2025ApJ...990..118L

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 H I. 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 being possible from the ionized gas (${\dot{M}}_{{\rm{ion}}}$) than the neutral gas (${\dot{M}}_{{\rm{neut}}}$), 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, finding that ${\dot{M}}_{{\rm{ion}}}$ scales with ${\dot{M}}_{{\rm{neut}}}$ and both scale with the star formation rate of the galaxy. Finally, we find that, on average, ${\dot{M}}_{{\rm{ion}}}$ could account for 81% of the galactic star formation rate (assuming the material can sufficiently cool and condense), while ${\dot{M}}_{{\rm{neut}}}$ 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.