New evidence for halo gas accretion onto disk galaxies

TL;DR

This paper presents evidence that much of the gas accretion onto disk galaxies is hidden within galactic fountain material, explaining the discrepancy in observed accretion rates and matching halo gas kinematics.

Contribution

It introduces a model where most accretion is concealed as mixing with fountain gas, resolving the missing gas problem and explaining halo gas kinematics.

Findings

Most accretion is hidden as mixing with fountain gas.

The model reproduces the observed vertical rotation gradient.

Provides a new explanation for halo gas accretion evidence.

Abstract

Studies of the halo gas in the Milky Way and in nearby spiral galaxies show the presence of gas complexes that cannot be reconciled with an internal (galactic fountain) origin and are direct evidence of gas accretion. Estimating gas accretion rates from these features consistently gives values, which are one order of magnitude lower than what is needed to feed the star formation. I show that this problem can be overcome if most of the accretion is in fact "hidden" as it mixes with the galactic fountain material coming from the disk. This model not only provides an explanation for the missing gas accretion but also reproduces the peculiar kinematics of the halo gas in particular the vertical rotation gradient. In this view this gradient becomes indirect evidence for gas accretion.