The principle of the Hubble classification of spiral galaxies is that the bulge-to-disk ratio is higher in early-type galaxies (Sa-Sb) than in late-type galaxies (Sc-Sd). The ratio of the current to the historical star formation rate is higher in late-type galaxies than early-type galaxies (Kennicutt 1983).
Devereux, Becklin, and Scoville (1987) present evidence that the IRAS emission from spiral galaxies is mainly a disk phenomenon. They find that their measurements of the 1.65 µm nuclear (5.5 arcsec or 500 pc diameter) flux densities of Virgo cluster spirals is much better correlated with the 60 µm flux densities in disk-dominated late-type spirals than in bulge-dominated early-type spirals. On the whole, however, the dependence of infrared luminosity on Hubble type is weak for spiral galaxies. Devereux (1986) finds no statistically significant differences between the fractional luminosity functions of galaxies of Hubble types in the range Sa to Sc, though there appear to be deficiencies of high-luminosity galaxies in the very late and very early classes. There are strong resemblances between the infrared fractional luminosity function and the radio continuum fractional luminosity function of spiral galaxy disks, as derived by Hummel (1981).
The 60-100 µm color temperature shows more dependence on Hubble type than does the bolometric luminosity. Table 1 summarizes an analysis by Devereux (1986) of a sample consisting of all 227 known galaxies in the distance range 15-40 Mpc, the galactic latitude range |bII| 20°, and with a 60 µm luminosity ( S) 2 x 109 L. There is a statistically significant excess of ``hot'' galaxies among early types. He has also measured the ``compactness'' of many of the sample galaxies at 10 µm by comparing the flux in a 5.5 arcsec IRTF beam with the color-corrected 12 µm flux measured through the much larger IRAS beam. He finds that galaxies in which the 10 µm emission is compact are significantly more common among early-type than among late-type galaxies. A similar result was found by Hummel (1981) for the central radio sources of disk galaxies, but observations in the 2.6 mm CO emission line show a different trend with a number of early-type galaxies displaying a central hole rather than an enhancement (Young 1986).
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