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Register a new userImpacts of a Flaring Star-forming Disc and Stellar Radial Mixing on the Vertical Metallicity Gradient
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D. Kawata
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Using idealised N-body simulations of a Milky Way-sized disc galaxy, we qualitatively study how the metallicity distributions of the thin disc star particles are modified by the formation of the bar and spiral arm structures. The thin disc in our numerical experiments initially has a tight negative radial metallicity gradient and a constant vertical scale-height. We show that the radial mixing of stars drives a positive vertical metallicity gradient in the thin disc. On the other hand, if the initial thin disc is flared, with vertical scale-height increasing with galactocentric radius, the metal poor stars originally in the outer disc become dominant in regions above the disc plane at every radii. This process can drive a negative vertical metallicity gradient, which is consistent with the current observed trend. This model mimics a scenario where the star-forming thin disc was flared in the outer region at earlier epochs. Our numerical experiment with an initial flared disc predicts that the negative vertical metallicity gradient of the mono-age relatively young thin disc population should be steeper in the inner disc, and the radial metallicity gradient of the mono-age population should be shallower at greater heights above the disc plane. We also predict that the metallicity distribution function of mono-age young thin disc populations above the disc plane would be more positively skewed in the inner disc compared to the outer disc.
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Using N-body simulations of the Galactic disks, we qualitatively study how the metallicity distributions of the thick and thin disk stars are modified by radial mixing induced by the bar and spiral arms. We show that radial mixing drives a positive vertical metallicity gradient in the mono-age disk population whose initial scale-height is constant and initial radial metallicity gradient is tight and negative. On the other hand, if the initial disk is flaring, with scale-height increasing with galactocentric radius, radial mixing leads to a negative vertical metallicity gradient, which is consistent with the current observed trend. We also discuss impacts of radial mixing on the metallicity distribution of the thick disk stars. By matching the metallicity distribution of N-body models to the SDSS/APOGEE data, we argue that the progenitor of the Milky Ways thick disk should not have a steep negative metallicity gradient.
We study oxygen abundance profiles of the gaseous disc components in simulated galaxies in a hierarchical universe. We analyse the disc metallicity gradients in relation to the stellar masses and star formation rates of the simulated galaxies. We find a trend for galaxies with low stellar masses to have steeper metallicity gradients than galaxies with high stellar masses at z ~0. We also detect that the gas-phase metallicity slopes and the specific star formation rate (sSFR) of our simulated disc galaxies are consistent with recently reported observations at z ~0. Simulated galaxies with high stellar masses reproduce the observed relationship at all analysed redshifts and have an increasing contribution of discs with positive metallicity slopes with increasing redshift. Simulated galaxies with low stellar masses a have larger fraction of negative metallicity gradients with increasing redshift. Simulated galaxies with positive or very negative metallicity slopes exhibit disturbed morphologies and/or have a close neighbour. We analyse the evolution of the slope of the oxygen profile and sSFR for a gas-rich galaxy-galaxy encounter, finding that this kind of events could generate either positive and negative gas-phase oxygen profiles depending on their state of evolution. Our results support claims that the determination of reliable metallicity gradients as a function of redshift is a key piece of information to understand galaxy formation and set constrains on the subgrid physics.
Context. The xGASS and xCOLD GASS surveys have measured the atomic (HI) and molecular gas (H2) content of a large and representative sample of nearby galaxies (redshift range of 0.01 $lt$ z $lt$ 0.05). Aims. We present optical longslit spectra for a subset of the xGASS and xCOLD GASS galaxies to investigate the correlation between radial metallicity profiles and cold gas content. In addition to data from Moran et al. (2012), this paper presents new optical spectra for 27 galaxies in the stellar mass range of 9.0 $leq$ log Mstar/Msun $leq$ 10.0. Methods. The longslit spectra were taken along the major axis of the galaxies, allowing us to obtain radial profiles of the gas-phase oxygen abundance (12 + log(O/H)). The slope of a linear fit to these radial profiles is defined as the metallicity gradient. We investigated correlations between these gradients and global galaxy properties, such as star formation activity and gas content. In addition, we examined the correlation of local metallicity measurements and the global HI mass fraction. Results. We obtained two main results: (i) the local metallicity is correlated with the global HI mass fraction, which is in good agreement with previous results. A simple toy model suggests that this correlation points towards a local gas regulator model; (ii) the primary driver of metallicity gradients appears to be stellar mass surface density (as a proxy for morphology). Conclusions. This work comprises one of the few systematic observational studies of the influence of the cold gas on the chemical evolution of star-forming galaxies, as considered via metallicity gradients and local measurements of the gas-phase oxygen abundance. Our results suggest that local density and local HI mass fraction are drivers of chemical evolution and the gas-phase metallicity.
We explore the chemical distribution of stars in a simulated galaxy. Using simulations of the same initial conditions but with two different feedback schemes (MUGS and MaGICC), we examine the features of the age-metallicity relation (AMR), and the three-dimensional age-metallicity-[O/Fe] distribution, both for the galaxy as a whole and decomposed into disc, bulge, halo, and satellites. The MUGS simulation, which uses traditional supernova feedback, is replete with chemical substructure. This sub- structure is absent from the MaGICC simulation, which includes early feedback from stellar winds, a modified IMF and more efficient feedback. The reduced amount of substructure is due to the almost complete lack of satellites in MaGICC. We identify a significant separation between the bulge and disc AMRs, where the bulge is considerably more metal-rich with a smaller spread in metallicity at any given time than the disc. Our results suggest, however, that identifying the substructure in observations will require exquisite age resolution, on the order of 0.25 Gyr. Certain satellites show exotic features in the AMR, even forming a sawtooth shape of increasing metallicity followed by sharp declines which correspond to pericentric passages. This fact, along with the large spread in stellar age at a given metallicity, compromises the use of metallicity as an age indicator, although alpha abundance provides a more robust clock at early times. This may also impact algorithms that are used to reconstruct star formation histories from resolved stellar populations, which frequently assume a monotonically-increasing AMR.
Using G dwarfs from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey, we have determined a vertical metallicity gradient over a large volume of the Milky Ways disk, and examined how this gradient varies for different [a/Fe] subsamples. This sample contains over 40,000 stars with low-resolution spectroscopy over 144 lines of sight. We employ the SEGUE Stellar Parameter Pipeline (SSPP) to obtain estimates of effective temperature, surface gravity, [Fe/H], and [a/Fe] for each star and extract multiple volume-complete subsamples of approximately 1000 stars each. Based on the surveys consistent target-selection algorithm, we adjust each subsample to determine an unbiased picture of the disk in [Fe/H] and [a/Fe]; consequently, each individual star represents the properties of many. The SEGUE sample allows us to constrain the vertical metallicity gradient for a large number of stars over a significant volume of the disk, between ~0.3 and 1.6 kpc from the Galactic plane, and examine the in situ structure, in contrast to previous analyses which are more limited in scope. This work does not pre-suppose a disk structure, whether composed of a single complex population or a distinct thin and thick disk component. The metallicity gradient is -0.243 +0.039 -0.053 dex/kpc for the sample as a whole, which we compare to various literature results. Each [a/Fe] subsample dominates at a different range of heights above the plane of the Galaxy, which is exhibited in the gradient found in the sample as a whole. Stars over a limited range in [a/Fe] show little change in median [Fe/H] with height. If we associate [a/Fe] with age, our consistent vertical metallicity gradients with [a/Fe] suggest that stars formed in different epochs exhibit comparable vertical structure, implying similar star-formation processes and evolution.
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