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Register a new userThe Evolution of Gas-Phase Metallicity and Resolved Abundances in Star-forming Galaxies at $z approx0.6-1.8$
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S. Gillman
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We present an analysis of the chemical abundance properties of $approx$650 star-forming galaxies at $z approx0.6-1.8$. Using integral-field observations from the $K$-band Multi-Object Spectrograph (KMOS), we quantify the [NII]/H$alpha$ emission-line ratio, a proxy for the gas-phase Oxygen abundance within the interstellar medium. We define the stellar mass-metallicity relation at $z approx0.6-1.0$ and $z approx1.2-1.8$ and analyse the correlation between the scatter in the relation and fundamental galaxy properties (e.g. H$alpha$ star-formation rate, H$alpha$ specific star-formation rate, rotation dominance, stellar continuum half-light radius and Hubble-type morphology). We find that for a given stellar mass, more highly star-forming, larger and irregular galaxies have lower gas-phase metallicities, which may be attributable to their lower surface mass densities and the higher gas fractions of irregular systems. We measure the radial dependence of gas-phase metallicity in the galaxies, establishing a median, beam smearing-corrected, metallicity gradient of $ Delta Z/ Delta R=0.002 pm0.004$ dex kpc$^{-1}$, indicating on average there is no significant dependence on radius. The metallicity gradient of a galaxy is independent of its rest-frame optical morphology, whilst correlating with its stellar mass and specific star-formation rate, in agreement with an inside-out model of galaxy evolution, as well as its rotation dominance. We quantify the evolution of metallicity gradients, comparing the distribution of $Delta Z/ Delta R$ in our sample with numerical simulations and observations at $z approx0-3$. Galaxies in our sample exhibit flatter metallicity gradients than local star-forming galaxies, in agreement with numerical models in which stellar feedback plays a crucial role redistributing metals.
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We report on the gas-phase metallicity gradients of a sample of 264 star-forming galaxies at 0.6 < z < 2.6, measured through deep near-infrared Hubble Space Telescope slitless spectroscopy. The observations include 12-orbit depth Hubble/WFC3 G102 grism spectra taken as a part of the CANDELS Lya Emission at Reionization (CLEAR) survey, and archival WFC3 G102+G141 grism spectra overlapping the CLEAR footprint. The majority of galaxies (84%) in this sample are consistent with a zero or slightly positive metallicity gradient across the full mass range probed (8.5 < log M_*/M_sun < 10.5). We measure the intrinsic population scatter of the metallicity gradients, and show that it increases with decreasing stellar mass---consistent with previous reports in the literature, but confirmed here with a much larger sample. To understand the physical mechanisms governing this scatter, we search for correlations between the observed gradient and various stellar population properties at fixed mass. However, we find no evidence for a correlation with the galaxy properties we consider---including star-formation rates, sizes, star-formation rate surface densities, and star-formation rates per gravitational potential energy. We use the observed weakness of these correlations to provide material constraints for predicted intrinsic correlations from theoretical models.
We have measured the radial profiles of isophotal ellipticity ($varepsilon$) and disky/boxy parameter A$_4$ out to radii of about three times the semi-major axes for $sim4,600$ star-forming galaxies (SFGs) at intermediate redshifts $0.5<z<1.8$ in the CANDELS/GOODS-S and UDS fields. Based on the average size versus stellar-mass relation in each redshift bin, we divide our galaxies into Small SFGs (SSFGs), i.e., smaller than average for its mass, and Large SFGs (LSFGs), i.e., larger than average. We find that, at low masses ($M_{ast} < 10^{10}M_{odot}$), the SSFGs generally have nearly flat $varepsilon$ and A$_4$ profiles for both edge-on and face-on views, especially at redshifts $z>1$. Moreover, the median A$_4$ values at all radii are almost zero. In contrast, the highly-inclined, low-mass LSFGs in the same mass-redshift bins generally have monotonically increasing $varepsilon$ with radius and are dominated by disky values at intermediate radii. These findings at intermediate redshifts imply that low-mass SSFGs are not disk-like, while low-mass LSFGs appear to harbour disk-like components flattened by significant rotation. At high masses ($M_{ast} > 10^{10}M_{odot}$), highly-inclined SSFGs and LSFGs both exhibit a general, distinct trend for both $varepsilon$ and A$_4$ profiles: increasing values with radius at lower radii, reaching maxima at intermediate radii, and then decreasing values at larger radii. Such a trend is more prevalent for more massive ($M_{ast} > 10^{10.5}M_{odot}$) galaxies or those at lower redshifts ($z<1.4$). The distinct trend in $varepsilon$ and A$_4$ can be simply explained if galaxies possess all three components: central bulges, disks in the intermediate regions, and halo-like stellar components in the outskirts.
We report a Giant Metrewave Radio Telescope (GMRT) search for HI 21cm emission from a large sample of star-forming galaxies at $z approx 1.18 - 1.34$, lying in sub-fields of the DEEP2 Redshift Survey. The search was carried out by co-adding (stacking) the HI 21cm emission spectra of 857 galaxies, after shifting each galaxys HI 21cm spectrum to its rest frame. We obtain the $3sigma$ upper limit S$_{rm{HI}} < 2.5 mu$Jy on the average HI 21cm flux density of the 857 galaxies, at a velocity resolution of $approx 315$ km s$^{-1}$. This yields the $3sigma$ constraint M$_{rm{HI}} < 2.1 times 10^{10} times left[Delta {rm V}/315 rm{km/s} right]^{1/2} textrm{M}_odot$ on the average HI mass of the 857 stacked galaxies, the first direct constraint on the atomic gas mass of galaxies at $z > 1$. The implied limit on the average atomic gas mass fraction (relative to stars) is ${rm M}_{rm GAS}/{rm M}_* < 0.5$, comparable to the cold molecular gas mass fraction in similar star-forming galaxies at these redshifts. We find that the cosmological mass density of neutral atomic gas in massive star-forming galaxies at $z approx 1.3$ is $Omega_{rm GAS} < 3.7 times 10^{-4}$, significantly lower than $Omega_{rm GAS}$ estimates in both galaxies in the local Universe and damped Lyman-$alpha$ absorbers at $z geq 2.2$. Massive blue star-forming galaxies thus do not appear to dominate the neutral atomic gas content of the Universe at $z approx 1.3$.
We study the properties of star-forming galaxies selected at 610 MHz with the GMRT in a survey covering $sim$1.86 deg$^2$ down to a noise of $sim$7.1,$mu$Jy / beam. These were identified by combining multiple classification diagnostics: optical, X-ray, infrared and radio data. Of the 1685 SFGs from the GMRT sample, 496 have spectroscopic redshifts whereas 1189 have photometric redshifts. We find that the IRRC of star-forming galaxies, quantified by the infrared-to-1.4 GHz radio luminosity ratio $rm{q_{IR}}$, decreases with increasing redshift: $rm{q_{IR},=,2.86pm0.04(1,+,z)^{-0.20pm0.02}}$ out to $z sim 1.8$. We use the $rm{V/V_{max}}$ statistic to quantify the evolution of the co-moving space density of the SFG sample. Averaged over luminosity our results indicate $rm{langle V/V_{max} rangle}$ to be $rm{0.51,pm, 0.06}$, which is consistent with no evolution in overall space density. However we find $rm V/V_{max}$ to be a function of radio luminosity, indicating strong luminosity evolution with redshift. We explore the evolution of the SFGs radio luminosity function by separating the source into five redshift bins and comparing to theoretical model predictions. We find a strong redshift trend that can be fitted with a pure luminosity evolution of the form $rm{L_{610,MHz},propto,(,1+,z)^{(2.95pm0.19)-(0.50pm0.15)z}}$. We calculate the cosmic SFR density since $rm{z sim 1.5}$ by integrating the parametric fits of the evolved 610,MHz luminosity function. Our sample reproduces the expected steep decline in the star formation rate density since $rm{z,sim,1}$.
While all models for the evolution of galaxies require the accretion of gas to sustain their growth via on-going star formation, it has proven difficult to directly detect this inflowing material. In this paper we use data of nearby star-forming galaxies in the SDSS IV Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to search for evidence of accretion imprinted in the chemical composition of the interstellar medium. We measure both the O/H and N/O abundance ratios in regions previously identified as having anomalously low values of O/H. We show that the unusual locations of these regions in the N/O vs. O/H plane indicate that they have been created through the mixing of disk gas having higher metallicity with accreted gas having lower metallicity. Taken together with previous analysis on these anomalously low-metallicity regions, these results imply that accretion of metal-poor gas can probably sustain star formation in present-day late-type galaxies.
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