Fitting the Density Substructure of the Stellar Halo with MilkyWay@home

We propose and test a method for applying statistical photometric parallax to main-sequence turnoff stars in the Sloan Digital Sky Survey (SDSS). Using simulated data, we show that if our density model is similar to the actual density distribution of our data, we can reliably determine the density model parameters of three major substructures in the Milky Way halo using the computational resources available on MilkyWay@home (a 20-parameter fit). We fit the stellar density in SDSS stripe 19 with a smooth stellar spheroid component and three major streams. One of these streams is consistent with the Sagittarius tidal stream at 21.1 kpc away, one is consistent with the trailing tail of the Sagittarius tidal stream in the north Galactic cap at 48 kpc away, and one is possibly part of the Virgo Overdensity at 6 kpc away. We find the 1σ widths of these three streams to be 1.0 kpc, 17.6 kpc, and 6.1 kpc, respectively. The width of the trailing tail is extremely wide (41 kpc full width at half maximum). This large width could have implications for the shape of the Milky Way dark matter halo. The width of the Virgo Overdensity-like structure is consistent with what we might expect for a “cloudlike” structure; analysis of additional stripes of data are needed to outline the full extent of this structure and confirm its association with the Virgo Overdensity.