Towards the Unbound Stellar Population in the Sloan Digital Sky Survey

Abstract

This thesis uses two complementary techniques to mine the Sloan Digital Sky Survey (SDSS) data to search for stars that are unbound to the Milky Way.

A significant fraction of a galaxy's light may come from a population of unbound stars, and this population can inform us about the dynamical state and stellar content of the galaxy. In galaxy clusters this unbound fraction is known as intracluster light (ICL), identified as a diffuse excess of light in the outskirts of the galaxy. ICL has been observed around distant galaxies and a similar intracluster stellar glow ought to exist around the Milky Way. We developed a technique to identify distant M-giants within SDSS based solely on color. Using this technique, we identified 700 candidate M-giant stars at intragroup distances, beyond 300 kpc, that we call intragroup stars (IGS). These IGS may constitute rare tracers of an underlying ICL population surrounding the Milky Way. One possible explanation for the origin of these IGS is that they were ejected from the center of the Galaxy through interactions with our supermassive black hole as hypervelocity stars (HVSs).

There are only 18 confirmed HVSs and all are young, massive, B-type stars that are thought to be boosted to extreme velocities through interactions with the supermassive black hole at the Galactic Center. We identify candidate HVSs from a sample of SEGUE G- and K-dwarfs. Previous searches for HVSs have only focused on large radial velocities; in these studies we also use proper motions to select the candidates. We determine the hypervelocity likelihood of each candidate via Monte Carlo simulations. Using the observed 6-d positions and velocities, we also calculate the orbits of these candidates in order to determine their place of origin within the Galaxy. We find that nearly half of the candidates exceed their escape velocities with at least 98% probability and no candidate's orbit is consistent with a Galactic Center origin. The lack of HVS candidates originating from the Galactic Center is an indication that either the ejection mechanism is mass-dependent or the initial mass function at the center of the Galaxy is top-heavy.