Global HI asymmetries in IllustrisTNG: a diversity of physical processes disturb the cold gas in galaxies

TL;DR

This study uses the IllustrisTNG simulations to analyze the origins of asymmetries in the neutral hydrogen profiles of galaxies, revealing multiple physical processes contribute to these asymmetries beyond environmental effects.

Contribution

It demonstrates that various physical processes, not just environment, cause HI asymmetries in galaxies within cosmological simulations, aligning with observational findings.

Findings

Over 50% of simulated galaxies show at least 10% flux asymmetry.

Satellite galaxies are more asymmetric than centrals, especially in massive groups.

Ram pressure and other physical processes drive HI asymmetries in both central and satellite galaxies.

Abstract

Observations of the cold neutral atomic hydrogen (HI) in and around disc galaxies have revealed that spatial and kinematic asymmetries are commonplace, and are reflected in the global HI spectra. We use the TNG100 box from the IllustrisTNG suite of cosmological simulations to study the conditions under which these asymmetries may arise in current theoretical galaxy formation models. We find that more than 50% of the sample has at least a 10% difference in integrated flux between the high- and low-velocity half of the spectrum, thus the typical TNG100 galaxy has an HI profile that is not fully symmetric. We find that satellite galaxies are a more asymmetric population than centrals, consistent with observational results. Using halo mass as a proxy for environment, this trend appears to be driven by the satellite population within the virial radius of haloes more massive than $10^{13} M_{\odot}$, typical of medium/large groups. We show that, while the excess of HI asymmetry in group satellites is likely driven by ram pressure, the bulk of the asymmetric HI profiles observed in TNG100 are driven by physical processes able to affect both the central and satellite populations. Our results highlight how asymmetries are not driven solely by environment, and multiple physical processes can produce the same asymmetric shape in global HI spectra.