UC Irvine

Observations of low-mass satellite galaxies in the nearby Universe point towards a strong dichotomy in their star-forming properties relative to systems with similar mass in the field. Specifically, satellite galaxies are preferentially gas-poor and no longer forming stars, while their field counterparts are largely gas-rich and actively forming stars. This dichotomy implies that environmental processes play the dominant role in suppressing star formation within this low-mass regime ($\mstar \sim 10^{5.5-8} \msun$). The high satellite quenched fraction observed in the Local Group demands an extremely short quenching timescale ($\sim2$~Gyr) for dwarf satellites in the mass range $\mstar \sim 10^{6}-10^{8}~\msun$. This quenching timescale is significantly shorter than that required to explain the quenched fraction of more massive satellites ($\sim 8$~Gyr), both in the Local Group and in more massive host halos, suggesting a dramatic change in the dominant satellite quenching mechanism at $\mstar \lesssim 10^{8}~\msun$. This work posits that ram-pressure stripping can naturally explain this behavior, with the critical mass (of $\mstar \sim 10^{8}~\msun$) corresponding to halos with gravitational restoring forces that are too weak to overcome the drag force encountered when moving through an extended, hot circumgalactic medium. Physical models of this mechanism combined with the accretion history of subhalos around a Milky Way-like host suggest that these "environmental" trends can extend well beyond the virial radius of the host, thereby providing an explanation for the population of quenched dwarf galaxies in the nearby Local Volume.

Finally, in a complementary effort, this work also utilizes the newly-available \emph{Gaia} DR2 proper motion measurements along with a suite of high-resolution \emph{N}-body simulations in order to study low-mass satellite quenching around the Milky Way on an object-by-object basis. To this end, we derive constraints on the infall times for $37$ of the known low-mass satellite galaxies of the Milky Way. We find that $\gtrsim~70\%$ of the ``classical'' satellites of the Milky Way are consistent with the very short quenching timescales inferred from the total population in previous works. The remaining classical Milky Way satellites have quenching timescales noticeably longer, with $\tau_{\rm quench} \sim 6 - 8~{\rm Gyr}$, highlighting how detailed orbital modeling is likely necessary to understand the specifics of environmental quenching for individual satellite galaxies. Additionally, we find that the $6$ ultra-faint dwarf galaxies with publicly available {\it HST}-based star formation histories are all consistent with having their star formation shut down prior to infall onto the Milky Way -- which, combined with their very early quenching times, strongly favors quenching driven by reionization. Overall, this work builds and puts forward a consistent picture of environmental quenching across roughly $5$ orders of magnitude in satellite stellar mass in Milky Way-like environments.