|Annu. Rev. Astron. Astrophys. 1994. 32:
Copyright © 1994 by . All rights reserved
Some of the systematics in the properties of galaxies point to a common thread--that of star formation. In this section, we investigate trends along the Hubble sequence and consider also the critical parameter of local environment.
We have discussed 18 measures of the properties of galaxies. For five of these there is a clear monotonic variation with type over the entire spiral-irregular range. A sixth, that of H II regions, also shows such a monotonic type-dependent variation though here the data are far fewer. All six are normalized quantities: T, HI, MHI/LB, MHI/MT, (B-V), and H II region luminosity functions. However not all of the normalized (nor absolute) quantites vary systematically and monotonically, e.g., MT/LB, B, FIR. Five of the six can be identified with star formation activity: past, present, or future potential. The sixth, the mass surface density, is also likely related to such activity as a triggering mechanism.
Thus, (B-V) measures past and current formation activity, H II regions relate current activity, and the cool gas content measures the reservoir for future star formation. In addition, those parameters for which the data are too few to describe any detailed type dependence other than elliptical versus spiral - radio, X-ray, and chemical abundance - are also star formation related. What is surprisingly lacking from this overall list is the FIR surface density. It does have the elliptical versus spiral dependence of others but shows no variation within the classical spiral category (and only an uncertain trend in the later types due to fewer data). This lends support to the two-component model for the origin of the FIR described previously. Clearly, the morphological sequence of normal, isolated galaxies measures star formation activity.