Annu. Rev. Astron. Astrophys. 1984. 22: 37-74 Copyright © 1984 by Annual Reviews. All rights reserved

3.5 Usable Gas

In measuring star formation rates and other parameters that depend on the gas content, the gas mass used is the total mass detected. The proper value to consider is really the mass that has the potential to engage in star formation. Irrs often have halos of neutral hydrogen extending to several optical diameters. An extreme example is the dwarf Irr IC 10, which has a hydrogen envelope 20 times its optical diameter (326); however, this galaxy suffers heavy Galactic extinction, so it is possible that the optical size has been underestimated. It is not clear that this outer gas necessarily participates in the star formation process (an idea suggested for M33 in 235). If such is the case, our concept of the Irrs as systems that homogeneously evolve as a single spatial unit must be altered.

In a sample of 21 Irrs (mostly low surface brightness), Huchtmeier et al. (183) found that the FWHM of the HI distribution occurred at the Holmberg radius but that HI usually extended to several Holmberg radii at a density of 10¹⁹ atoms cm^-2. Spirals, on the other hand, can have H I values to 1.5 times the Holmberg dimension at 10²⁰ atoms cm^-2 (37, 38). However, there are important exceptions. NGC 4449 still has a column density ~ 10²⁰ atoms cm^-2 at 4 times its Holmberg radius (376) and IC 10 at 2-3 times (326). Yet, in neither galaxy is there direct evidence for ongoing star formation at such large radii. All of this suggests that much of the gas in the outer parts of many Irrs may not be able to contribute to the star formation as effectively as the inner gas. This provides the empirical basis for our earlier assumption that condensation of at least delectably young OB stars is a threshold phenomenon that does not occur (or takes place with much lower frequency) in gas below a critical density (94, 244). Based on the data for spirals given by Bosma (37, 38) and the properties of Irrs, a rough empirical guess is that gas in HI disks is below the OB star formation threshold density when _HI 5 x 10²⁰ atoms cm^-2.