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

TL;DR

This study uses the TNG50 simulation to show that ionized envelopes around high-velocity clouds significantly contribute to galaxy star formation, with ionized gas mass being much greater than neutral gas.

Contribution

It reveals that ionized gas envelopes are prevalent around HVCs and dominate accretion, a novel insight into galaxy fueling mechanisms.

Findings

Ionized gas envelopes are prolate in 73% of HVCs.

Ionized mass is approximately six times the neutral mass.

Ionized accretion can account for up to 81% of star formation rate.

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.