Milky Way archaeology using RR Lyrae and type II Cepheids

Abstract

We report the discovery of high-velocity candidates among RR Lyrae stars found in the Milky Way halo. We identified nine RR Lyrae stars with Galactocentric velocities exceeding the local escape velocity based on the assumed Galaxy potential. Furthermore, based on a close examination of their orbits’, we ruled out their ejection location in the Milky Way disk and bulge. The spatial distribution revealed that seven out of nine pulsators overlap with the position of the Sagittarius stellar stream. Two out of these seven RR Lyrae stars can be tentatively linked to the Sagittarius dwarf spheroidal galaxy on the basis of their orbits. Focusing on the high-velocity tail of the RR Lyrae velocity distribution, we estimated the escape velocity in the Solar neighborhood to be vesc = 512−37+94 km s−1 (4 to 12 kpc); and beyond the Solar neighborhood as vesc = 436−22+44 km s−1 and vesc = 393−26+53 km s−1 (for distances between 12 to 20 kpc and 20 to 28 kpc), respectively. We utilized three escape velocity estimates together with the local circular velocity to estimate the Milky Way mass. The resulting measurement M200 = 0.83−0.16+0.29 × 1012 M⊙ falls on the lower end of the current Milky Way mass estimates, but once corrected for the likely bias in the escape velocity (an increase of approximately 10% in terms of the escape velocity), our mass estimate yields M200 = 1.26−0.22+0.40 × 1012 M⊙, which is in agreement with estimates based on different diagnostics of the Milky Way (MW) mass. The MW mass of within 20 kpc then corresponds to MMW(r < 20 kpc) = 1.9−0.1+0.2 × 1011 M⊙ without any correction for bias, and MMW(r < 20 kpc) = 2.1−0.1+0.2 × 1011 M⊙ corrected for a likely offset in escape velocities.