Structure and Evolution of the Milky Way

Abstract

This review discusses the structure and evolution of the Milky Way, in the context of opportunities provided by asteroseismology of red giants. The review is structured according to the main Galactic components: the thin disk, thick disk, stellar halo, and the Galactic bar/bulge. The review concludes with an overview of Galactic archaeology and chemical tagging, and a brief account of the upcoming HERMES survey with the AAT. The Thin Disk: Formation and EvolutionHere are some of the issues related to the formation and evolution of the Galactic thin disk:• Building the thin disk: its exponential radial structure, and the role of mergers.• The star formation history: chemical evolution and continued gas accretion.• Evolutionary processes in the disk: disk heating, radial mixing.• The outer disk: chemical properties and chemical gradients.Many of the basic observational constraints on the properties of the Galactic disk are still uncertain. At this time, we do not have reliable information about the star formation history of the disk. We do not know how the metallicity distribution and the stellar velocity dispersions in the disk have evolved with time. One might have expected that these observational questions were well understood by now, but this is not yet so. The basic observational problem is the difficulty of measuring ages for individual stars.The younger stars of the Galactic disk show a clear abundance gradient of about 0.07 dex kpc −1 , outlined nicely by the cepheids (Luck et al., 2006). In the outer disk, for the older stars, the abundance gradient appears to be even stronger: the

Origin of the metallicity distribution in the thick disc

“…- Abadi et al (2003b) postulate that the thick disc formed from the direct accretion of debris from a now-disrupted SMC-mass satellite; such a satellite mass is required to give the correct stellar metallicities (Freeman 2012). - Quinn & Binney (1992), Kazantzidis et al (2008), Villalobos & Helmi (2008) and Qu et al (2011) favour a scenario in which the thick disc originated from kinematic heating of a pre-existing thin disc.…”

The kinematics of the white dwarf population from the SDSS DR12

“…This strongly suggests that the thin and thick disc have a different physical origin. Moreover, most of the thick Galactic disc population is kinematically hotter than that of the thin disc (Freeman 2012). However, the strong gravities of white dwarfs do not allow to use the metallicity to classify white dwarfs, because all metals are diffused inwards in short timescales, resulting in atmospheres made of pristine hydrogen.…”

Asteroseismology of Red Giants and Galactic Archaeology

“…If this is indeed happening the bulge formation takes 2-3 Gyrs to act after the disk has been formed. Hence in this scenario the bulge structure is younger than the bulge stars that were originally part of the inner disk (Freeman, 2012).…”

“…However, there is much discussion about the existence of such relations (e.g. Freeman, 2012;Bergemann et al, 2014;Kumamoto et al, 2017) and it may be that such relations can only be measured for young stars in the solar neighbourhood.…”