Invisible Accretion: Ionized Envelopes of TNG50 HVCs can Sustain Star Formation

Abstract: Galactic high-velocity clouds (HVCs) are known to be complex, multiphase systems consisting of neutral and/or ionized gas moving at high velocities relative to the rotation of the disk. In this work, we investigate Milky Way-like galaxies from the TNG50 simulation to characterize the properties, morphology, and accretion rates of the warm and hot ionized material comoving with neutral HVCs visible in HI. We find that the ionized gas forms an envelope around the neutral material, and in most cases (73% of the HVCs) it is prolate in morphology. We also find that the ionized mass is ~6 times greater than the neutral mass, which leads to significantly more accretion possible from the ionized gas ($\dot{M}\mathrm{ion}$) than the neutral gas ($\dot{M}\mathrm{neut}$), consistent with estimates made from observations of our own Galaxy. We investigate the accretion rates from both phases of HVCs around 47 Milky Way-like galaxies and find that $\dot{M}\mathrm{ion}$ scales with $\dot{M}\mathrm{neut}$, and both scale with the star formation rate of the galaxy. Finally, we find that, on average, $\dot{M}\mathrm{ion}$ could account for 81% of the galactic star formation rate (assuming the material can sufficiently cool and condense), while $\dot{M}\mathrm{neut}$ can only balance 11%. Thus, the diffuse, ionized, high-velocity circumgalactic medium plays a defining role in the evolution and growth of galaxies at low redshift.