11.2.2. Correlations
a) Mass Luminosity
The masses and luminosities of spiral and
irregular galaxies cover about four orders of magnitude, and a plot of
mass against luminosity (e.g.,
Roberts, 1969)
shows that the brighter galaxies are more massive. The mass-to-light
ratio is approximately constant with one order of magnitude dispersion
(M / L 
4
solar units
M
/
L).
There is no
separation according to structural type in the mass-luminosity diagram,
except in the case of the irregulars, which tend to be low-mass
intrinsically faint systems. Without the irregulars the relationship
between mass, luminosity, and morphological type is not obvious.
b) Hydrogen Mass Luminosity
The distance to an external galaxy is generally ill-determined and distance-independent parameters are of great value in establishing correlations between the integral properties of galaxies. The neutral hydrogen-to-luminosity ratio, MH / L, is such a parameter and tends to be higher in the bluer later-type galaxies (e.g., Epstein, 1964). The neutral hydrogen content is itself correlated with the luminosity, and these two correlations probably signify that star formation is proceeding rapidly in galaxies containing more hydrogen.
c) Fractional Hydrogen Content
A plot of neutral hydrogen mass versus total mass (e.g., Roberts, 1969) again shows a correlation, but one which is highly dependent on the presence of the low-mass irregular systems. There is a clear relationship between the fractional hydrogen content and the structural type. The HI content ranges from 1 to 2% hydrogen in the Sa spirals to over 30% in the irregular systems. This is the most striking relationship in the integral properties of galaxies and probably indicates that the early-type spirals are more evolved than the late-type spirals and irregulars.
d) Ionized Hydrogen
Except for the nearest galaxies, ionized hydrogen has been discussed only in terms of large HII regions. Sersic (1960) has made a survey of HII regions in 66 galaxies and finds that the mean size of the three largest HII regions increases progressively with neutral hydrogen content from the earlier types to the Sc- galaxies and then decreases again in the dwarf Sc and irregulars. Hodge (1967) has cataloged the positions of HII regions in 66 galaxies and finds in an analysis of 25 of these (Hodge, 1969) that the distribution of HII regions within a galaxy tends to be a function of the morphological type, possibly indicating a difference in the state, or rate of evolution, of the galaxies.
e) Radio Continuum
Normal spiral galaxies are sources of nonthermal radio continuum, with flux densities of typically 0.1 to 1 flux unit, at 408 MHz. The optical and radio luminosities of spiral galaxies are closely correlated and recent data (Cameron, 1971) has shown that this correlation is even stronger for the restricted set of galaxies of Morgan type f. The radio continuum power is better correlated with the van den Bergh and Morgan classifications than with the Hubble type. The correlation with the van den Bergh classification and hence with spiral arm development is particularly interesting in the light of recent maps of spiral galaxies, e.g., M31 (Pooley, 1969) and M51 (Mathewson, et. al., 1972), showing that spiral arms are sources of radio continuum emission.