# HI content in the galactic discs: the role of gravitational instability

## TL;DR

This study investigates how hydrogen content in galactic discs relates to rotation speed and size, suggesting gravitational stability influences gas regulation and galaxy evolution, with implications for gas accretion and consumption history.

## Contribution

It demonstrates a close correlation between hydrogen mass and specific angular momentum, proposing gravitational stability as a key regulator of gas content in galaxies.

## Key findings

- Hydrogen mass correlates with angular momentum ($V_{rot}D_{25}$ or $V_{rot}R_0$).
- Flat and isolated galaxies follow similar $V_{rot}R_0$ sequences.
- Gas stability conditions may differ from standard assumptions or reflect past higher gas content.

## Abstract

We examine the dependence between hydrogen total mass $M_{HI}$ and rotation speed $V_{rot}$, optical size $D_{25}$ or disc radial scale $R_0$ for two samples of late-type galaxies: a) isolated galaxy (AMIGA sample), and b) the edge-on galaxies (flat galaxies of Karachentsev et al. 1999). Estimates of $M_{HI}$, given in the HYPERLEDA database for flat galaxies appear to be on average higher at $\sim $0.2 dex, than for isolated galaxies with similar $V_{rot}$ or $D_{25}$ values, most probably, due to the overvaluation of self-absorption in the HI line. We confirm that the hydrogen mass for both samples closely correlates with galactic disc integral specific angular momentum $J$, which is proportional to $V_{rot}D_{25}$ or $V_{rot}R_0$, with low surface brightness galaxies lie along a common $V_{rot}R_0$ sequence. This relationship can be explained, assuming that gas mass in the disc is regulated by marginal gravitational stability condition of gas layer. A comparison of the observed and theoretically expected dependences leads to a conclusion that either gravitational stability corresponds to higher values of Toomre parameter than is usually assumed, or the threshold stability condition for most galaxies took place only in the past, when gas mass in discs was 2-4 times higher than at present (with the exception of galaxies with abnormally high $HI$ content). The last condition requires that the gas accretion was not compensated by gas consumption during the evolution of most of galaxies.