The Dual Origin of Stellar Halos

Abstract

We investigate the formation of the stellar halos of four simulated disk galaxies using high resolution, cosmological SPH + N-Body simulations. These simulations include a self-consistent treatment of all the major physical processes involved in galaxy formation. The simulated galaxies presented here each have a total mass of ∼ 10 12 M ⊙ , but span a range of merger histories. These simulations allow us to study the competing importance of in-situ star formation (stars formed in the primary galaxy) and accretion of stars from subhalos in the building of stellar halos in a ΛCDM universe. All four simulated galaxies are surrounded by a stellar halo, whose inner regions (r< 20 kpc) contain both accreted stars, and an in-situ stellar population. The outer regions of the galaxies' halos were assembled through pure accretion and disruption of satellites. Most of the in-situ halo stars formed at high redshift out of smoothly accreted cold gas in the inner 1 kpc of the galaxies' potential wells, possibly as part of their primordial disks. These stars were displaced from their central locations into the halos through a succession of major mergers. We find that the two galaxies with recently quiescent merger histories have a higher fraction of in-situ stars (∼ 20 − 50%) in their inner halos than the two galaxies with many recent mergers (∼ 5 − 10% in-situ fraction). Observational studies concentrating on stellar populations in the inner halo of the Milky Way will be the most affected by the presence of in-situ stars with halo kinematics, as we find that their existence in the inner few tens of kpc is a generic feature of galaxy formation.

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

“…This confirms the notion that the Galactic halo was formed and assembled very early on as suggested by both theory (Robertson et al 2005;Font et al 2006;Zolotov et al 2009Zolotov et al , 2010Font et al 2011) and observations (e.g. Schuster et al 2012).…”

“…Schuster et al (2012) attempted to answer this question by finding that the α-poor stars in their sample are ∼ 2-3 Gyr younger than the α-rich population. This is in favor of the models of Zolotov et al (2009Zolotov et al ( , 2010). Yet, the recent simulations of Font et al (2011) found that in situ stars can be as much as 3-4 Gyr younger than the accreted population.…”

The age of the Milky Way inner halo

“…The latest SPH + N--body simulations of the formation of stellar halos are also finding evidence for a dual origin to the halo. In addition to an outer halo that is dominated by accreted stars from satellite disruption, the inner few tens of kiloparsecs of galaxy halos can contain 50% stars that formed in--situ 26,27 . The local white dwarf population in this study is akin to this latter population, and the derived age measurement of 11.4 +/-- 0.7 billion years agrees very well with the prediction that 70% of the in--situ population in galaxy formation simulations formed by a redshift of 3 (i.e., 11.5 billion years ago) 26 .…”

Building blocks of the Milky Way's accreted spheroid

“…We investigate when they merged and how they relate to the surviving satellite population. In the past decade several efforts have already focussed on the build-up of Milky Way stellar haloes and/or their chemical evolution, either using hydrodynamical simulations or with semi-analytic techniques (e.g., Salvadori et al 2007;Tumlinson 2006Tumlinson , 2010Zolotov et al 2009;Cooper et al 2010;Font et al 2011;Tissera et al 2013Tissera et al , 2014Cooper et al 2015;Pillepich et al 2014;Lowing et al 2015;Pillepich et al 2015). A specific focus on chemical evolution has been provided by Robertson et al (2005); Font et al (2006a,b), using the hybrid semi-analytic plus N-body approach of .…”