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Modeling Milky Way History

Periodic Report Summary 1 - MMWH (Modeling Milky Way History)

This project aims to develop theoretical tools to aid in our understanding of the history of the Milky Way Galaxy. In particular, the project focuses on the theory of radial migration in galactic disks. The process of radial migration in disk galaxies has in recent years gained much attention, as it has potentially deep implications for the information conveyed in currently-observed stellar populations. If radial migration is efficient, stars from all parts of the disk may end up in the same place in the galaxy today. The project also aims to develop methodologies for comparing the simulated results with observational data from upcoming surveys such as Gaia.

To this end, we have undertaken an ambitious project, in which we focused on extending our deconstruction of the radial mixing process in our previous simulations, as well as created a new suite of simulations extending our previous work. In 2012, we published a comprehensive paper detailing the dynamics of this process, which extended our previous work on the impact of radial migration on disk stellar populations. The main finding was that migration in our models was due to the exchange of angular momentum and energy through the corotation resonance with spirals.

In addition to migrating radially, much attention in the literature in recent years has been devoted to the idea that stars moving radially from the galactic interior may end up in the thicker component of the disk. This is an important consideration because the stars far away from the plane are assumed to be associated with some past event from the early epochs of disk formation. For this reason we focused specifically on the vertical evolution of migrated stars and found that indeed radial migration causes individual stellar populations to change their vertical distribution as a consequence of the migration.

However, many other effects such as interactions with substructure can influence the vertical distribution of disks. We are therefore currently investigating whether mergers can be disentangled from internal evolution using a new set of models extending our previous modeling approach. This has resulted in an entirely new suite of simulations. Finally, we carried out an extended study of galaxy formation in the cosmological context, focusing on the problem of excessive baryon retention in Milky Way-sized halos. We found that when amount of feedback was sufficiently high to satisfy various observational constraints, our disk morphologies were severely affected to the point of essentially destroying any semblance of thin disks. This work highlighted the need to develop our understanding of feedback processes further before realistic galaxies can be formed in the cosmological context.

One of the early triumphs of this project was the successful organization of the first-ever international workshop dealing specifically with the process of radial mixing that took place in 2012. Considerable time has also been invested in developing a secondary goal of the project, which is methods of comparison between simulations and observations.