Supernova-driven Turbulent Metal Mixing in High-redshift Galactic Disks: Metallicity Fluctuations in the Interstellar Medium and its Imprints on Metal-poor Stars in the Milky Way (original) (raw)
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The Metallicity Evolution of Interacting Galaxies
2011
Nuclear inflows of metal-poor interstellar gas triggered by galaxy interactions can account for the systematically lower central oxygen abundances observed in local interacting galaxies. Here, we investigate the metallicity evolution of a large set of simulations of colliding galaxies. Our models include cooling, star formation, feedback, and a new stochastic method for tracking the mass recycled back to the interstellar medium from stellar winds and supernovae. We study the influence of merger-induced inflows, enrichment, gas consumption, and galactic winds in determining the nuclear metallicity. The central metallicity is primarily a competition between the inflow of low-metallicity gas and enrichment from star formation. An average depression in the nuclear metallicity of ∼ 0.07 is found for gas-poor disk-disk interactions. Gas-rich disk-disk interactions, on the other hand, typically have an enhancement in the central metallicity that is positively correlated with the gas content. The simulations fare reasonably well when compared to the observed mass-metallicity and separation-metallicity relationships, but further study is warranted.
Mixing and transport of metals by gravitational instability-driven turbulence in galactic discs
Monthly Notices of the Royal Astronomical Society
Metal production in galaxies traces star formation, and is highly concentrated toward the centers of galactic discs. This suggests that galaxies should have inhomogeneous metal distributions with strong radial gradients, but observations of present-day galaxies show only shallow gradients with little azimuthal variation, implying the existence of a redistribution mechanism. We study the role of gravitational instability-driven turbulence as a mixing mechanism by simulating an isolated galactic disc at high resolution, including metal fields treated as passive scalars. Since any cylindrical field can be decomposed into a sum of Fourier-Bessel basis functions, we set up initial metal fields characterized by these functions and study how different modes mix. We find both shear and turbulence contribute to mixing, but the mixing strongly depends on the symmetries of the mode. Non-axisymmetric modes have decay times smaller than the galactic orbital period because shear winds them up to small spatial scales, where they are erased by turbulence. The decay timescales for axisymmetric modes are much greater, though for all but the largest-scale inhomogeneities the mixing timescale is still short enough to erase chemical inhomogeneities over cosmological times. These different timescales provide an explanation for why galaxies retain metallicity gradients while there is almost no variation at a fixed radius. Moreover, the comparatively long timescales required for mixing axisymmetric modes may explain the greater diversity of metallicity gradients observed in high redshift galaxies as compared to local ones: these systems have not yet reached equilibrium between metal production and diffusion. 1 INTRODUCTION Understanding the dynamics of the flow of metals through and around galaxies is a key problem in the study of galaxy formation. Metals trace the history of the gas flows in galaxies such as the young Milky Way (Ivezić et al. 2012) and record the buildup of stellar populations in their progenitors at high redshift (Tremonti et al. 2004; Erb et al. 2006). Moreover, metals are not just passive tracers. They change the chemical and radiative cooling properties of the interstellar medium, altering how stars form (
Metal Enrichment in Galactic Winds
Observations give evidences of the presence of metals in the intergalactic medium (IGM). The stars responsible for transforming hydrogen and helium into more complex atoms do not form outside the galaxies in the standard scenario of galaxy formation. Supernovaedriven winds and their associated feedback was proposed as a possible solution to explain such enrichment of the IGM. It turned out that a proper modelling of supernovae explosions within a turbulent interstellar medium (ISM) is a difficult task. Recent advances have been obtained using a multiphase approach to solve for the thermal state of the ISM, plus some additional recipes to account for the kinetic effect of supernovae on the galactic gas. We briefly describe here our implementation of supernovae feedback within the RAMSES code, and apply it to the formation and evolution of isolated galaxies of various masses and angular momenta. We have explored under what conditions a galactic wind can develop, if one considers only a quiescent mode of star formation. We have also characterized the distribution and evolution of metallicity in the gas outflow spreading in the IGM.
The Astrophysical Journal, 2011
We study the influence of gas metallicity, turbulence, and non-equilibrium chemistry on the evolution of the two-phase interstellar medium (warm and cold atomic phases), and thereby constrain the initial conditions for star formation prevailing in turbulent gas. We perform highresolution simulations in three dimensions, including a realistic non-equilibrium treatment of the ionization state of the gas, and examine both driven and decaying turbulence. This allows us to explore variations in the metallicity Z. In this paper, we study solar metallicity, Z = Z , and low metallicity, Z = 10 −3 Z , gas. For driven, large-scale turbulence, we find that the influence of the metallicity on the amount of mass in the cold gas component is small. However, in decaying turbulent conditions this picture is much changed. While cold regions survive in the case of solar metallicity, they are quickly heated and dispersed in low-metallicity gas. This result suggests that star formation can be suppressed in environments of low metallicity, unless a strong turbulent driver is acting on time scales shorter than a few turbulent crossing times. Inter alia this finding could explain the overall inefficient star formation as well as the burst-like mode of star formation found in metal-poor, gas-rich systems like dwarf galaxies.
Gas Inflows, Star Formation and Metallicity Evolution in Galaxy Pairs
Proceedings of the International Astronomical Union, 2010
It has been known since many decades that galaxy interactions can induce star formation (hereafter SF) enhancements and that one of the driving mechanisms of this enhancement is related to gas inflows into the central galaxy regions, induced by asymmetries in the stellar component, like bars. In the last years many evidences have been accumulating, showing that interacting pairs have central gas-phase metallicities lower than those of field galaxies, by ∼ 0.2-0.3 dex on average. These diluted ISM metallicities have been explained as the result of inflows of metal-poor gas from the outer disk to the galaxy central regions. A number of questions arises: What's the timing and the duration of this dilution? How and when does the SF induced by the gas inflow enrich the circum-nuclear gas with re-processed material? Is there any correlation between the timing and strength of the dilution and the timing and intensity of the SF? By means of Tree-SPH simulations of galaxy major interactions, we have studied the effect that gas inflows have on the ISM dilution, and the effect that the induced SF has, subsequently, in re-enriching the nuclear gas. In this contribution, we present the main results of this study.
Monthly Notices of the Royal Astronomical Society, 2014
We analyse the properties of the circum-galactic medium and the relation between its metal content and that of the stars comprising the central galaxy in eight hydrodynamical 'zoom-in' simulations of disc galaxy formation. The simulations employ the moving-mesh code arepo combined with a comprehensive model for the galaxy formation physics, and succeed in forming realistic late-type spirals in the set of 'Aquarius' initial conditions of Milky Way-sized haloes. Galactic winds significantly influence the morphology of the circum-galactic medium and induce bipolar features in the distribution of heavy elements. They also affect the thermodynamic properties of the circum-galactic gas by supplying an energy input that sustains its radiative losses. Although a significant fraction of the heavy elements are transferred from the central galaxy to the halo, and even beyond the virial radius, the overall stellar metallicity distribution is broadly consistent with observations, apart from an overestimate of the [O/Fe] ratio in our default runs, an effect that can however be rectified by an increase of the adopted SN type Ia rate. All our simulations have difficulty in producing stellar metallicity gradients of the same strength as observed in the Milky Way.
Star formation and metallicity gradients in semi-analytic models of disc galaxy formation
Monthly Notices of the Royal Astronomical Society, 2013
We have updated our radially resolved semi-analytic models (SAMs) of galaxy formation, which track both the atomic and molecular gas phases of the interstellar medium. The models are adapted from those of Guo et al. using similar methodology as by Fu et al. and are run on halo merger trees from the Millennium and Millennium-II simulations with the following main changes. (1) We adopt a simple star formation law SFR ∝ H 2. (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 disc. (3) We include radial gas inflows in discs 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 surface density profiles of molecular gas in L * galaxies place strong constraints on inflow velocities, favouring models where v inflow ∼ 7 km s −1 at a galactocentric radius of 10 kpc. 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 per cent 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 , as well as the relations between gas-phase metallicity and specific star formation rate in the outer parts of galactic discs. We examine the correlation between the 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.
Galaxy evolution in cosmological simulations with outflows - II. Metallicities and gas fractions
Monthly Notices of the Royal Astronomical Society, 2011
We use cosmological hydrodynamic simulations to investigate how inflows, star formation and outflows govern the gaseous and metal content of galaxies within a hierarchical structure formation context. In our simulations, galaxy metallicities are established by a balance between inflows and outflows as governed by the mass outflow rate, implying that the mass-metallicity relation reflects how the outflow rate varies with stellar mass. Gas content, meanwhile, is set by a competition between inflow into and gas consumption within the interstellar medium, the latter being governed by the star formation law, while the former is impacted by both wind recycling and preventive feedback. Stochastic variations in the inflow rate move galaxies off the equilibrium mass-metallicity and mass-gas fraction relations in a manner correlated with the star formation rate, and the scatter is set by the timescale to re-equilibrate. The evolution of both relations from z = 3 → 0 is slow, as individual galaxies tend to evolve mostly along the relations. Gas fractions at a given stellar mass slowly decrease with time because the cosmic inflow rate diminishes faster than the consumption rate, while metallicities slowly increase as infalling gas becomes more enriched. Observations from z ∼ 3 → 0 are better matched by simulations employing momentum-driven wind scalings rather than constant wind speeds, but all models predict too low gas fractions at low masses and too high metallicities at high masses. All our models reproduce observed second-parameter trends of the mass-metallicity relation with the star formation rate and environment, indicating that these are a consequence of equilibrium and not feedback. Overall, the analytical framework of our equilibrium scenario broadly captures the relevant physics establishing the galaxy gas and metal content in simulations, which suggests that the cycle of baryonic inflows and outflows centrally governs the cosmic evolution of these properties in typical star-forming galaxies.
The metal enrichment of the intracluster medium in hierarchical galaxy formation models
Monthly Notices of the Royal Astronomical Society, 2005
We investigate the metal enrichment of the intracluster medium (ICM) in the framework of hierarchical models of galaxy formation. We calculate the formation and evolution of galaxies and clusters using a semi-analytical model which includes the effects of flows of gas and metals both into and out of galaxies. For the first time in a semi-analytical model, we calculate the production of both α and iron-peak elements based on theoretical models for the lifetimes and ejecta of type Ia and type II supernovae (SNe Ia and SNe II). It is essential to include the long lifetimes of the SNIa progenitors in order to correctly model the evolution of the iron-peak elements. We find that if all stars form with an IMF similar to that found in the solar neighbourhood, then the metallicities of O, Mg, Si and Fe in the ICM are predicted to be 2-3 times lower than observed values. In contrast, a model (also favoured on other grounds) in which stars formed in bursts triggered by galaxy mergers have a top-heavy IMF reproduces the observed ICM abundances of O, Mg, Si and Fe. The same model predicts ratios of ICM mass to total stellar luminosity in clusters which agree well with observations. According to our model, the bulk of the metals in clusters are produced by L * and brighter galaxies. We predict only mild evolution of [Fe/H] in the ICM with redshift out to z ∼ 1, consistent with the sparse data available on high-z clusters. By contrast, the [O/Fe] ratio is predicted to gradually decrease with time because of the delayed production of iron compared with oxygen. We find that, at a given redshift, the scatter in global metallicity for clusters of a given mass is quite small, even though the formation histories of individual clusters show wide variations. The observed diversity in ICM metallicities may thus result from the range in metallicity gradients induced by the scatter in the assembly histories of clusters of galaxies.
Atomic diffusion and mixing in old stars
Astronomy & Astrophysics, 2008
Context. Evolutionary trends in the surface abundances of heavier elements have recently been identified in the globular cluster NGC 6397 ([Fe/H] = −2), indicating the operation of atomic diffusion in these stars. Such trends constitute important constraints for the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars. Aims. We perform an independent check of the reality and size of abundance variations within this metal-poor globular cluster. Methods. Observational data covering a large stellar sample, located between the cluster turn-off point and the base of the red giant branch, are homogeneously analysed. The spectroscopic data were obtained with the medium-high resolution spectrograph FLAMES/GIRAFFE on VLT-UT2 (R ∼ 27 000). We derive independent effective-temperature scales from profile fitting of Balmer lines and by applying colour-T eff calibrations to Strömgren uvby and broad-band BV I photometry. An automated spectral analysis code is used together with a grid of MARCS model atmospheres to derive stellar surface abundances of Mg, Ca, Ti, and Fe. Results. We identify systematically higher iron abundances for more evolved stars. The turn-off point stars are found to have 0.13 dex lower surface abundances of iron compared to the coolest, most evolved stars in our sample. There is a strong indication of a similar trend in magnesium, whereas calcium and titanium abundances are more homogeneous. Within reasonable error limits, the obtained abundance trends are in agreement with the predictions of stellar structure models including diffusive processes (sedimentation, levitation), if additional turbulent mixing below the outer convection zone is included.