Abundance Gradients and the Formation of the Milky Way (original) (raw)
Related papers
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.
The origin of abundance gradients in the Milky Way: the predictions of different models
Astronomy and Astrophysics, 2009
Aims. We aim at studying the abundance gradients along the Galactic disk and their dependence upon several parameters: a threshold in the surface gas density regulating star formation, the star formation efficiency, the timescale for the formation of the thin disk and the total surface mass density of the stellar halo. We use chemical evolution models already tested on a large number of observational constraints. Methods. We test a model which considers a cosmological infall law. This law does not predict an inside-out disk formation, but it allows to well fit the properties of the solar vicinity. To check whether this model can reproduce the properties of the galactic disk, we study several cases. We drop the threshold in the surface gas density and assume a star formation efficiency varying with radius. We also test the same parameters in the two-infall model for the Galaxy. Finally, we perform some additional analysis on our simulations to test if the cosmological infall law can account for the inside-out formation of the disk. Results. We find that to reproduce at the same time the abundance, star formation rate and surface gas density gradients along the Galactic disk it is necessary to assume an inside-out formation for the disk. The threshold in the gas density is not necessary and the same effect could be reached by assuming a variable star formation efficiency. The derived new cosmological infall law contains a mild inside-out formation and is still not enough to reproduce the disk properties at best. Conclusions. A cosmologically derived infall law with an inside-out process for the disk formation and a variable star formation efficiency can indeed well reproduce all the properties of the disk. However, the cosmological model presented here does not have sufficient resolution to capture the requested inside-out formation for the disk. High resolution cosmological simulations should be performed to better capture such a behavior.
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.
On star formation and chemical evolution in the Galactic disc
Astronomy and Astrophysics
The abundance gradients and the radial gas profile of the Galactic disc are analysed by means of a model for the chemical evolution of galaxies. As one of the major uncertainties in models for galaxy evolution is the star formation (SF) process, various SF laws are considered, to assess the response of model predictions to the different assumptions. Only some SF laws are successful in reproducing the metallicity gradient, and only if combined with a suitable infall timescale increasing outward (insideout formation scenario). Still, it is difficult to reproduce at the same time also the observed gas distribution; we therefore suggest further improvements for the models.
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.
Connecting Galaxies, Halos, and Star Formation Rates Across Cosmic Time
Astrophysical Journal, 2009
A simple, observationally-motivated model is presented for understanding how halo masses, galaxy stellar masses, and star formation rates are related, and how these relations evolve with time. The relation between halo mass and galaxy stellar mass is determined by matching the observed spatial abundance of galaxies to the expected spatial abundance of halos at multiple epochs -i.e. more massive galaxies are assigned to more massive halos at each epoch. This "abundance matching" technique has been shown previously to reproduce the observed luminosity-and scale-dependence of galaxy clustering over a range of epochs. Halos at different epochs are connected by halo mass accretion histories estimated from N-body simulations. The halo-galaxy connection at fixed epochs in conjunction with the connection between halos across time provides a connection between observed galaxies across time. With approximations for the impact of merging and accretion on the growth of galaxies, one can then directly infer the star formation histories of galaxies as a function of stellar and halo mass. This model is tuned to match both the observed evolution of the stellar mass function and the normalization of the observed star formation rate -stellar mass relation to z ∼ 1. The data demands, for example, that the star formation rate density is dominated by galaxies with M star ≈ 10 10.0−10.5 M ⊙ from 0 < z < 1, and that such galaxies over these epochs reside in halos with M vir ≈ 10 11.5−12.5 M ⊙ . The star formation ratehalo mass relation is approximately Gaussian over the range 0 < z < 1 with a mildly evolving mean and normalization. This model is then used to shed light on a number of issues, including 1) a clarification of "downsizing", 2) the lack of a sharp characteristic halo mass at which star formation is truncated, and 3) the dominance of star formation over merging to the stellar build-up of galaxies with M star 10 11 M ⊙ at z < 1.
The Dominant Epoch of Star Formation in the Milky Way Formed the Thick Disk
The Astrophysical Journal, 2014
We report the first robust measurement of the Milky Way star formation history using the imprint left on chemical abundances of long-lived stars. The formation of the Galactic thick disc occurs during an intense star formation phase between 9.0 (z∼1.5) and 12.5 Gyr (z∼4.5) ago and is followed by a dip (at z∼1.1) lasting about 1 Gyr. Our results imply that the thick disc is as massive as the Milky Way's thin disc, suggesting a fundamental role of this component in the genesis of our Galaxy, something that had been largely unrecognized. This new picture implies that huge quantities of gas necessary to feed the building of the thick disc must have been present at these epochs, in contradiction with the long-term infall assumed by chemical evolution models in the last two decades. These results allow us to fit the Milky Way within the emerging features of the evolution of disc galaxies in the early Universe.
The Astrophysical Journal, 2008
This paper explores the mapping between the observable properties of a stellar halo in phase-and abundance-space and the parent galaxy's accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor objects. The study utilizes a suite of eleven stellar halo models constructed within the context of a standard ΛCDM cosmology. The results demonstrate that coordinate-space studies are sensitive to the recent (0-8 Gyears ago) merger histories of galaxies (this timescale corresponds to the last few to tens of percent of mass accretion for a Milky-Way-type galaxy). Specifically, the frequency, sky coverage and fraction of stars in substructures in the stellar halo as a function of surface brightness are indicators of the importance of recent merging and of the luminosity function of infalling dwarfs. The morphology of features serves as a guide to the orbital distribution of those dwarfs. Constraints on the earlier merger history (> 8 Gyears ago) can be gleaned from the abundance patterns in halo stars: within our models, dramatic differences in the dominant epoch of accretion or luminosity function of progenitor objects leave clear signatures in the [α/Fe] and [Fe/H] distributions of the stellar halo -halos dominated by very early accretion have higher average [α/Fe], while those dominated by high luminosity satellites have higher [Fe/H]. This intuition can be applied to reconstruct much about the merger histories of nearby galaxies from current and future data sets.
Tracing Galaxy Formation with Stellar Halos. I. Methods
The Astrophysical Journal, 2005
If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted ∼ 100 − 200 luminous satellite galaxies in the past ∼ 12 Gyr. We model this process using a hybrid semi-analytic plus N-body approach which distinguishes explicitly between the evolution of light and dark matter in accreted satellites. This distinction is essential to our ability to produce a realistic stellar halo, with mass and density profile much like that of our own Galaxy, and a surviving satellite population that matches the observed number counts and structural parameter distributions of the satellite galaxies of the Milky Way. Our model stellar halos have density profiles which typically drop off with radius faster than those of the dark matter. They are assembled from the inside out, with the majority of mass (∼ 80%) coming from the ∼ 15 most massive accretion events. The satellites that contribute to the stellar halo have median accretion times of ∼ 9 Gyr in the past, while surviving satellite systems have median accretion times of ∼ 5 Gyr in the past. This implies that stars associated with the inner halo should be quite different chemically from stars in surviving satellites and also from stars in the outer halo or those liberated in recent disruption events. We briefly discuss the expected spatial structure and phase space structure for halos formed in this manner. Searches for this type of structure offer a direct test of whether cosmology is indeed hierarchical on small scales.
On the early evolution of the Galactic halo
Astronomy & Astrophysics, 2003
It is shown that the low-metallicity tail of the stellar metallicity distribution predicted by simple Outflow models for the Milky Way halo depends sensitively on whether instantaneous recycling is adopted or relaxed. In both cases, current-and still preliminary-data suggest a "G-dwarf problem" for the halo (reminiscent of the local disk). We suggest that the problem can be solved by introducing a (physically motivated) early infall phase. We point out several important implications of such a modification, concerning: the putative Pop. III (super)massive stars, the number of stars expected at very low metallicities, the questions of primary nitrogen and of the dispersion in abundance ratios of halo stars.