Unweaving the tangled web : using BHB stars to trace tidal streams in the Galactic halo

Abstract

Mapping and analyzing substructure in the halo of the Milky Way is our primary way of investigating the morphology and composition of the Milky Way's dark matter halo, the Galaxy's accretion history, and ultimately, models of galaxy formation. We search for structure in blue horizontal branch (BHB) stars in the Sloan Digital Sky Survey (SDSS) crossmatched with the \textit{Gaia} Survey. Due to their intrinsic brightness and narrow range of luminosities, these stars are vital for exploring gravitationally-disrupted tidal streams in both the near and distant halo.

We also develop a new algorithm to search for tidal debris both orthogonal and parallel to our line of sight. This algorithm exploits the characteristic shape of an orbit's track when plotted in line-of-sight velocity vs. heliocentric distance along with the fact that tidal debris tends to build up at apogalacticon. When we fit parabolas to these line-of-sight velocity tracks, we are able to create an algorithm that ``moves'' stars to their largest heliocentric distance (which is usually near their apogalacticon) and search for buildups of stars there by comparing against a synthetic halo background. We present a number of potential structures found by this algorithm and focus on the three most signficiant. One of these is a detection of the Hercules-Corona Borealis Stream which has an orbit fit to it for the first time. This orbit's optimized starting point is $(\mathbf{X},\mathbf{V}) = (7.2,28.5,37.0~\text{kpc};85,-104,23~\text{km}~\text{s}^{-1})$ in a Galactocentric Cartesian frame. The second is a tentative detection of a small, previously-undiscovered stream with a starting point of $(\mathbf{X},\mathbf{V}) = (9.2,14.4,22.3~\text{kpc};89,-173,-61~\text{km}~\text{s}^{-1})$. The third is a complex structure close in to the Galactic Center which we argue is potentially associated with other substructure recently discovered there.

The region of the Hyllus Stream has several interesting velocity structures in it. We fit an orbit to a statistically significant excess in line-of-sight velocity from 14 to 17 kpc. This orbit has an optimized starting point of $(\mathbf{X},\mathbf{V}) = (2.5,7.3,10.9~\text{kpc};60,101,168)~\text{km}~\text{s}^{-1})$ in a Galactocentric Cartesian frame. This orbit is highly radial in nature and is likely connected to a second group of stars that appear as an excess in $\mu_\delta$ and as a distinct bimodal structure in line-of-sight velocity. Using $N$-body simulations and additional orbit fits, we suggest that what was originally identified as the Hyllus Stream is in fact either one large radial structure or possibly two independent structures: one a highly radial stream and the other possibly a detection of some portions of the Virgo Radial Merger.

This thesis has two major goals. Our first is to improve our understanding of already-discovered tidal streams by using velocity data from SDSS DR10 and \textit{Gaia} DR2 to separate them kinematically, fit orbits, and characterize their progenitors. The second goal is to develop a new way to search for tidal streams. Much of the lower-hanging fruit has been found, as prominent streams have been discovered as overdensities in magitude/distance or have had their tails directly observed. However, the smaller-scale structure of the halo and the possibility of many smaller streams leaving their impact on its stars has not been explored as deeply. We thus need a new method of separating out tidal structure that is sensitive to more tenuous or smaller streams.

To progress our understanding of known structure, we identify kinematic moving groups in the vicinity of the Hermus Stream, the Hyllus Stream, and the Hercules-Corona Borealis Stream. We fit orbits to these streams and in the case of the Hermus Stream, attempt to characterize its progenitor. We find that the Hermus Stream is best fit by an orbit starting at $(\mathbf{X},\mathbf{V}) = (-2.3,5.7,8.7~\text{kpc};-123,155,59~\text{km}~\text{s}^{-1})$ in a Galactocentric Cartesian frame and its morphology as measured by its angular and distance dispersions are best matched by a $10^6~M_{\odot}$ progenitor with a scale radius of 50 pc that fell into the Milky Way about 4 Gyr ago.