Milky Way archaeology using RR Lyrae and type II Cepheids II. High velocity RR Lyrae stars, and mass of the Milky Way

Abstract

We report the discovery of high velocity candidates among RR~Lyrae stars found in the Milky Way halo. We identified 9 RR~Lyrae stars with Galactocentric velocities exceeding the local escape velocity based on the assumed Galaxy potential. Based on 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 9 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 estimate the escape velocity in the Solar neighborhood to be $v_{\mathrm{esc}}=512_{-37}^{+94}$\,km\,s${}^{-1}$~($4$ to $12$\,kpc), and beyond the Solar neighborhood as $v_{\mathrm{esc}}=436_{-22}^{+44}$\,km\,s${}^{-1}$~and $v_{\mathrm{esc}}=393_{-26}^{+53}$\,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 $M_{200}=0.83_{-0.16}^{+0.29}\cdot 10^{12}$\,M${}_{\odot }$ falls on the lower end of the current Milky Way mass estimates, but once corrected for the likely bias in the escape velocity (approximately $10$ percent increase of the escape velocity), our mass estimate yields $M_{200}=1.26_{-0.22}^{+0.40}\cdot 10^{12}$\,M${}_{\odot }$, which is in agreement with estimates based on different diagnostics of the Milky Way mass. The MW mass within $20$\,kpc then corresponds to $M_{\mathrm{MW}}\left(r<20\text{kpc}\right)=1.9_{-0.1}^{+0.2}\times 10^{11}$\,M${}_{\odot }$ without correction for bias, and $M_{\mathrm{MW}}\left(r<20\text{kpc}\right)=2.1_{-0.1}^{+0.2}\times 10^{11}$\,M${}_{\odot }$ corrected for a likely offset in escape velocities.

Accepted for publication in A&A

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