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Register a new userThe VIMOS Ultra-Deep Survey: evidence for AGN feedback in galaxies with CIII]-$lambda$1908AA ~emission 10.8 to 12.5 Gyr ago
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We analyze the CIII]-1908AA emission properties in a sample of 3899 star-forming galaxies (SFGs) at 2<z<3.8 from the VIMOS Ultra-Deep Survey (VUDS). We find a median EW(CIII])=2.0$pm$0.2 to 2.2$pm$0.2AA ~for the SFG population at 2<z<3 and 3<z<4, resp. About 24% of SFGs are showing EW(CIII])>3AA, including 20% with 3<EW(CIII])<10AA ~and 4% with strong emission EW(CIII])>10AA. A significant fraction of 1.2% of SFGs presents strong CIII] emission 20<EW(CIII])<40AA. This makes CIII] the second most-frequent emission line in the UV-rest spectra of SFGs after Lyman-$alpha$. We find a large dispersion in the weak EW(CIII]) - EW(Lyalpha) correlation, with galaxies showing strong CIII] and no Ly$alpha$, and vice-versa. SFGs with 10<EW(CIII])<0AA ~present strong emission lines including CIV-1549, HeII-1640, OIII-1664, but also weaker emission of highly ionized elements SiIV-1403, NIV-1485, NIII-1750, or SiIII-1888, indicating a hard radiation field. We present a broad range of observational evidence supporting the presence of AGN in the strong CIII] emitting population. As EW(CIII]) is rising, we identify powerful outflows with velocities up to 1014 km/s. The strongest CIII] emitters are preferentially located below the SFGs main sequence, with the SFR reduced by x2. In addition, the median stellar age of the strongest emitters is ~0.8 Gyr, three times that of galaxies with EW(CIII])<10AA. Spectral line analysis presented in a joint paper by Nakajima et al. (2017) confirms that the strongest emitters require an AGN. We conclude that these properties are indicative of AGN feedback in SFGs at 2<z<3.8, contributing to star-formation quenching. We find that quenching timescales of 0.25-0.5x10^9 years are necessary for such AGN feedback to turn part of the star-forming galaxy population with Mstar>10^10 MSun at z~3 into the population of quiescent galaxies observed at redshift z~1-2.
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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]
Using an unconventional single line diagnostic that unambiguously identifies AGNs in composite galaxies we report statistical differences in the properties (stellar age, [OII] luminosity, colour) between active and inactive galaxies at 0.62<z<1.2 extracted from the VIMOS Public Extragalactic Redshift Survey (VIPERS). The nuclear activity is probed by the high-ionization [NeV] emission line and along with their parent samples, the galaxies are properly selected according to their stellar mass, redshift, and colour distributions. We report younger underlying stellar ages and higher [OII] luminosities of active galaxies in the green valley and in the blue cloud compared to control samples. We observe higher fractions of green galaxies hosting AGN activity at progressively bluer (r-K) colours. Depending on the location of the host galaxy in the NUVrK colour diagram we find higher AGN fractions in massive blue galaxies and in the least massive red galaxies, in agreement with the picture that black holes vary their properties when hosted in either star-forming or passive galaxies. Exactly where the fast quenching processes are expected to play a role, we identify a novel class of active galaxies in the blue cloud with signatures typical for a suddenly suppression of their star formation activity after a burst happening in the recent past. Their optical spectra resemble those of post-starburst galaxies, that would never be identified in a spectroscopic search using classical selection techniques. Broadly, these active galaxies selected on the [NeV] line are not commonly represented in shallow X-ray, mid-IR, or classical line diagnostics. If we consider that our results are limited by the shallow observational limits and rapid AGN variability, the impact of AGN feedback on galaxy formation and evolution may represent an important channel of fast-transiting galaxies moving to the red sequence.
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.
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 used the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) to observe the semi-forbidden CIII] emission in Green Pea galaxies at 0.13 < z < 0.3. We detect CIII] emission in 7/10 galaxies with CIII] equivalent widths that range from 2-10AA~. The observed CIII] emission line strengths are consistent with the predictions from photoionization models which incorporate the effects of binary stellar evolution with young stellar ages < 3-5 Myrs, and high ionization parameters (logU > -2). The hard ionizing radiation from young massive stars, and high nebular temperatures at low-metallicities can account for the observed high equivalent widths of CIII] and [OIII] emission lines. The Green Pea galaxies do not show a significant correlation between the Ly$alpha$ and CIII] equivalent widths, and the observed scatter is likely due to the variations in the optical depth of Ly$alpha$ to the neutral gas. Green Pea galaxies are likely to be density-bounded, and we examined the dependence of CIII] emission on the Lyman continuum optical depth. The potential LyC leaker galaxies in our sample have high CIII] equivalent widths that can only be reproduced by starburst ages as young as < 3 Myrs and harder ionizing spectra than the non-leakers. Among the galaxies with similar metallicities and ionization parameters, the CIII] equivalent width appears to be stronger for those with higher optical depth to LyC, as expected from the photoionization models. Further investigation of a larger sample of CIII]-emitters is necessary to calibrate the dependence of CIII] emission on the escape of LyC radiation, and to enable application of the CIII] diagnostics to galaxies in the reionization epoch.
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