Chemodynamical evolution of the Milky Way disk

Abstract

In the first paper of this series, we present a new approach for studying the chemo-dynamical evolution in disk galaxies, which consists of fusing disk chemical evolution models with compatible numerical simulations of galactic disks. This method avoids known star formation and chemical enrichment problems encountered in simulations. Here we focus on the Milky Way, by using a detailed thindisk chemical evolution model (matching local observables, which are weakly affected by radial migration) and a simulation in the cosmological context, with dynamical properties close to those of our Galaxy. We examine in detail the interplay between in situ chemical enrichment and radial migration and their impact on key observables in the solar neighborhood, e.g., the age-metallicityvelocity relation, the metallicity distribution, and gradients in the radial and vertical directions. We show that, due to radial migration from mergers at high redshift and the central bar at later times, a sizable fraction of old metal-poor high-[α/Fe] stars can reach the solar vicinity. This naturally accounts for a number of recent observations related to both the thin and thick disks, despite the fact that we use thin-disk chemistry only. Although significant radial mixing is present, the slope in the age-metallicity relation is only weakly affected, with a scatter compatible with recent observational work. While we find a smooth density distribution in the [O/Fe]-[Fe/H] plane, we can recover the observed discontinuity by selecting particles according to kinematic criteria used in high-resolution samples to define the thin and thick disks. We outline a new method for estimating the birth place of the Sun and predict that the most likely radius lies in the range 4.4 < r < 7.7 kpc (for a current location at r = 8 kpc). A new, unifying model for the Milky Way thick disk is offered, where both mergers and radial migration play a role at different stages of the disk evolution. We show that in the absence of early-on massive mergers the vertical velocity dispersion of the oldest stars is underestimated by a factor of ∼2 compared with observations. We can, therefore, argue that the Milky Way thick disk is unlikely to have been formed through a quiescent disk evolution. An observational test involving both chemical and kinematic information must be devised to ascertain this possibility.

Radial migration of the Sun in the Milky Way: a statistical study

“…Based on this result we use dm = 1.7 kpc in the discussions of the results below. Considering changes in the Sun's radial distance larger than 1.7 kpc as significant migration is consistent with the estimates of the Sun's migration made by Wielen (1996) and Minchev et al (2013).…”

“…Other studies also support the idea that the Sun has migrated from its birth place. Based on chemo-dynamical simulations of Galactic disks, Minchev et al (2013) found that the most likely region in which the Sun was born is between 4.4 and 7.7 kpc from the Galactic centre.…”

The evolution of the Milky Way: new insights from open clusters

“…The radial metallicity gradient traced by the field stars and Cepheids lying close to the midplane (| z| < 0.5 kpc) are steeper while that measured for field stars located away from the midplane (0.5 < | z| < 2.0 kpc) are shallower over the entire radial extent of the disc and are comparable to similar gradients measured for the sample of OCs. The chemodynamical models of Minchev et al (2013) produce a steep gradient for samples of stars younger than 2 Gyr and a flat trend for stars older than 2 Gyr throughout the radial range 5 to 16 kpc of the disc. In their simulations, a signature of flattening of the radial gradient arise naturally due to the radial mixing of stars but the young population is hardly affected as the radial mixing operates at a slow pace.…”

“…They also found a signature of flattening of the radial gradient due to radial migration induced by the strong interation of stars older than 2 Gyr with the bar, but the young population is hardly affected out to Rgc < 12 kpc, where some flattening occurs (see figure 5 in Minchev et al 2013). As a result, the initial and the present-day gradient derived for the younger populations are very similar out to 12 kpc.…”

“…Therefore, measurement of the chemical abundance distribution in the disc (i.e. the radial abundance gradient) and the gradient's temporal variation over the disc's lifetime present strong observational constraints on the theoretical models of Galactic chemical evolution (GCE; Magrini et al 2009;Minchev et al 2013;Kubryk et al 2015).…”