Estimate of the Mass and Radial Profile of the Orphan Stream's Dwarf Galaxy Progenitor Using MilkyWay@home

We fit the mass and radial profile of the Orphan Stream's dwarf galaxy progenitor using turnoff stars the Sloan Digital Sky Survey (SDSS) and the Dark Energy Camera (DEC), using N-body simulations of the Orphan Stream progenitor falling into the Milky Way on the 1.5 PetaFLOPS MilkyWay@home distributed supercomputer. We optimize the evolution time, the baryonic scale radius, the dark matter radius, the baryonic mass, and the dark matter mass of the progenitor dwarf galaxy using a differential evolution algorithm. The likelihood score for each set of parameters is determined by comparing the resulting tidal stream to the stellar data we observe in the sky. Our optimizations fix the Milky Way potential, the Orphan Stream's orbit, and the form of the radial profile of the progenitor dwarf galaxy. Under these assumptions, we measure the Orphan Stream progenitor's baryonic Plummer scale radius to be 0.161 ± 0.001 kpc and its dark matter Plummer scale radius to be 0.578 ± 0.004 kpc. We also find the best fit total mass of the Orphan Stream's original progenitor to be (6.0 ± 0.1) × 10⁶ M_⊙ with a mass-to-light ratio of γ = 19.4 ± 0.4 and (7.7 ± 0.1) × 10⁵ M_⊙ within 300 pc of its center. Within the original progenitor's half-light radius, we calculate a total mass of (2.20 ± 0.04) × 10⁵ M_⊙. This is on the low end of previous measurements, and if confirmed lowers the mass range of ultrafaint dwarf galaxies. We also deduce the progenitor started undergoing tidal disruption ~3.6 Gyr ago. The systematic errors introduced by our assumptions will be explored in a future paper.