The Origin of the 300 km s−1 Stream near Segue 1.

We present a search for new members of the 300 km s−1 stream (300S) near the dwarf galaxy Segue 1 using wide-field survey data. We identify 11 previously unknown bright stream members in the APOGEE-2 and SEGUE-1 and 2 spectroscopic surveys. Based on the spatial distribution of the high-velocity stars, we confirm for the first time that this kinematic structure is associated with a 24°-long stream seen in SDSS and Pan-STARRS imaging data. The 300S stars display a metallicity range of −2.17 < [Fe/H] < −1.24, with an intrinsic dispersion of dex. They also have chemical abundance patterns similar to those of Local Group dwarf galaxies, as well as that of the Milky Way halo. Using the open-source code galpy to model the orbit of the stream, we find that the progenitor of the stream passed perigalacticon about 70 Myr ago, with a closest approach to the Galactic center of about 4.1 kpc. Using Pan-STARRS DR1 data, we obtain an integrated stream luminosity of 4 × 103L⊙. We conclude that the progenitor of the stream was a dwarf galaxy that is probably similar to the satellites that were accreted to build the present-day Milky Way halo. [ABSTRACT FROM AUTHOR]