Orbital pericenters and the inferred dark matter halo structure of satellite galaxies

Abstract

Using the phat-ELVIS suite of Milky Way-size halo simulations, we show that subhalo orbital pericenters, $r_{\rm peri}$, correlate with their dark matter halo structural properties. Specifically, at fixed maximum circular velocity, $V_{\rm max}$, subhalos with smaller $r_{\rm peri}$ are more concentrated (have smaller $r_{\rm max}$ values) and have lost more mass, with larger peak circular velocities, $V_{\rm peak}$, prior to infall. These trends provide information that can tighten constraints on the inferred $V_{\rm max}$ and $V_{\rm peak}$ values for known Milky Way satellites. We illustrate this using published pericenter estimates enabled by Gaia for the nine classical Milky Way dwarf spheroidal satellites. The two densest dSph satellites (Draco and Ursa Minor) have relatively small pericenters, and this pushes their inferred $r_{\rm max}$ and $V_{\rm max}$ values lower than they would have been without pericenter information. For Draco, we infer $V_{\rm max} = 23.5 \, \pm 3.3$ km s$^{-1}$ (compared to $27.3 \, \pm 7.1$ km s$^{-1}$ without pericenter information). Such a shift exacerbates the traditional Too Big to Fail problem. Draco's peak circular velocity range prior to infall narrows from $V_{\rm peak} = 21 - 49$ km s$^{-1}$ without pericenter information to $V_{\rm peak} = 25-37$ km s$^{-1}$ with the constraint. Over the full population of classical dwarf spheroidals, we find no correlation between $V_{\rm peak}$ and stellar mass today, indicative of a high level of stochasticity in galaxy formation at stellar masses below $\sim 10^7$ M$_\odot$. As proper motion measurements for dwarf satellites become more precise, they should enable useful priors on the expected structure and evolution of their host dark matter subhalos.