The outermost edges of the Milky Way halo from galaxy kinematics

Abstract

We measure for the first time the outermost edges of the Milky Way (MW) halo in terms of the depletion and turnaround radii. The inner depletion radius, $r_\mathrm{id}$, identified at the location of maximum infall velocity, separates a growing halo from the draining environment, while the turnaround radius, $r_\mathrm{ta}$, marks the outermost edge of infalling material towards the halo, both of which are located well outside the virial radius. Using the motions of nearby dwarf galaxies within $3\mathrm{Mpc}$, we obtain a marginal detection of the infall zone around the MW with a maximum velocity of $v_\mathrm{inf, max}=-46_{-39}^{+24}\mathrm{km s^{-1}}$. This enables us to measure $r_\mathrm{id}=559\pm 107 \mathrm{kpc}$ and $r_\mathrm{ta}=839\pm 121 \mathrm{kpc}$. The measured depletion radius is about 1.5 times the MW virial radius ($R_\mathrm{200m}$) measured from internal dynamics. Compared with halos in the cosmological simulation Illustris TNG100, the factor 1.5 is consistent with that of halos with similar masses and dynamical environments to the MW but slightly smaller than typical values of Local Group analogs, potentially indicating the unique evolution history of the MW. These measurements of halo edges directly quantify the ongoing evolution of the MW outer halo and provide constraints on the current dynamical state of the MW that are independent from internal dynamics.