A, ~30% are likely dominated by AGNs. Their derived metallicity is low, Z=0.02-0.2Zsun, and the ionization parameter higher (logU=-1.7). To explain the average UV observations of the strongest but rarest CIII] emitters (EW(CIII])>20A), we find that stellar photoionization is clearly insufficient. A radiation field consisting of a mix of a young stellar population (log(xi_ion/[Hz/erg])~25.7) plus an AGN component is required. Furthermore an enhanced C/O abundance ratio is needed for metallicities Z=0.1-0.2Zsun and logU=-1.7 to -1.5. Conclusions: The UV diagnostics we propose should serve as an important basis for the interpretation of observations of high-redshift galaxies. [abridged]
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The Lyman-$alpha$ (Ly$alpha$) emission line has been ubiquitously used to confirm and study high redshift galaxies. We report on the line morphology as seen in the 2D spectra from the VIMOS Ultra Deep Survey in a sample of 914 Ly$alpha$ emitters from a parent sample of 4192 star-forming galaxies at $2<z_mathrm{spec}lesssim6$. The study of the spatial extent of Ly$alpha$ emission provides insight into the escape of Ly$alpha$ photons from galaxies. We classify the line emission as either non-existent, coincident, projected spatial offset, or extended with respect to the observed 2D UV continuum emission. The line emitters in our sample are classified as ~45% coincident, ~24% extended and ~11% offset emitters. For galaxies with detected UV continuum, we show that extended Ly$alpha$ emitters (LAEs) correspond to the highest equivalent width galaxies (with an average $W_mathrm{Lyalpha}sim-22${AA}). This means that this class of objects is the most common in narrow-band selected samples, which usually select high equivalent width LAEs, $<-20${AA}. Extended Ly$alpha$ emitters are found to be less massive, less star-forming, with lower dust content, and smaller UV continuum sizes ($r_{50}sim0.9$kpc) of all the classes considered here. We also find that galaxies with larger UV-sizes have lower fractions of Ly$alpha$ emitters. By stacking the spectra per emitter class we find that the weaker Ly$alpha$ emitters have stronger low ionization inter-stellar medium (ISM) absorption lines. Interestingly, we find that galaxies with Ly$alpha$ offset emission (median separation of $1.1_{-0.8}^{+1.3}$kpc from UV continuum) show similar velocity offsets in the ISM as those with no visible emission (and different from other Ly$alpha$ emitting classes). This class of objects may hint at episodes of gas accretion, bright offset clumps, or on-going merging activity into the larger galaxies.
Aims. The aim of this work is to constrain the evolution of the fraction of Lya emitters among UV selected star forming galaxies at 2<z<6, and to measure the stellar escape fraction of Lya photons over the same redshift range. Methods. We exploit the ultradeep spectroscopic observations collected by the VIMOS Ultra Deep Survey (VUDS) to build an unique, complete and unbiased sample of 4000 spectroscopically confirmed star forming galaxies at 2<z<6. Our galaxy sample UV luminosities brighter than M* at 2<z<6, and luminosities down to one magnitude fainter than M* at 2<z<3.5. Results. We find that 80% of the star forming galaxies in our sample have EW0(Lya)<10A, and correspondingly fesc(Lya)<1%. By comparing these results with literature, we conclude that the bulk of the Lya luminosity at 2<z<6 comes from galaxies that are fainter in the UV than those we sample in this work. The strong Lya emitters constitute, at each redshift, the tail of the distribution of the galaxies with extreme EW0(Lya) and fesc(Lya) . This tail of large EW0 and fesc(Lya) becomes more important as the redshift increases, and causes the fraction of Lya with EW0> 25A to increase from 5% at z=2 to 30% at z=6, with the increase being relatively stronger beyond z=4. We observe no difference, for the narrow range of UV luminosities explored in this work, between the fraction of strong Lya emitters among galaxies fainter or brighter than M*, although the fraction for the FUV faint galaxies evolves faster, at 2<z<3.5, than for the bright ones. We do observe an anticorrelation between E(B-V) and fesc(Lya): generally galaxies with high fesc(Lya) have also small amounts of dust (and viceversa). However, when the dust content is low (E(B-V)<0.05) we observe a very broad range of fesc(Lya), ranging from 10^-3 to 1. This implies that the dust alone is not the only regulator of the amount of escaping Lya photons.
Utilizing spectroscopic observations taken for the VIMOS Ultra-Deep Survey (VUDS), new observations from Keck/DEIMOS, and publicly available observations of large samples of star-forming galaxies, we report here on the relationship between the star formation rate (SFR) and the local environment ($delta_{gal}$) of galaxies in the early universe ($2<z<5$). Unlike what is observed at lower redshifts ($z<2$), we observe a definite, nearly monotonic increase in the average SFR with increasing galaxy overdensity over more than an order of magnitude in $delta_{gal}$. The robustness of this trend is quantified by accounting for both uncertainties in our measurements and galaxy populations that are either underrepresented or not present in our sample finding that the trend remains significant under all circumstances. This trend appears to be primarily driven by the fractional increase of galaxies in high density environments that are more massive in their stellar content and are forming stars at a higher rate than their less massive counterparts. We find that, even after stellar mass effects are accounted for, there remains a weak but significant SFR-$delta_{gal}$ trend in our sample implying that additional environmentally-related processes are helping to drive this trend. We also find clear evidence that the average SFR of galaxies in the densest environments increases with increasing redshift. These results lend themselves to a picture in which massive gas-rich galaxies coalesce into proto-cluster environments at $zsim3$, interact with other galaxies or with a forming large-scale medium, subsequently using or losing most of their gas in the process, and begin to seed the nascent red sequence that is present in clusters at slightly lower redshifts.
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 selected a sample of 76 Lya emitting galaxies from the VIMOS Ultra Deep Survey (VUDS) at 2<z<4. We estimated the velocity of the neutral gas flowing out of the interstellar medium as the velocity offset, Deltav, between the systemic redshift (zsys) and the center of low-ionization absorption line systems (LIS). To increase the SN of VUDS spectra, we stacked subsamples. We measured the systemic redshift from the rest-frame UV spectroscopic data using the CIII]1908 nebular emission line, and we considered SiII1526 as the highest signal-to-noise LIS line. We calculated the Lya peak shift with respect to the zsys, the EW(Lya), and the Lya spatial extension, Ext(Lya-C), from the profiles in the 2D stacked spectra. The galaxies that are faint in the rest-frame UV continuum, strong in Lya and CIII], with compact UV morphology, and localized in an underdense environment are characterized by outflow velocities of the order of a few hundreds of km/sec. The subsamples with smaller Deltav are characterized by larger Lya peak shifts, larger Ext(Lya-C), and smaller EW(Lya). In general we find that EW(Lya) anti-correlates with Ext(Lya-C) and Lya peak shift. We interpret these trends using a radiative-transfer shell model. The model predicts that an HI gas with a column density larger than 10^20/cm^2 is able to produce Lya peak shifts larger than >300km/sec. An ISM with this value of NHI would favour a large amount of scattering events, especially when the medium is static, so it can explain large values of Ext(Lya-C) and small EW(Lya). On the contrary, an ISM with a lower NHI, but large velocity outflows would lead to a Lya spatial profile peaked at the galaxy center (i.e. low values of Ext(Lya-C)) and to a large EW(Lya), as we see in our data. Our results and their interpretation via radiative-transfer models tell us that it is possible to use Lya to study the properties of the HI gas.
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