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Register a new userCharacterization of star-forming dwarf galaxies at 0.1 $lesssim z lesssim$ 0.9 in VUDS: Probing the low-mass end of the mass-metallicity relation
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Added by
Antonello Calabr\\`o
Publication date
2017
fields
Physics
and research's language is
English
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We present the discovery and spectrophotometric characterization of a large sample of 164 faint ($i_{AB}$ $sim$ $23$-$25$ mag) star-forming dwarf galaxies (SFDGs) at redshift $0.13$ $leq z leq$ $0.88$ selected by the presence of bright optical emission lines in the VIMOS Ultra Deep Survey (VUDS). We investigate their integrated physical properties and ionization conditions, which are used to discuss the low-mass end of the mass-metallicity relation (MZR) and other key scaling relations. We use optical VUDS spectra in the COSMOS, VVDS-02h, and ECDF-S fields, as well as deep multiwavelength photometry, to derive stellar masses, star formation rates (SFR) and gas-phase metallicities. The VUDS SFDGs are compact (median $r_{e}$ $sim$ $1.2$ kpc), low-mass ($M_{*}$ $sim$ $10^7-10^9$ $M_{odot}$) galaxies with a wide range of star formation rates (SFR($Halpha$) $sim 10^{-3}-10^{1}$ $M_{odot}/yr$) and morphologies. Overall, they show a broad range of subsolar metallicities (12+log(O/H)=$7.26$-$8.7$; $0.04$ $lesssim Z/Z_{odot} lesssim$ $1$). The MZR of SFDGs shows a flatter slope compared to previous studies of galaxies in the same mass range and redshift. We find the scatter of the MZR partly explained in the low mass range by varying specific SFRs and gas fractions amongst the galaxies in our sample. Compared with simple chemical evolution models we find that most SFDGs do not follow the predictions of a closed-box model, but those from a gas regulating model in which gas flows are considered. While strong stellar feedback may produce large-scale outflows favoring the cessation of vigorous star formation and promoting the removal of metals, younger and more metal-poor dwarfs may have recently accreted large amounts of fresh, very metal-poor gas, that is used to fuel current star formation.
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Galaxies at low-redshift typically possess negative gas-phase metallicity gradients (centres more metal-rich than their outskirts). Whereas, it is not uncommon to observe positive metallicity gradients in higher-redshift galaxies ($z gtrsim 0.6$). Bridging these epochs, we present gas-phase metallicity gradients of 84 star-forming galaxies between $0.08 < z < 0.84$. Using the galaxies with reliably determined metallicity gradients, we measure the median metallicity gradient to be negative ($-0.039^{+0.007}_{-0.009}$ dex/kpc). Underlying this, however, is significant scatter: $(8pm3)% [7]$ of galaxies have significantly positive metallicity gradients, $(38 pm 5)% [32]$ have significantly negative gradients, $(31pm5)% [26]$ have gradients consistent with being flat. (The remaining $(23pm5)% [19]$ have unreliable gradient estimates.) We notice a slight trend for a more negative metallicity gradient with both increasing stellar mass and increasing star formation rate (SFR). However, given the potential redshift and size selection effects, we do not consider these trends to be significant. Indeed, once we normalize the SFR relative to that of the main sequence, we do not observe any trend between the metallicity gradient and the normalized SFR. This is contrary to recent studies of galaxies at similar and higher redshifts. We do, however, identify a novel trend between the metallicity gradient of a galaxy and its size. Small galaxies ($r_d < 3$ kpc) present a large spread in observed metallicity gradients (both negative and positive gradients). In contrast, we find no large galaxies ($r_d > 3$ kpc) with positive metallicity gradients, and overall there is less scatter in the metallicity gradient amongst the large galaxies. These large (well-evolved) galaxies may be analogues of present-day galaxies, which also show a common negative metallicity gradient.
We quantify the star formation (SF) in the inner cores ($mathcal{R}$/$R_{200}$$leq$0.3) of 24 massive galaxy clusters at 0.2$lesssim$$z$$lesssim$0.9 observed by the $Herschel$ Lensing Survey and the Cluster Lensing and Supernova survey with $Hubble$. These programmes, covering the rest-frame ultraviolet to far-infrared regimes, allow us to accurately characterize stellar mass-limited ($mathcal{M}_{*}$$>$$10^{10}$ $M_{odot}$) samples of star-forming cluster members (not)-detected in the mid- and/or far-infrared. We release the catalogues with the photometry, photometric redshifts, and physical properties of these samples. We also quantify the SF displayed by comparable field samples from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. We find that in intermediate-$z$ cluster cores, the SF activity is suppressed with respect the field in terms of both the fraction ($mathcal{F}$) of star-forming galaxies (SFG) and the rate at which they form stars ($mathcal{SFR}$ and $smathcal{SFR} = mathcal{SFR}/mathcal{M}_{*}$). On average, the $mathcal{F}$ of SFGs is a factor $sim$$2$ smaller in cluster cores than in the field. Furthermore, SFGs present average $mathcal{SFR}$ and $smathcal{SFR}$ typically $sim$0.3 dex smaller in the clusters than in the field along the whole redshift range probed. Our results favour long time-scale quenching physical processes as the main driver of SF suppression in the inner cores of clusters since $z$$sim$0.9, with shorter time-scale processes being very likely responsible for a fraction of the missing SFG population.
We present Lyman continuum (LyC) radiation escape fraction $f_{rm{esc}}$ measurements for 183 spectroscopically confirmed star-forming galaxies in the redshift range $3.11 < z < 3.53$ in the textit{Chandra} Deep Field South. We use ground-based imaging to measure $f_{rm{esc}}$, and use ground- and space-based photometry to derive galaxy physical properties using spectral energy distribution (SED) fitting. We additionally derive [O,textsc{iii}],+,H$beta$ equivalent widths (that fall in the observed $K$ band) by including nebular emission in the SED fitting. After removing foreground contaminants, we report the discovery of 11 new candidate LyC leakers, with absolute LyC escape fractions, $f_{rm{esc}}$ in the range $0.07-0.52$. Most galaxies in our sample ($approx94%$) do not show any LyC leakage, and we place $1sigma$ upper limits of $f_{rm{esc}} < 0.07$ through weighted averaging, where the Lyman-break selected galaxies have $f_{rm{esc}} < 0.07$ and `blindly discovered galaxies with no prior photometric selection have $f_{rm{esc}} < 0.10$. We additionally measure $f_{rm{esc}} < 0.09$ for extreme emission line galaxies in our sample with rest-frame [O,textsc{iii}],+,H$beta$ equivalent widths $>300$,AA. For the candidate LyC leakers, we do not find a strong dependence of $f_{rm{esc}}$ on their stellar masses and/or specific star-formation rates, and no correlation between $f_{rm{esc}}$ and EW$_0$([O,textsc{iii}],+,H$beta$). We suggest that this lack of correlations may be explained by viewing angle and/or non-coincident timescales of starburst activity and periods of high $f_{rm{esc}}$. Alternatively, escaping radiation may predominantly occur in highly localised star-forming regions, thereby obscuring any global trends with galaxy properties. Both hypotheses have important consequences for models of reionisation.
We study the origin and cosmic evolution of the mass-metallicity relation (MZR) in star-forming galaxies based on a full, numerical chemical evolution model. The model was designed to match the local MZRs for both gas and stars simultaneously. This is achieved by invoking a time-dependent metal enrichment process which assumes either a time-dependent metal outflow with larger metal loading factors in galactic winds at early times, or a time-dependent Initial Mass Function (IMF) with steeper slopes at early times. We compare the predictions from this model with data sets covering redshifts 0<z<3.5. The data suggests a two-phase evolution with a transition point around z ~ 1.5. Before that epoch the MZRgas has been evolving parallel with no evolution in the slope. After z ~ 1.5 the MZRgas started flattening until today. We show that the predictions of both the variable metal outflow and the variable IMF model match these observations very well. Our model also reproduces the evolution of the main sequence, hence the correlation between galaxy mass and star formation rate. We also compare the predicted redshift evolution of the MZRstar with data from the literature. As the latter mostly contains data of massive, quenched early-type galaxies, stellar metallicities at high redshifts tend to be higher in the data than predicted by our model. Data of stellar metallicities of lower-mass (< 10^11 solar mass), star-forming galaxies at high redshift is required to test our model.
To investigate the growth history of galaxies, we measure the rest-frame radio, ultraviolet (UV), and optical sizes of 98 radio-selected, star-forming galaxies (SFGs) distributed over $0.3 lesssim z lesssim 3$ and median stellar mass of $log(M_star/ rm M_odot)approx10.4$. We compare the size of galaxy stellar disks, traced by rest-frame optical emission, relative to the overall extent of star formation activity that is traced by radio continuum emission. Galaxies in our sample are identified in three Hubble Frontier Fields: MACSJ0416.1$-$2403, MACSJ0717.5+3745, and MACSJ1149.5+2223. Radio continuum sizes are derived from 3 GHz and 6 GHz radio images ($lesssim 0$$.6$ resolution, $approx0.9, rm mu Jy, beam^{-1}$ noise level) from the Karl G. Jansky Very Large Array. Rest-frame UV and optical sizes are derived using observations from the Hubble Space Telescope and the ACS and WFC3 instruments. We find no clear dependence between the 3 GHz radio size and stellar mass of SFGs, which contrasts with the positive correlation between the UV/optical size and stellar mass of galaxies. Focusing on SFGs with $log(M_star/rm M_odot)>10$, we find that the radio/UV/optical emission tends to be more compact in galaxies with high star-formation rates ($rm SFRgtrsim 100,M_odot,yr^{-1}$), suggesting that a central, compact starburst (and/or an Active Galactic Nucleus) resides in the most luminous galaxies of our sample. We also find that the physical radio/UV/optical size of radio-selected SFGs with $log(M_star/rm M_odot)>10$ increases by a factor of $1.5-2$ from $zapprox 3$ to $zapprox0.3$, yet the radio emission remains two-to-three times more compact than that from the UV/optical. These findings indicate that these massive, {radio-selected} SFGs at $0.3 lesssim z lesssim 3$ tend to harbor centrally enhanced star formation activity relative to their outer-disks.
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