On star formation and chemical evolution in the Galactic disc (original) (raw)
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The Chemical Evolution of the Galaxy: The Two‐Infall Model
The Astrophysical Journal, 1997
In this paper we present a new chemical evolution model for the Galaxy which assumes two main infall episodes for the formation of halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo since our model is better suited for computing the evolution of the disk (thick plus thin) but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin-disk is much longer than that of the thick disk, implying that the infalling gas forming the thin-disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin-disk is assumed to be a function of the galactocentric distance, leading to an inside-out picture for the Galaxy building. The model takes into account the most up to date nucleosynthesis prescriptions and adopts a threshold in the star formation process which naturally produces a hiatus in the star formation rate at the end of the thick disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints both for the solar neighbourhood and the whole disk. Among these constraints, the tightest one is the metallicity distribution of the G-dwarf stars for which new data are now available. Our model fits very well these new data. The model predicts also the evolution of the gas mass, the star formation rate, the supernova rates and the abundances of 16 chemical elements as functions of time and galactocentric distance. We show that in order to reproduce most of these constraints a timescale ≤ 1 Gyr for the (halo)-thick-disk and of 8 Gyr for the thin-disk formation in the solar vicinity are required.-3-We predict that the radial abundance gradients in the inner regions of the disk (R < R ⊙) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner ones steepen in time during the Galactic lifetime. The importance and the advantages of assuming a threshold gas density for the onset of the star formation process is discussed.
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 chemical evolution models: the role of molecular gas formation
Monthly Notices of the Royal Astronomical Society, 2017
In our classical grid of multiphase chemical evolution models, star formation in the disc occurs in two steps: first, molecular gas forms, and then stars are created by cloud-cloud collisions or interactions of massive stars with the surrounding molecular clouds. The formation of both molecular clouds and stars are treated through the use of free parameters we refer to as efficiencies. In this work, we modify the formation of molecular clouds based on several new prescriptions existing in the literature, and we compare the results obtained for a chemical evolution model of the Milky Way Galaxy regarding the evolution of the Solar region, the radial structure of the Galactic disc and the ratio between the diffuse and molecular components, H I/H 2. Our results show that the six prescriptions we have tested reproduce fairly consistent most of the observed trends, differing mostly in their predictions for the (poorly constrained) outskirts of the Milky Way and the evolution in time of its radial structure. Among them, the model proposed by Ascasibar et al. (in preparation), where the conversion of diffuse gas into molecular clouds depends on the local stellar and gas densities as well as on the gas metallicity, seems to provide the best overall match to the observed data.
Numerical simulations of the formation and chemical evolution of galaxies
Monthly Notices of the Royal Astronomical Society, 2001
We present the results of a set of three-dimensional SPH-Treecode simulations which model the formation and early evolution of disc galaxies, including the generation and return of heavy elements to the interstellar medium by star formation. Starting from simple initial conditions which are given by a uniform density sphere of gas which is embedded in a dark matter halo and in solid-body rotation, we are able to form realistic disc galaxies, and find that an exponential gas disc is quickly formed. Star formation within this exponential disc naturally leads to the formation of abundance gradients which are in broad agreement with those observed, although they are slightly shallower than some observations. We investigate the systematic effects of variation of mass, rotation and star formation parameters on the abundance gradients. We find that the abundance gradients are most sensitive to changes in the star formation parameters or rotation. Including a critical-density cutoff in the star formation law causes abundance gradients to be steepened. Analysis of gas flows within the models shows radial flows which are a function of angle of azimuth around the galaxies, with alternating inward and outward flows. This motion is linked to the presence of a bar, whose strength is related to the amount of star formation in the models, and there is a gentle drift of mass inwards. The shallow abundance gradients may be linked to these radial flows.
Gas Flows, Star Formation and Galaxy Evolution
Astrophysics and Space Science Library, 2004
In the first part of this article we show how observations of the chemical evolution of the Galaxy: G-and K-dwarf numbers as functions of metallicity, and abundances of the light elements, D, Li, Be and B, in both stars and the interstellar medium (ISM), lead to the conclusion that metal poor H i gas has been accreting to the Galactic disc during the whole of its lifetime, and is accreting today at a measurable rate, ∼ 2M⊙ per year across the full disc. Estimates of the local star formation rate (SFR) using methods based on stellar activity, support this picture. The best fits to all these data are for models where the accretion rate is constant, or slowly rising with epoch. We explain here how this conclusion, for a galaxy in a small bound group, is not in conflict with graphs such as the Madau plot, which show that the universal SFR has declined steadily from z = 1 to the present day. We also show that a model in which disc galaxies in general evolve by accreting major clouds of low metallicity gas from their surroundings can explain many observations, notably that the SFR for whole galaxies tends to show obvious variability, and fractionally more for early than for late types, and yields lower dark to baryonic matter ratios for large disc galaxies than for dwarfs. In the second part of the article we use NGC 1530 as a template object, showing from Fabry-Pérot observations of its Hα emission how strong shear in this strongly barred galaxy acts to inhibit star formation, while compression acts to stimulate it.
A simple chemical evolution model for the Milky Way disc with radial gas flows and stellar migration
We introduce a simple treatment of stellar migration in a detailed chemical evolution model for the thin disc of the Milky Way that already includes gas radial flows and reproduces several observational constraints for the solar vicinity, as well as the whole disc. We find that stellar migration has a negligible effect on the G-dwarf metallicity distribution in the solar neighbourhood, even in presence of a significant drift from the innermost regions. Therefore we conclude that the G-dwarf metallicity distribution hardly gives any information to be used to quantify the extent of migration. On the other hand, a large fraction of the spread observed in the age-metallicity relation of solar neighbourhood stars can be explained by the presence of stars that originated at different Galactocentric distances, though part of the observed spread could still be due to errors in the determination of stellar ages. Finally, we show that a substantial stellar migration can significantly affect the observed distribution of stars along the disk, so that the stellar surface density seems to be another important constraint to stellar migration models. In conclusion, our simulations suggest that, while stellar migration should be present at some extent, its amount has been probably overestimated in previous works.
We present results concerning the internal structure and kinematics of disk galaxies formed in cosmologically motivated simulations. The calculations include dark matter, gas dynamics, radiative cooling, star formation, supernova feedback and metal enrichment. The initial model is a rigidly rotating overdense sphere with a mass of about 8 10 11 M which is perturbed by small scale uctuations according to a biased CDM power spectrum. Converging, Jeans unstable and rapidly cooling regions are allowed to form stars. Via supernovae, metal enriched gas is returned to the interstellar medium. From these initial conditions a galaxy forms which shows the main properties of spiral galaxies: a rotationally supported exponential disk which consists of young stars with about solar metallicity, a slowly rotating halo of old metal poor stars, a bulge of old metal rich stars and a slowly rotating extended halo of dark matter. Bulge, stellar and dark halo are supported by an anisotropic velocity dispersion and have a de Vaucouleurs surface density pro le. The attening of the dark and stellar halo is too large to be explained by rotation only. Whether the attening of the bulge is caused by an anisotropic velocity dispersion or by its rotation cannot be answered, because of the limited numerical resolution due to gravitational softening. The velocity dispersion and the thickness of the stellar disk increase with the age of the stars. Considering only the young stellar component, the disk is cold ( = 20 km/sec) and thin (z < 1 kpc). The dynamical formation process ends after about 4 Gyr, when the disk reaches a quasi{stationary state. During the subsequent 8 Gyr mainly gas is transformed into stars decreasing the gas fraction from 40% to 10%. The star formation rate successively decreases during the quasi{stationary state from about 5 M /yr at z = 1 to less than 0.5 M /yr at the end of the simulation (z = 0).
An analytic model for the evolution of the stellar, gas and metal content of galaxies
Monthly Notices of the Royal Astronomical Society, 2011
We present an analytic formalism that describes the evolution of the stellar, gas and metal content of galaxies. It is based on the idea, inspired by hydrodynamic simulations, that galaxies live in a slowly evolving equilibrium between inflow, outflow and star formation. We argue that this formalism broadly captures the behaviour of galaxy properties evolving in simulations. The resulting equilibrium equations for the star formation rate, gas fraction and metallicity depend on three key free parameters that represent ejective feedback, preventive feedback and reaccretion of ejected material. We schematically describe how these parameters are constrained by models and observations. Galaxies perturbed off the equilibrium relations owing to inflow stochasticity tend to be driven back towards equilibrium, such that deviations in star formation rate at a given mass are correlated with gas fraction and anticorrelated with metallicity. After an early gas accumulation epoch, quiescently star-forming galaxies are expected to be in equilibrium over most of cosmic time. The equilibrium model provides a simple intuitive framework for understanding the cosmic evolution of galaxy properties, and centrally features the cycle of baryons between galaxies and surrounding gas as the driver of galaxy growth.
Abundance gradients in the galactic disk
Science in China Series G: Physics …, 2003
The relationship between abundances and orbital parameters for 235 F-and G-type intermediate-and low-mass stars in the Galaxy is analyzed. We found that there are abundance gradients in the thin disk in both radial and vertical directions (−0.116 dex kpc −1 and −0.309 dex kpc −1 respectively). The gradients appear to be flatter as the Galaxy evolves. No gradient is found in the thick disk based on 18 thick disk stars. These results indicate that the ELS model is mainly suitable for the evolution of the thin disk, while the SZ model is more suitable for the evolution of the thick disk. Additionally, these results indicate that in-fall and out-flow processes play important roles in the chemical evolution of the Galaxy.
Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances
Astronomy & Astrophysics, 2015
We develop a chemical evolution model in order to study the star formation history of the Milky Way. Our model assumes that the Milky Way is formed from a closed box-like system in the inner regions, while the outer parts of the disc experience some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) in order to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age in order to recover the star formation history of the Galaxy. Our method enables one to recover with unprecedented accuracy the star formation history of the Milky Way in the first Gyrs, in both the inner (R<7-8 kpc) and outer (R>9-10 kpc) discs as sampled in the solar vicinity. We show that, in the inner disc, half of the stellar mass formed during the thick disc phase, in the first 4-5 Gyr. This phase was followed by a significant dip in the star formation activity (at 8-9 Gyr) and a period of roughly constant lower level star formation for the remaining 8 Gyr. The thick disc phase has produced as many metals in 4 Gyr as the thin disc in the remaining 8 Gyr. Our results suggest that a closed box model is able to fit all the available constraints in the inner disc. A closed box system is qualitatively equivalent to a regime where the accretion rate, at high redshift, maintains a high gas fraction in the inner disc. In such conditions, the SFR is mainly governed by the high turbulence of the ISM. By z∼1 it is possible that most of the accretion takes place in the outer disc, while the star formation activity in the inner disc is mostly sustained by the gas not consumed during the thick disc phase, and the continuous ejecta from earlier generations of stars. The outer disc follows a star formation history very similar to that of the inner disc, although initiated at z∼2, about 2 Gyr before the onset of the thin disc formation in the inner disc.