Inside Out and Upside Down: Tracing the Assembly of a Simulated Disk Galaxy Using Mono-Age Stellar Populations

Abstract

We analyze the present-day structure and assembly history of a high resolution hydrodynamic simulation of the formation of a Milky Way (MW)-like disk galaxy, from the "Eris" simulation suite, dissecting it into cohorts of stars formed at different epochs of cosmic history. At z = 0, stars with t form < 2 Gyr mainly occupy the stellar spheroid, with the oldest (earliest forming) stars having more centrally concentrated profiles. The younger age cohorts populate disks of progressively longer radial scale length and shorter vertical scale height. At a given radius, the vertical density profiles and velocity dispersions of stars vary smoothly as a function of age, and the superposition of old, vertically-extended and young, vertically-compact cohorts gives rise to a double-exponential profile like that observed in the MW. Turning to formation history, we find that the trends of spatial structure and kinematics with stellar age are largely imprinted at birth, or immediately thereafter. Stars that form during the active merger phase at z > 3 are quickly scattered into rounded, kinematically hot configurations. The oldest disk cohorts form in structures that are radially compact and relatively thick, while subsequent cohorts form in progressively larger, thinner, colder configurations from gas with increasing levels of rotational support. The disk thus forms "inside-out" in a radial sense and "upside-down" in a vertical sense. Secular heating and radial migration influence the final state of each age cohort, but the changes they produce are small compared to the trends established at formation. The predicted correlations of stellar age with spatial and kinematic structure are in good qualitative agreement with the correlations observed for mono-abundance stellar populations in the MW.

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

“…Moreover, early discs may form hotter directly (e.g. Bird et al 2013). As we showed in the star-forming simulation, at early times even 2 Gyr is enough time to produce a significant difference in the random motions between younger and older stars, and in the Milky Way the bar may have required even longer to form.…”

“…Therefore, we argue that a pre-existing thick disc at the time of bar formation, whether as part of a continuum with the thin disc (Brook et al 2004;Schönrich & Binney 2009;Loebman et al 2011;Bird et al 2013;Bovy et al 2012), or as the separate distinct structure envisaged by Bekki & Tsujimoto (2011), is not necessary to produce the chemical trends observed in the Milky Way bulge. However, it is not inconceivable that the oldest population evolves into what, structurally, is consistent with the present-day thick disc.…”

“…Other studies (e.g. Samland 2002;Brook et al 2004;House et al 2011;Guedes et al 2013;Bird et al 2013) have considered the evolution of various aspects of the Milky Way via simulations in which the disc forms from gas, but the bar and bulge have received comparatively less attention in such work. The simulation tracks the dispersal of iron and oxygen, using the yields of Raiteri et al (1996, SN II), Thielemann et al (1986, SN Ia), and Weidemann (1987, stellar winds), allowing us to dissect the model not just by age, but also by metallicity, [Fe/H], and α-enhancement, [O/Fe].…”

Chemical Cartography With Apogee: Large-Scale Mean Metallicity Maps of the Milky Way Disk

“…Many models predict that galaxies form inside-out, with star formation proceeding more rapidly, and perhaps starting earlier, in the inner regions of disks (e.g., Larson 1976;Kobayashi & Nakasato 2011;Bird et al 2013). Models of chemical and dynamical evolution are becoming increasingly sophisticated, and it is now recognized that the structure of disks likely evolves with time.…”

“…Chemical evolution models from Hou et al (2000) predict that radial metallicity gradients arise naturally in the disk and flatten with time, while models from Chiappini et al (2001) predict that metallicity gradients present in the disk will steepen with time. Recent models and simulations postulate that migration of stars within disks may be important to understanding the observed chemical structure of the disk (e.g., Sellwood & Binney 2002;Schönrich & Binney 2009;Loebman et al 2011;Bird et al 2013;Kubryk et al 2013). Observations of chemical abundance gradients for a range of populations of stars are critical to our understanding of the evolution of the Milky Way.…”