Galactic Stellar Populations in the Era of the Sloan Digital Sky Survey and Other Large Surveys

Abstract

Studies of stellar populations, understood to mean collections of stars with common spatial, 2 Ivezić, Beers & Jurić kinematic, chemical, and/or age distributions, have been reinvigorated during the last decade by the advent of large-area sky surveys such as SDSS, 2MASS, RAVE, and others. We review recent analyses of these data that, together with theoretical and modeling advances, are revolutionizing our understanding of the nature of the Milky Way, and galaxy formation and evolution in general.The formation of galaxies like the Milky Way was long thought to be a steady process leading to a smooth distribution of stars. However, the abundance of substructure in the multi-dimensional space of various observables, such as position, kinematics, and metallicity, is by now proven beyond doubt, and demonstrates the importance of mergers in the growth of galaxies. Unlike smooth models that involve simple components, the new data reviewed here clearly exhibit many irregular structures, such as the Sagittarius dwarf tidal stream and the Virgo and Pisces overdensities in the halo, and the Monoceros stream closer to the Galactic plane. These recent developments have made it clear that the Milky Way is a complex and dynamic structure, one that is still being shaped by the merging of neighboring smaller galaxies. We also briefly discuss the next generation of wide-field sky surveys, such as SkyMapper, Pan-STARRS, Gaia, and LSST, which will improve measurement precision manyfold, and comprise billions of individual stars. The ultimate goal, development of a coherent and detailed story of the assembly and evolutionary history of the Milky Way and other large spirals like it, now appears well within reach.

Kinematics and chemical properties of the Galactic stellar populations

“…Bovy et al (2012a) find that mass-weighted scale-height distribution smoothly varies and that there is no thin-thick disk bi-modality (i.e., the MW has no distinct thick disk). We refer the reader to Ivezić et al (2012) and Rix & Bovy (2013) for more recent reviews of the stellar disk(s) and populations. While considering that the thick disk was formed by one single mechanism is most probably too semplicistic, several different scenarios have been proposed for the formation of thick disk: the heating of a pre-existing old thin disk via minor mergers (e.g., Quinn et al 1993;Villalobos & Helmi 2008), the direct accretion of stars from disrupted satellites (e.g., Abadi et al 2003), gas accretion at high redshift and stars formed in situ (e.g., Brook et al 2005), and radial migration of stellar orbits (e.g., Schönrich & Binney 2009a,b;Loebman et al 2011).…”

Deep SDSS optical spectroscopy of distant halo stars

“…In the half century since metal-poor stars were identified in the halo of the Galaxy (Chamberlain & Aller 1951), a great deal of effort has been devoted to clarifying their origin, detailed chemical abundance patterns, and observed kinematics (Beers & Christlieb 2005;Ivezić et al 2012;Frebel & Norris 2013). One long standing debate was focused on whether the Galaxy was formed from the monolithic collapse of a protogalactic cloud (Eggen et al 1962) or assembled from smaller substructures (Press & Schechter 1974;Searle & Zinn 1978).…”

On the relative ages of the α-rich and α-poor stellar populations in the Galactic halo

“…The works of Carollo et al (2007Carollo et al ( , 2010, for example, have shown observational evidence that there is an outer stellar halo, which is dominant at galactocen-⋆ E-mail: khawkins@ast.cam.ac.uk tric distances larger than 15 kpc, mostly formed from accreted sub-units and an inner stellar halo, confined to galactocentric distances less than 15 kpc, which seems to be rather smooth, with no predominant sub-unit like the outer halo. However, these results are still debated (see discussion in Ivezić, Beers & Jurić 2012;Schönrich, Asplund & Casagrande 2011).…”