4.1.1. Neutral and Molecular Gas
Neutral gas is the largest component of gas in most spirals and irregulars, as determined from H I 21-cm hyperfine line measurements. The 21-cm line has been well-mapped in many nearby galaxies. With regard to determining gas fractions and surface densities, a few points should be kept in mind:
Fortunately, many nearby galaxies have been well mapped in H I at kiloparsec scale resolution or better.
Molecular gas, mostly in the form H2, is the important phase associated with star formation. Although H2 may not dominate the total gas mass, it is often found to be the main component in the inner disk of Sbc or later type spirals, and so can be the main contribution to the gas surface density in such regions.
H2 has no dipole moment and thus emits no strong dipole radiation of its own. The usual tracer of molecular gas is the abundant CO molecule, typically the 12CO (J = 1-0) millimeter-wave transition. The conversion from the measured I(CO) to the column density N(H2), X(CO), must be calibrated largely without the benefits of direct measurement of H2 column densities. The result has been a long-standing controversy over the value of the CO-H2 conversion factor and its dependence on metallicity. For example, Wilson (1995) has studied the CO-N(H2) relation in a variety of environments in Local Group galaxies, comparing I(CO) with molecular cloud masses derived from the velocity dispersion assuming the clouds are in virial equilibrium. From her data Wilson found a roughly linear relation between X(CO) and 12 + log O/H corresponding to approximately a factor ten increase in X(CO) for factor ten decrease in O/H, from solar O/H to 0.1 times solar O/H. On the other hand, Israel (1997a,b) argues that virial equilibrium is a poor assumption for short-lived molecular clouds. He instead uses the FIR dust emission surface brightness and H I maps to determine the dust/N(H) ratio, then uses the FIR surface brightness to estimate N(H2) in regions where CO is detected. With this method Israel derives a variation in X(CO) with O/H which is much steeper than that obtained by Wilson: a factor of approximately 100 decrease in X for a factor 10 increase in O/H.
Both of these methods likely suffer from errors due to assumptions made. Virial equilibrium may very well be a poor assumption for molecular clouds. On the other hand, the FIR calibration depends on the assumption that the dust-to-gas ratio is the same in neutral gas and in molecular clouds; however, grains may be preferentially destroyed by shocks in the lower density neutral component. The FIR model is also very sensitive to the dust temperature. The relation between I(CO) and N(H2) likely depends on a variety of factors besides metallicity (Maloney & Black 1988). More work needs to be done to determine the best method to obtain H2 masses.