Reignition of Star Formation in Dwarf Galaxies
  The Local Group hosts a number of star-forming dwarf galaxies that show
evidence of periods of little to no star formation. We use a suite of
cosmological simulations to study how star formation is reignited in such
galaxies. We focus on isolated galaxies at $z=0$ with halo masses between
9.2$\times$10$^8$ M$_\odot$ and 8.4$\times$10$^9$ M$_\odot$, where star
formation is typically shut off by reionization or by supernova feedback.
Nearly 20% of these simulated galaxies later restart star formation, due to
interactions with streams of gas in the intergalactic medium, indicating that
this mechanism is relatively common in this mass range and that many isolated
dwarfs at $z=0$ may not have been isolated throughout their histories. The
source of this gas is not necessarily cosmic filaments. Rather, the dwarfs
interact with gas thrown off by nearby galaxy mergers or streams extending from
neighboring galaxies. While high ram pressure interactions of this nature lead
to stripping, the encounters that reignite star formation are low density
and/or low velocity and thus low ram pressure, resulting in compression of the
hot gas in the halos of our dwarfs. The gas mass bound up in hot halos can be
substantial -- at least an order of magnitude greater than the mass contained
in HI. Consequently, we find that dwarfs that have experienced reignition tend
to be more HI-rich and have a higher M$_{HI}$/M$_{*}$ ratio at $z=0$ than
galaxies with continuous star formation. Using this fact, we identify galaxies
in the Local Volume that might have "gappy" star formation histories, and can
be studied by the Hubble Space Telescope or the James Webb Space Telescope.
