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 .
Much of the recent work to understand this dichotomy has been statistical in nature , determining not just that environmental processes are most likely responsible for quenching these low-mass systems but also that they must operate very quickly after infall onto the host system , with quenching timescales \lesssim 2 ~ { } { Gyr } at { M } _ { \star } \lesssim 10 ^ { 8 } ~ { } { M } _ { \odot } .
This work utilizes the newly-available Gaia DR2 proper motion measurements along with the Phat ELVIS suite of high-resolution , cosmological , zoom-in simulations to study low-mass satellite quenching around the Milky Way on an object-by-object basis .
We derive constraints on the infall times for 37 of the known low-mass satellite galaxies of the Milky Way , finding 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 _ { quench } \sim 6 - 8 ~ { } { 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 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 .