Milky Way Mass Through Escape Velocity Curve from LAMOST K Giants

Abstract

Escape velocity has long been used to constrain the mass of the Dark Matter (DM) halo in the Milky Way (MW). Here we present a study of the escape velocity curve using a sample of high-velocity K giants with full 6D phase-space information and relatively good quality, selected from LAMOST DR8 and cross-matched with Gaia DR3. To expand the high-velocity stars to larger distances, we used radius-dependent criteria of total velocity, that is, $v_{\mathrm{GC}}>300\mathrm{km}{\mathrm{s}}^{-1}$ for the solar neighborhood; $v_{\mathrm{GC}}>v_{\min }\sim 0.6\times v_{\mathrm{esc}}(r_{\mathrm{GC}})$ for outer region. We also selected halo stars based on $v_{\ddot{I}}•-[Fe/H]$ information to ensure that the sample is isotropic. We modeled the velocity distribution with traditional power-law models to determine the escape velocity in each radial bin. For the first time, we have directly measured a relatively continuous escape velocity curve that can extend to Galactocentric radii of $\sim 50$ kpc, finding a decline in agreement with previous studies. The escape velocity at the solar position yielded by our measurements is $523.74_{-13.47}^{+12.83}\mathrm{km}{\mathrm{s}}^{-1}$. Combined with the local circular velocity, we estimated the mass of the MW assuming a Navarro-Frenk-White DM profile, which resulted in a total mass of $M_{200,\mathrm{total}}=0.90_{-0.07}^{+0.06}\times 10^{12}{M}_{\odot }$, with a concentration of $c_{200}=13.47_{-1.70}^{+1.85}$. The small uncertainty implies that including the escape velocities beyond the solar neighborhood can result in a more precise mass estimate. Our derived MW mass is consistent with some recent studies using the escape velocity as well as other tracers, which may support a lower mass of the DM halo than in the past.

13 pages, 8 figures, published in ApJ