Life in the Outer Limits: Insights on Hierarchical Assembly from Stellar Halos in the Local Universe

Abstract

ivOwing to their large dynamical timescales, the stellar haloes of Milky Way (MW) sized galaxies represent ideal environments to test modern theories of galaxy formation within the Λ-CDM paradigm. Only in stellar haloes can the remnants of hierarchical accretion both be preserved over long timescales as in-tact dwarf satellites or as tidal debris and be easily distinguished from the underlying smooth structure. Stellar haloes, however, remain some of the most difficult galactic structures to study due to their large angular extent (to R virial ) and extremely low surface brightness (∼> 30 mag arcsec 2 ). Thus, the basic properties of stellar haloes -the overall stellar distribution, substructure fraction, global kinematics and detailed stellar contentremain relatively unconstrained.First, we revisit the development of the modern theory of stellar evolutionin particular, the visualization of this evolution from the Hertzsprung-Russell dia- allowed the SPLASH project to identify bone fide M 31 halo member stars at 32 mag arcsec −2 , which are buried beneath a Galactic foreground of many times higher surface brightness. This thesis presents a campaign that has resulted in the largest sample of stellar halo member stars beyond R proj ∼ 120 kpc (∼1/3 R virial ) in any galaxy. We utilize the large angular extent of these member stars on the sky (R > 8 • )to measure the transverse motion of the M 31 system, using the variable projection of this motion onto its member stars at large angular separations. The technique is found to be comparable in precision to other statistical and direct proper motion studies and this effort is the first step to a global kinematical modelling of the halo using this unique kinematic sample.Third, we use deep imaging to 29 mag arcsec −2 to reveal merger debris NGC 5387, an edge-on disk disk galaxy similar to the Milky Way. The debris are found to be consistent with the merger of a dwarf satellite galaxy at a 1:10 mass ratio. The progenitor of the tidal debris is identified as a relatively compact (∼1 kpc) region undergoing extreme star formation at a rate of 2 M ⊙ year −1 , consistent with a massive star density of 1200 stars kpc −1 -making it similar to the conditions observed in super star clusters typically identified in more massive mergers. Despite showing chemical properties consistent with a galaxy of its original mass, we show that this galaxy is unique among samples of nearby dwarf galaxies regardless of environment, both in its integrated colors and in its star forming properties. From comparison to detailed numerical modelling, we infer the extreme star formation was induced by a recent passage through the NGC 5387 disk and that the satellite remnant could continue forming stars through an additional disk passage. We show that were this galaxy undisturbed, i.e., had its stellar halo not been stripped, it would have colors vi consistent with those objects in the "green valley" for dwarf satellites, suggesting opportunities to probe extreme star formation environments nee...