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