arXiv subjects

Robert M. Yates

Publications and source records attributed to Robert M. Yates.

41 records · Page 3Linked to original sources

arXiv2013

Dilution in elliptical galaxies: Implications for the relation between metallicity, stellar mass and star formation rate

We investigate whether gradual dilution of the gas in some elliptical galaxies is the cause of a positive correlation between star formation rate (SFR) and gas-phase metallicity (Zg) at high stellar mass (M*) in the local Universe. To do this, two classes of massive (M* >= 10^10.5 Msun) galaxy are selected from the Sloan Digital Sky Survey (SDSS) and the Munich semi-analytic model of galaxy formation, L-Galaxies. The first class is selected by high specific star formation rates (sSFR) and high Zg, and the second class by low sSFR and low Zg. These criteria roughly distinguish disc-dominant galaxies from metal-poor, elliptical galaxies. In the semi-analytic model, the second class of galaxies obtain low sSFR and low Zg due to gradual dilution of the interstellar medium by accretion of metal-poor gas via infalling clumps and low-mass satellites. This occurs after a merger-induced starburst and the associated supernova feedback have quenched most of the original gas reservoir. A number of signatures of this evolution are present in these model galaxies at z=0, including low gas fractions, large central black holes, elliptical morphologies, old ages, and importantly, low (Zg-Z*) indicating dilution after star formation. Remarkably, all of these properties are also found in low-sSFR, low-Zg, massive galaxies in the SDSS-DR7. This provides strong, indirect evidence that some elliptical galaxies are undergoing gradual dilution after a gas-rich merger in the local Universe. This dilution scenario also explains the positive correlation between SFR and Zg measured in high-M* galaxies, and therefore has consequences for the local fundamental metallicity relation (FMR), which assumes a weak anti-correlation between SFR and Zg above 10^10.5 Msun.

astro-ph.CO↗

arXiv2013

Star Formation and Metallicity Gradients in Semi-analytic Models of Disk Galaxy Formation

We have updated our radially-resolved SAMs of galaxy formation, which track both the atomic and molecular gas phases of the ISM. The models are adapted from those of Guo et al. using similar methodology as in Fu et al. and are run on halo merger trees from the MS and MS II with the following main changes: (1) We adopt a simple star formation law where Σ_SFR \propto Σ_H2 (2) We inject the heavy elements produced by supernovae directly into the halo hot gas, instead of first mixing them with the cold gas in the disk. (3) We include radial gas inflows in disks using a model of the form v_inflow = αr. The models are used to study the radial profiles of star formation rate and gas-phase metallicity in present-day galaxies. The Σ_H2 profiles in L* galaxies place strong constraints on inflow velocities, favouring models where v_inflow~7km/s at a galactocentric radius of 10kpc. Radial gas inflow has little influence on gas-phase and stellar metallicity gradients, which are affected much more strongly by the fraction of metals that are directly injected into the halo gas, rather than mixed with the cold gas. Metals ejected out of the galaxy in early epochs result in late infall of pre-enriched gas and flatter present-day gas-phase metallicity gradients. A prescription in which 80% of the metals are injected into the halo gas results in good fits to the flat observed metallicity gradients in galaxies with stellar masses greater than 10^10 M_sun, as well as the relations between gas-phase metallicity and sSFR in the outer parts of galactic disks. We examine the correlation between gas-phase metallicity gradient and global galaxy properties, finding that it is most strongly correlated with the bulge-to-total ratio of the galaxy. This is because gas is consumed when the bulge forms during galaxy mergers, and the gas-phase metallicity gradient is then set by newly-accreted gas.

astro-ph.CO↗

arXiv2013

The Observed Relation between Stellar Mass, Dust Extinction and Star Formation Rate in Local Galaxies

In this study we investigate the relation between stellar mass, dust extinction and star formation rate (SFR) using ~150,000 star-forming galaxies from the SDSS DR7. We show that the relation between dust extinction and SFR changes with stellar mass. For galaxies at the same stellar mass dust extinction is anti-correlated with the SFR at stellar masses <10^10 M_solar. There is a sharp transition in the relation at a stellar mass of 10^10 M_solar. At larger stellar masses dust extinction is positively correlated with the SFR for galaxies at the same stellar mass. The observed relation between stellar mass, dust extinction and SFR presented in this study helps to confirm similar trends observed in the relation between stellar mass, metallicity and SFR. The relation reported in this study provides important new constraints on the physical processes governing the chemical evolution of galaxies. The correlation between SFR and dust extinction for galaxies with stellar masses >10^10 M_solar is shown to extend to the population of quiescent galaxies suggesting that the physical processes responsible for the observed relation between stellar mass, dust extinction and SFR may be related to the processes leading to the shut down of star formation in galaxies.

astro-ph.CO↗

arXiv2013

Modelling Element Abundances in Semi-analytic Models of Galaxy Formation

We update the treatment of chemical evolution in the Munich semi-analytic model, L-GALAXIES. Our new implementation includes delayed enrichment from stellar winds, supernovae type II (SNe-II) and supernovae type Ia (SNe-Ia), as well as metallicity-dependent yields and a reformulation of the associated supernova feedback. Two different sets of SN-II yields and three different SN-Ia delay-time distributions (DTDs) are considered, and eleven heavy elements (including O, Mg and Fe) are self-consistently tracked. We compare the results of this new implementation with data on a) local, star-forming galaxies, b) Milky Way disc G dwarfs, and c) local, elliptical galaxies. We find that the z=0 gas-phase mass-metallicity relation is very well reproduced for all forms of DTD considered, as is the [Fe/H] distribution in the Milky Way disc. The [O/Fe] distribution in the Milky Way disc is best reproduced when using a DTD with less than or equal to 50 per cent of SNe-Ia exploding within ~400 Myrs. Positive slopes in the mass-[alpha/Fe] relations of local ellipticals are also obtained when using a DTD with such a minor `prompt' component. Alternatively, metal-rich winds that drive light alpha elements directly out into the circumgalactic medium also produce positive slopes for all forms of DTD and SN-II yields considered. Overall, we find that the best model for matching the wide range of observational data considered here should include a power-law SN-Ia DTD, SN-II yields that take account of prior mass loss through stellar winds, and some direct ejection of light alpha elements out of galaxies.

astro-ph.CO↗

arXiv2011

The relation between metallicity, stellar mass and star formation in galaxies: an analysis of observational and model data

We study relations between stellar mass, star formation and gas-phase metallicity in a sample of 177,071 unique emission line galaxies from the SDSS-DR7, as well as in a sample of 43,767 star forming galaxies at z=0 from the cosmological semi-analytic model L-GALAXIES. We demonstrate that metallicity is dependent on star formation rate at fixed mass, but that the trend is opposite for low and for high mass galaxies. Low-mass galaxies that are actively forming stars are more metal-poor than quiescent low-mass galaxies. High-mass galaxies, on the other hand, have lower gas-phase metallicities if their star formation rates are small. Remarkably, the same trends are found for our sample of model galaxies. We find that massive model galaxies with low gas-phase metallicities have undergone a gas-rich merger in the past, inducing a starburst which exhausted their cold gas reservoirs and shut down star formation. This led to a gradual dilution in the gas-phase metallicities of these systems via accretion of gas. These model galaxies have lower-than-average gas-to-stellar mass ratios and higher-than-average central black hole masses. We confirm that massive galaxies with low gas-phase metallicities in our observational sample also have very massive black holes. We propose that accretion may therefore play a significant role in regulating the gas-phase metallicities of present-day massive galaxies.

astro-ph.CO↗