The Inside-out Growth of the Galactic Disk (original) (raw)
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Tracing Chemical Evolution Over the Extent of the Milky Way's Disk with Apogee Red Clump Stars
The Astrophysical Journal, 2014
We employ the first two years of data from the near-infrared, high-resolution SDSS-III/APOGEE spectroscopic survey to investigate the distribution of metallicity and α-element abundances of stars over a large part of the Milky Way disk. Using a sample of ≈ 10, 000 kinematically-unbiased redclump stars with ∼5% distance accuracy as tracers, the [α/Fe] vs. [Fe/H] distribution of this sample exhibits a bimodality in [α/Fe] at intermediate metallicities, −0.9 <[Fe/H]< −0.2, but at higher metallicities ([Fe/H]∼+0.2) the two sequences smoothly merge. We investigate the effects of the APOGEE selection function and volume filling fraction and find that these have little qualitative impact on the α-element abundance patterns. The described abundance pattern is found throughout the range 5<R<11 kpc and 0<|Z|<2 kpc across the Galaxy. The [α/Fe] trend of the high-α sequence is surprisingly constant throughout the Galaxy, with little variation from region to region (∼10%). Using simple galactic chemical evolution models we derive an average star formation efficiency (SFE) in the high-α sequence of ∼4.5×10 −10 yr −1 , which is quite close to the nearly-constant value found in molecular-gas-dominated regions of nearby spirals. This result suggests that the early evolution of the Milky Way disk was characterized by stars that shared a similar star formation history and were formed in a well-mixed, turbulent, and molecular-dominated ISM with a gas consumption timescale (SFE −1) of ∼ 2 Gyr. Finally, while the two α-element sequences in the inner Galaxy can be explained by a single chemical evolutionary track this cannot hold in the outer Galaxy, requiring instead a mix of two or more populations with distinct enrichment histories.
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.
Evolution of the Milky Way with radial motions of stars and gas
Astronomy & Astrophysics, 2015
Context. We study the role of radial migration of stars on the chemical evolution of the Milky Way disk. Aims. We are interested in the impact of that process on the local properties of the disk (age-metallicity relation and its dispersion, metallicity distribution, evolution of abundance ratios) and on the morphological properties of the resulting thick and thin disks. Methods. We use a model with several new or updated ingredients: atomic and molecular gas phases, star formation that depends on molecular gas, yields from a recent homogeneous grid and observationally inferred SNIa rates. We describe radial migration with parametrised time-and radius-dependent diffusion coefficients, based on the analysis of an N-body+SPH simulation. We also consider parametrised radial gas flows, induced by the action of the Galactic bar. Results. Our model reproduces current values of most of the main global observables of the MW disk and bulge, and also the observed "stacked" evolution of MW-type galaxies. The azimuthally averaged radial velocity of gas inflow is constrained to less than a few tenths of km s −1. Radial migration is constrained by the observed dispersion in the age-metallicity relation. Assuming that the thick disk is the oldest (>9 Gyr) part of the disk, we find that the adopted radial migration scheme can quantitatively reproduce the main local properties of the thin and thick disk: metallicity distributions, "two-branch" behaviour in the O/Fe vs. Fe/H relation and the local surface densities of stars. The thick disk extends up to ∼11 kpc and has a scale length of 1.8 kpc, which is considerably shorter than the thin disk, because of the inside-out formation scheme. We also show how, in this framework, current and forthcoming spectroscopic observations can constrain the nucleosynthesis yields of massive stars for the metallicity range of 0.1 Z to 2−3 Z .
The Astrophysical Journal, 2015
We investigate the influence of stellar migration caused by minor mergers (mass ratio from 1:70 to 1:8) on the radial distribution of chemical abundances in the disks of Milky Way-like galaxies during the last four Gyr. A GPU-based pure N-body tree-code model without hydrodynamics and star formation was used. We computed a large set of mergers with different initial satellite masses, positions, and orbital velocities. We find that there is no significant metallicity change at any radius of the primary galaxy in the case of accretion of a low-mass satellite of 10 9 M ⊙ (mass ratio 1:70) except for the special case of prograde satellite motion in the disk plane of the host galaxy. The accretion of a satellite of a mass 3 × 10 9 M ⊙ (mass ratio 1:23) results in an appreciable increase of the chemical abundances at galactocentric distances larger than ∼ 10 kpc. The radial abundance gradient flattens in the range of galactocentric distances from 5 to 15 kpc in the case of a merger with a satellite with a mass 3 × 10 9 M ⊙ . There is no significant change in the abundance gradient slope in the outer disk (from ∼ 15 kpc up to 25 kpc) in any merger while the scatter in metallicities at a given radius significantly increases for most of the satellite's initial masses/positions compared to the case of an isolated galaxy. This argues against attributing the break (flattening) of the abundance gradient near the optical radius observed in the extended disks of Milky Way-like galaxies only to merger-induced stellar migration.
The Astrophysical Journal, 2011
A sample of very high resolution cosmological disk galaxy simulations is used to investigate the evolution of galaxy disk sizes back to redshift 1 within the ΛCDM cosmology. Artificial images in the rest frame B band are generated, allowing for a measurement of disk scale lengths using surface brightness profiles as observations would, and avoiding any assumption that light must follow mass as previous models have assumed. We demonstrate that these simulated disks are an excellent match to the observed magnitude -size relation for both local disks, and for disks at z=1 in the magnitude/mass range of overlap. We disentangle the evolution seen in the population as a whole from the evolution of individual disk galaxies. In agreement with observations, our simulated disks undergo roughly 1.5 magnitudes/arcsec 2 of surface brightness dimming since z=1. We find evidence that evolution in the magnitude -size plane varies by mass, such that galaxies with M * ≥ 10 9 M ⊙ undergo more evolution in size than luminosity, while dwarf galaxies tend to evolve potentially more in luminosity. The disks grow in such a way as to stay on roughly the same stellar mass -size relation with time. Finally, due to an evolving stellar mass -SFR relation, a galaxy at a given stellar mass (or size) at z=1 will reside in a more massive halo and have a higher SFR, and thus a higher luminosity, than a counterpart of the same stellar mass at z=0.
The Relationship between Age, Metallicity, and Abundances for Disk Stars in a Simulated Milky Way
The Astrophysical Journal
Observations of the Milky Way’s low-α disk show that several element abundances correlate with age at fixed metallicity, with unique slopes and small scatters around the age–[X/Fe] relations. In this study, we turn to simulations to explore the age–[X/Fe] relations for the elements C, N, O, Mg, Si, S, and Ca that are traced in a FIRE-2 cosmological zoom-in simulation of a Milky Way–like galaxy, m12i, and understand what physical conditions give rise to the observed age–[X/Fe] trends. We first explore the distributions of mono-age populations in their birth and current locations, [Fe/H], and [X/Fe], and find evidence for inside-out radial growth for stars with ages <7 Gyr. We then examine the age–[X/Fe] relations across m12i’s disk and find that the direction of the trends agrees with observations, apart from C, O, and Ca, with remarkably small intrinsic scatters, σ int (0.01 − 0.04 dex). This σ int measured in the simulations is also metallicity dependent, with σ int ≈ 0.025 dex ...
Measuring Radial Orbit Migration in the Galactic Disk
The Astrophysical Journal, 2018
We develop and apply a model to quantify the global efficiency of radial orbit migrationamong stars in the Milky Way disk. This model parameterizes the possible star formation and enrichment histories and radial birth profiles, and combines them with a migration model that relates present-day orbital radii to birth radii through a Gaussian probability, broadening with age τ as s t 8 Gyr RM8. Guided by observations, we assume that stars are born with an initially tight age-metallicity relation at given radius, which becomes subsequently scrambled by radial orbit migration, thereby providing a direct observational constraint on radial orbit migrationstrength s RM8. We fit this model with Markov Chain Monte Carlo sampling of the observed age-metallicity distribution of low-α red clump stars with Galactocentric radii between 5 and 14 kpc from APOGEE DR12, sidestepping the complex spatial selection function and accounting for the considerable age uncertainties. This simple model reproduces the observed data well, and we find a global (in radius and time) radial orbit migrationefficiency in the Milky Way of s RM8 =3.6±0.1 kpc when marginalizing over all other aspects of the model. This shows that radial orbit migrationin the Milky Way's main disk is indeed rather strong, in line with theoretical expectations: stars migrate by about a half-mass radius over the age of the disk. The model finds the Sun's birth radius at ∼5.2 kpc. If such strong radial orbit migrationis typical, this mechanism indeed plays an important role in setting the structural regularity of disk galaxies.
The evolution of massive disc galaxies with environment and redshift
2006
ix Acknowledgements xi Published work 5.1 Cluster versus field offsets for relations of stellar and emission-line scalelengths against rotation velocities and magnitudes . . . . . . . . 124 5.2 Statistical comparison of cluster and field EW sample distributions . 129 viii Abstract
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.
1998
We present a scenario for the formation and evolution of disk galaxies within the framework of an inflationary CDM universe, and we compare the results with observations ranking from the present-day up to z~1. Galactic disks are built-up inside-out by gas infall with accretion rates driven by the cosmological mass aggregation history (MAH). We generate the MAHs for a Gaussian density fluctuation field, and we calculate the gravitational collapse and virialization of the dark halos. Assuming detailed angular momentum conservation, disks in centrifugal equilibrium are built-up within them. The disk galactic evolution is followed through a physically self-consistent approach. The main disk galaxy properties and their correlations are determined by the mass, the MAH, and the spin parameter. The models give exponential disk surface brightness (SB) profiles with realistic parameters (including LSB galaxies), nearly flat rotation curves, and negative gradients in the B-V radial profile. Th...