HI content in galactic disks: The role of gravitational instability

We examine the dependence of the total hydrogen mass M _HI in late-type star-forming galaxies on rotation velocity V _rot and optical size D ₂₅ or radial scale length R ₀ of the disk for two samples of galaxies: (i) isolated galaxies (AMIGA) and (ii) galaxies with edge-on disks (flat galaxies according to Karachentsev et al.). M _HI given in the HYPERLEDA database for flat galaxies have turned out to be, on average, overestimated by ~0.2 dex compared to isolated galaxies with similar V _rot or D ₂₅, which is apparently due to an overestimation of the self-absorption in the HI line. The hydrogen mass in the galaxies of both samples closely correlates with the total specific angular momentum of the galactic disk J, which is proportional to V _rot D ₂₅ or V _rot R ₀, with the low-surface-brightness galaxies lying along the common V _rot R ₀ sequence. We discuss the possibility of explaining the relationship between M _HI and V _rot D ₂₅ by assuming that the gas mass in the disk is regulated by the marginal gravitational stability condition for the gas layer. Comparison of the observed and theoretically expected dependences leads us to conclude that either the gravitational stability corresponds to higher values of the Toomre parameter than is usually assumed, or the threshold stability condition formost galaxies was fulfilled only in the past, when the gasmass in the disks was a factor of 2–4 higher than that at present (except for the galaxies with an anomalously high observed HI content). The latter condition requires that for most galaxies the conversion of gas into stars be not compensated by the external accretion of gas onto the disk.