HST/COS Observations of Ionized Gas Accretion at the Disk-halo Interface
  of M33
  We report the detection of accreting ionized gas at the disk-halo interface
of the nearby galaxy M33. We analyze HST/COS absorption-line spectra of seven
ultraviolet-bright stars evenly distributed across the disk of M33. We find Si
IV absorption components consistently redshifted relative to the bulk M33's ISM
absorption along all the sightlines. The Si IV detection indicates an enriched,
disk-wide, ionized gas inflow toward the disk. This inflow is most likely
multi-phase as the redshifted components can also be observed in ions with
lower ionization states (e.g., S II, P II, Fe II, Si II). Kinematic modeling of
the inflow is consistent with an accreting layer at the disk-halo interface of
M33, which has an accretion velocity of 110$^{+15}_{-20}$ km s$^{-1}$ at a
distance of 1.5$^{+1.0}_{-1.0}$ kiloparsec above the disk. The modeling
indicates a total mass of $\sim3.9\times10^7$ M$_{\odot}$ for the accreting
material at the disk-halo interface on the near side of the M33 disk , with an
accretion rate of $\sim2.9$ M$_{\odot}$ yr$^{-1}$. The high accretion rate and
the level of metal-enrichment suggest the inflow is likely to be the fall back
of M33 gas from a galactic fountain and/or the gas pulled loosed during a close
interaction between M31 and M33. Our study of M33 is the first to unambiguously
reveal the existence of a disk-wide, ionized gas inflow beyond the Milky Way,
providing a better understanding of gas accretion in the vicinity of a galaxy
disk.
