THE FAR-INFRARED PROPERTIES OF THE CFA GALAXY SAMPLE .2. GAS, DUST, AND STAR-FORMATION ALONG THE HUBBLE SEQUENCE

The FIR properties of normal, non-infrared-bright galaxies along the H ubble sequence are studied as a function of their morphological type, using a complete sample of galaxies in the magnitude-limited (m(Zwicky ) less-than-or-equal-to 14.5) CfA sample detected in the Faint Source Survey, a total of 1544 galaxies. We find that the short-wave-length F IR emission is best explained as having mainly an interstellar origin, and not as coming mostly from nonthermal sources or from circumstella r or photospheric emission from evolved stars. The FIR colors of galax ies from type E-SO to type Sbc, are mainly controlled by the spatial d istribution of the dust relative to the stars. The dust in elliptical galaxies is as hot as in Magellanic irregulars, because it is concentr ated in their central regions (r less than or similar to 1 kpc) where the UV energy density from post-AGB stars is the highest. The dust in Sbc galaxies is the coolest because it is most spread out in the disk where the UV energy density is the lowest. From type Sbc to type Sdm, the FIR colors are controlled by an increasing star formation efficien cy, modulated by a change in dust composition. The metallicity decreas e in Sdm galaxies leads to a deficiency of small grains relative to la rge grains and a reduction of the dust and H-2 masses relative to the H I masses. Conversely, in elliptical galaxies, the larger metallicity may increase the abundance of small grains relative to large grains. Most of the FIR color trends as a function of galaxian morphological t ype can be reproduced by dust models with three components (PAHs, inte rmediate-size grains, and large grains), combined with the appropriate dust heating spectral energy distribution.