|Annu. Rev. Astron. Astrophys. 1991. 29:
Copyright © 1991 by Annual Reviews. All rights reserved
6.2 Ratio of H2 to HI
In order to elucidate the processes that influence molecular cloud formation, it is helpful to analyze the relative amounts of atomic and molecular gas. That there is a morphological type dependence to the (normalized) total atomic gas content of galaxies has long been recognized. In particular, Roberts (1969) showed that the HI mass-to blue luminosity ratio, M (HI) / LB, increase by a factor of ~ 5 among spiral galaxies going from type Sa to Scd. (This trend is partially due to the contribution of the bulge to the blue luminosity in early type galaxies.)
Young & Knezek (1989) analyzed the global ratios of H2 to HI for 170 galaxies for which HI masses were available in the literature (Huchtmeier et al 1983). Within this sample, the mean ratio of total HI mass to blue luminosity increases with type by a factor of 5, but the mean ratio of H2 mass to blue luminosity is roughly the same for types Sa-Sc, then decreasing by a factor of at least 3 for types Scd-Sdm. The M (H2) / M (HI) ratio is shown in Figure 4 for the galaxies in several studies (Young & Knezek 1989, Thronson et al 1989b). While there is considerable scatter in the ratio of H2 to HI within a given type, the mean clearly changes systematically along the Hubble sequence. A volume limited sample of galaxies closer than 20 Mpc displays a similar decrease in the H2 / HI ratio. Thus, the ratio of molecular to atomic mass decreases by more than a factor of 10 as a function of morphological type for types Sa-Sd. The mean M (H2) / M (HI) ratio is 4.0 ± 1.9 for S0/Sa galaxies, and 0.2 ± 0.1 for Sd/Sm galaxies. Verter (1987) reached a somewhat different conclusion - that the CO/HI flux ratio peaks for intermediate morphological types - but her study was based on many fewer galaxies.
Figure 4. Ratio of molecular to atomic gas mass as a function of morphological type for spiral galaxies (Young & Knezek 1989, Thronson et al 1989). The vertical tick mark in each panel represents the median value for that type. Although there is a considerable range in the H2 / HI ratio within each type, the mean value clearly decreases for later type spirals.
The mean ratio of total ISM mass (HI + H2) to optical area is 17 M pc-2, with only a factor of 2 increases from early to late type spirals. Thus, late type galaxies have only marginally greater gas surface densities than the early type galaxies, but the phase of the gas for the cool ISM varies quite systematically along the Hubble sequence. In particular, the late type spiral galaxies have a lower global molecular gas fraction.
Variations in the H2 / HI ratio can result from changes in the efficiency of H2 cloud formation or the rate of cloud disruption (Wyse 1986, Tacconi & Young 1986, Shaya & Federman 1987, Wyse & Silk 1989, Wang 1990) The variation with morphological type could indicate that the efficiency of molecular cloud formation is higher in early-type than in late-type spirals, thus suggesting that large-scale gravitational effects may influence molecular cloud formation.
Recent observations of molecular and atomic gas in infrared bright galaxies indicate that the ISM in interacting systems may be predominately molecular (Mirabel & Sanders 1989, Knezek & Young 1989). Such a situation could arise through enhanced conversion of atomic to molecular gas during interactions, through removal of the loosely bound atomic gas, or both.
It is well known that the atomic gas content of many Virgo spirals is low by factors ranging from 2 to 10, compared with more isolated galaxies of the same type and optical size (Giovanelli & Haynes 1983, van Gorkom & Kotanyi 1985, Warmels 1986), and this depletion has been commonly interpreted as evidence of gas stripping in the cluster environment. Several surveys of CO emission have been conducted for the Virgo spiral galaxies (Kenney & Young 1986, 1989, Stark et al 1986). These studies show that the H2 is not deficient, that is, the mechanism removing the low density atomic gas has left the denser molecular clouds intact. Since the greatest HI depletion in the Virgo spirals occurs in the outer disks, well outside of the region in which most of the molecular gas is usually located in spiral galaxies, it is not surprising that the molecular gas does not show the same degree of depletion as the atomic gas. In addition, the molecular clouds have high column densities and, residing in the inner galaxy, are more tightly bound. The depletion of the HI in the outer disks of some Virgo spirals is approximately a factor of 10, whereas in the inner disk, it is usually a factor of 2 or less (Kenney & Young 1989). Thus, a possible interpretation of the HI and CO data for the Virgo spirals is that the gas depletion occurs entirely in the areas of the galaxy in which the gas is both tenuous and loosely bound by gravitation (i.e. the outer disk or at moderate above or below the galactic plane).