Inside Out and Upside-Down: The Roles of Gas Cooling and Dynamical Heating in Shaping the Stellar Age-Velocity Relation

Abstract

Kinematic studies of disk galaxies, using individual stars in the Milky Way or statistical studies of global disk kinematics over time, provide insight into how disks form and evolve. We use a high-resolution, cosmological zoom-simulation of a Milky Way-mass disk galaxy (h277) to tie together local disk kinematics and the evolution of the disk over time. The present-day stellar age-velocity relationship (AVR) of h277 is nearly identical to that of the analogous solar-neighborhood measurement in the Milky Way. A crucial element of this success is the simulation's dynamically cold multi-phase ISM, which allows young stars to form with a low velocity dispersion (σ birth ∼ 6 − 8 km s −1 ) at late times. Older stars are born kinematically hotter (i.e., the disk settles over time in an "upside-down" formation scenario), and are subsequently heated after birth. The disk also grows "inside-out", and many of the older stars in the solar neighborhood at z = 0 are present because of radial mixing. We demonstrate that the evolution of σ birth in h277 can be explained by the same model used to describe the general decrease in velocity dispersion observed in disk galaxies from z ∼ 2 − 3 to the present-day, in which the disk evolves in quasi-stable equilibrium and the ISM velocity dispersion decreases over time due to a decreasing gas fraction. Thus, our results tie together local observations of the Milky Way's AVR with observed kinematics of high z disk galaxies.

Stellar Migration and Chemical Enrichment in the Milky Way Disc: A Hybrid Model

“…Previous studies have shown that h277 and other disc galaxies evolved with similar physics have realistic rotation curves (Governato et al 2012;Christensen et al 2014a,b), stellar mass (Munshi et al 2013), metallicity (Christensen et al 2016), dwarf satellite populations (Zolotov et al 2012;Brooks & Zolotov 2014), and HI properties (Brooks et al 2017). Most directly relevant to this study, Bird et al (2020) demonstrate that h277 accurately reproduces the observed relation between stellar age and vertical velocity dispersion 𝜎 𝑧 . This relation arises as a consequence of "upside-down" disc formation in which the star-forming gas layer becomes thinner with time as well as the dynamical heating of stars as they age Bournaud, Elmegreen & Martig 2009;Forbes, Krumholz & Burkert 2012;Bird et al 2013;Vincenzo, Kobayashi & Yuan 2019;Yu et al 2021).…”

“…Vertical gradients arise because older populations have larger 𝜎 𝑧 and thus larger average |𝑧|, and also because radial migration is coupled to changes in 𝜎 𝑧 (Solway, Sellwood & Schönrich 2012). The good match to the observed age-velocity relation found by Bird et al (2020) allows us to use vertical trends of abundance ditributions as a further test of our chemical evolution model.…”

“…In this paper we make use of star particles from the h277 simulation (Christensen et al 2012;Zolotov et al 2012;Loebman et al 2012Loebman et al , 2014Brooks & Zolotov 2014). Recently employed to study the stellar age-velocity relationship, a synopsis of its detailed simulation parameters and cosmological model can be found in § 2 of Bird et al (2020). We do not repeat these details here, instead focusing on how we vet the sample of star particles for use in our chemical evolution models.…”

New families in our Solar neighborhood: applying Gaussian Mixture models for objective classification of structures in the Milky Way and in simulations

“…In an alternative to such kinematic selections, other studies have selected disk stars based on their elemental abundances, and from their spatial distributions tested the idea of a continuous transition between thin and thick disks (e.g., Bovy et al 2012aBovy et al ,b, 2016Mackereth et al 2017). Some cosmological simulations (e.g., Ma et al 2017a;Bird et al 2020) also do not predict a clean/sharp transition from the thick to the thin disk, but rather, a more gradual settling of the stellar disk (note, however, that some simulations such as the FIRE simulations analyzed in this paper suggest a sharper transition in the properties of the gas disk, as galaxies transition from highly bursty to more steady star formation rates, (e.g., Stern et al 2020)). Still others were interested in searching for local interlopers from the halo to assess the Milky Way's accretion history, simultaneously employing both kinematic and metallicity cuts to select this relatively small population from the overwhelmingly more numerous disk stars (e.g., Helmi et al 2017;Herzog-Arbeitman et al 2017).…”

An observational testbed for cosmological zoom-in simulations: constraining stellar migration in the solar cylinder using asteroseismology

“…The Auriga simulations used a density threshold of 0.13 cm −3 (Grand et al 2017), which was derived from the parameters describing the ISM and the desired star formation time-scale (Springel & Hernquist 2003). Recently, Bird et al (2020) used the highresolution cosmological zoom-in simulation h277 from Christensen et al (2012) to closely reproduce the measured solarneighbourhood AVDR of Casagrande et al (2011). They attributed this success mainly to the simulation's ability to form stars in dense and cold environment (n > 100 cm −3 and T < 1000 K, where n and T are number density and temperature, respectively), similar to those in giant molecular clouds.…”