The mystery of extremely HI rich galaxies

TL;DR

This study investigates extremely HI rich galaxies by analyzing their molecular gas content with ALMA, revealing lower-than-expected molecular gas fractions and high velocity dispersions, which sheds light on their unusual gas properties.

Contribution

The paper provides the first detailed molecular gas analysis of extremely HI rich galaxies, highlighting their atypical gas dynamics and fractions.

Findings

Molecular gas fractions are significantly lower than expected from scaling relations.

The CO gas exhibits relatively high velocity dispersion.

Sample galaxies have regularly rotating molecular gas disks.

Abstract

The properties of galaxies follow scaling relations related to the physics that govern galaxy evolution. Based on these, we can identify galaxies undergoing specific evolutionary processes such as HI-excess galaxies, which have relatively high HI mass compared to their stellar mass. The possible reasons for this could be either recent gas accretion or an inefficient conversion of the HI to molecular gas. Since recent gas accretion is difficult to prove conclusively, we investigated gas conversion by analysing the molecular gas content of five extremely HI rich galaxies from the HIX galaxy sample. For this, we obtained CO observations of the sample galaxies with the Atacama Large Millimeter Array (ALMA). While we find that our sample galaxies have relatively regularly rotating molecular gas disks, their molecular gas fraction is significantly lower than what is expected from scaling relations and the CO gas has relatively high velocity dispersion.