The Galaxy's Eating Habits

Tidal streams of disrupting dwarf galaxies orbiting around their host galaxy offer a unique way to constrain the shape of galactic gravitational potentials. Such streams can be used as leaning tower gravitational experiments on galactic scales. The most well motivated modification of gravity proposed as an alternative to dark matter on galactic scales is Milgromian dynamics (MOND), and we present here the first ever N-body simulations of the dynamical evolution of the disrupt...

M giants selected from the Two Micron All Sky Survey (2MASS) have been used to trace streams of tidal debris apparently associated with the Sagittarius dwarf spheroidal galaxy (Sgr) that entirely encircle the Galaxy. While the Sgr M giants are generally aligned with a single great circle on the sky, we measure a difference of 10.4 +- 2.6 degrees between the mean orbital poles of the great circles that best fit debris leading and trailing Sgr, which can be attributed to the pr...

We develop statistical methods for identifying star streams in the halo of the Milky Way galaxy that exploit observed spatial and radial velocity distributions. Within a great circle, departures of the observed spatial distribution from random provide a measure of the likelihood of a potential star stream. Comparisons between the radial velocity distribution within a great circle and the radial velocity distribution of the entire sample also measure the statistical significan...

Wrapping around the Milky Way, the Sagittarius stream is the dominant substructure in the halo. Our statistical selection method has allowed us to identify 106 highly likely members of the Sagittarius stream. Spectroscopic analysis of metallicity and kinematics of all members provides us with a new mapping of the Sagittarius stream. We find correspondence between the velocity distribution of stream stars and those computed for a triaxial model of the Milky Way dark matter hal...

The extensive span of the Sagittarius (Sgr) stream makes it a promising tool for studying the Milky Way gravitational potential. Characterizing its stellar kinematics can constrain halo properties and provide a benchmark for the Cold Dark Matter galaxy formation paradigm. Accurate models of the disruption dynamics of the Sgr progenitor are necessary to employ this tool. Using a combination of analytic modeling and N-body simulations, we build a new model of the Sgr orbit and ...

In this work, we study the phase-space and chemical properties of Sagittarius (Sgr) stream, the tidal tails produced by the ongoing destruction of Sgr dwarf spheroidal (dSph) galaxy, focusing on its very metal-poor (VMP; $\rm[Fe/H] < -2$) content. We combine spectroscopic and astrometric information from SEGUE and $Gaia$ EDR3, respectively, with data products from a new large-scale run of $\texttt{StarHorse}$ spectro-photometric code. Our selection criteria yields ${\sim}1600...

It is now generally believed that galaxies were built up through gravitational amplification of primordial fluctuations and the subsequent merging of smaller precursor structures. The stars of the structures that assembled to form the Milky Way should now make up much or all of its bulge and halo, in which case one hopes to find "fossil" evidence for those precursor structures in the present distribution of halo stars. Confirmation that this process is continuing came with th...

Numerical models of the tidal disruption of the Sagittarius (Sgr) dwarf galaxy have recently been developed that for the first time simultaneously satisfy most observational constraints on the angular position, distance, and radial velocity trends of both leading and trailing tidal streams emanating from the dwarf. We use these dynamical models in combination with extant 3-D position and velocity data for Galactic globular clusters and dSph galaxies to identify those Milky Wa...

We have assembled a large-area spectroscopic survey of giant stars in the Sagittarius (Sgr) dwarf galaxy core. Using medium resolution (R ~15,000), multifiber spectroscopy we have measured velocities of these stars, which extend up to 12 degrees from the galaxy's center (3.7 core radii or 0.4 times the King limiting radius). From these high quality spectra we identify 1310 Sgr members out of 2296 stars surveyed distributed across 24 different fields across the Sgr core. Addit...

We present two simple dynamical models for Sagittarius based on N-body simulations of the progressive disruption of a satellite galaxy orbiting for 12.5 Gyr within a realistic Galactic potential. In both models the satellite initially has observable properties similar to those of current outlying dwarfs; in one case it is purely stellar while in the other it is embedded in an extended massive halo. The purely stellar progenitor is a King model with a total velocity dispersion...