Dynamics and Detection of Tidal Debris
Creators: Hendel, David Anthony
Publisher: name: Columbia University
Year: 2018
Subjects: Astronomy
Astrophysics
Galaxies--Formation
Space debris

description: Tidal debris structures are striking evidence of hierarchical assembly -- the premise that the Milky Way and galaxies like it have been built over cosmic time through the coalescence of many smaller objects. In the prevailing Lambda -- Cold Dark Matter cosmology, the vast majority of mergers by number are minor; one dark matter halo, hosting a larger galaxy, dominates the interaction and a smaller object, the satellite, is stripped of mass by tidal forces. When the luminous component of the satellite is disrupted the debris may form structures such as stellar tidal streams or shells, depending on the parameters of the interaction. In this Thesis we examine the properties of this debris left behind by minor mergers theoretically, computationally, and observationally, making strides towards a more complete understanding of what tidal debris can tell us about the history of galaxy formation in the Universe.
      Around the Milky Way itself we have examined the properties of the Orphan Stream, a stellar tidal stream so named due to uncertainty about the position and current state of its progenitor. Using 3.6 um observations taken as part of the Spitzer Merger History and Shape of the Galactic Halo program, the latest period--luminosity--metallicity relations, and archival data, we compute precise distances to RR Lyrae stream members with state--of--the--art 2.5% relative uncertainties. Fitting an orbit to the data, we measure an enclosed mass for the Milky Way that is in good agreement with other recent results, once the biases in orbit fitting are taken into account. By applying the same technique to N--body simulations we determined that the Orphan progenitor is most likely similar to the classical dwarf spheroidal satellites. 
      We also examined tidal debris more generally, in particular by investigating the source of the morphological dichotomy between shells and streams. We find that the transition from a stream--like to a shell--like morphology occurs when the differential azimuthal precession between the orbits of stars exceeds the position angle subtended by individual petals of the progenitor orbit's rosette. This statement is cast more precisely in terms of scaling relations that control the dispersion of energy and angular momentum in the debris, and we find that the observed morphology can be predicted for a given host, orbit, and mass ratio. This leads us to the idea that the observed occurrence rates of different morphologies can be used to recover, at the population statistics level, the progenitor satellites' orbital infall distribution. This a part of the cosmological accretion history that is otherwise inaccessible. To achieve this in practice requires an unbiased and automated method to detect and classify substructure; we have developed just such a tool and demonstrate its effectiveness. In the upcoming era of LSST and WFIRST the methods and insights developed in this Thesis will be useful in decoding the information about the current state and assembly of galaxies encoded in tidal debris.