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We report the detection of accreting ionized gas at the disk-halo interfaceof the nearby galaxy M33. We analyze HST/COS absorption-line spectra of sevenultraviolet-bright stars evenly distributed across the disk of M33. We find SiIV absorption components consistently redshifted relative to the bulk M33's ISMabsorption along all the sightlines. The Si IV detection indicates an enriched,disk-wide, ionized gas inflow toward the disk. This inflow is most likelymulti-phase as the redshifted components can also be observed in ions withlower ionization states (e.g., S II, P II, Fe II, Si II). Kinematic modeling ofthe inflow is consistent with an accreting layer at the disk-halo interface ofM33, which has an accretion velocity of 110$^{+15}_{-20}$ km s$^{-1}$ at adistance of 1.5$^{+1.0}_{-1.0}$ kiloparsec above the disk. The modelingindicates a total mass of $\sim3.9\times10^7$ M$_{\odot}$ for the accretingmaterial at the disk-halo interface on the near side of the M33 disk , with anaccretion rate of $\sim2.9$ M$_{\odot}$ yr$^{-1}$. The high accretion rate andthe level of metal-enrichment suggest the inflow is likely to be the fall backof M33 gas from a galactic fountain and/or the gas pulled loosed during a closeinteraction between M31 and M33. Our study of M33 is the first to unambiguouslyreveal 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 galaxydisk.

HST/COS Observations of Ionized Gas Accretion at the Disk-halo Interface of M33