Antonela Monachesi (Universidad de La Serena)

How to decipher the accretion history of galaxies: Insights from stellar halos, bulges and brightest stellar streams

Large luminous galaxies like the Milky Way grow in mass hierarchically through accretion and merging.  Diffuse stellar halos around galaxies are formed primarily thanks to this process, and thus encode unique information about the merger history of galaxies. In addition, stellar halos may probe early chaotic phases of galaxy formation through possible in-situ stars formed in turbulent gas at early times and may also contain stars kicked out of galactic disks by interactions with satellite galaxies. Due to the extreme surface brightness of stellar halos in galaxies like the Milky Way ( muV > 28 mag/arcsec-2),  this component is hard to detect observationally. However, during the last decade, and thanks to exquisite observations, significant progress has been made to resolve the stellar halos of nearby galaxies.  These halos show a large diversity in their properties for Milky Way-like galaxies that are alike in terms of total luminosity and stellar mass; and understanding this diversity is crucial to gain insights on their accretion history.  In this seminar I will first present the properties that we find on the stellar halos of nearby Milky Way-like galaxies, mainly obtained from HST and Subaru observations. I will then present the results from the stellar halos of the Auriga simulations, a suite of thirty cosmological magneto-hydrodynamical zoom-in simulations of Milky Way-mass galaxies. The Auriga simulations represent one of the largest and highest resolution samples of simulated Milky Way-mass galaxies with which it is possible to investigate in detail the properties and origin of individual stellar halos. I will compare the results from the Auriga simulations with those obtained from observations of nearby galaxies and discuss observational signatures and properties of the stellar halos, brightest stellar streams and bulges that allow us to decode the accretion and merger history of observed galaxies.