  To study the distribution of star formation and dust emission within nearby
galaxies, we measured five morphological parameters in the 3.6 and 24 micron
wave bands for 65 galaxies in the Spitzer Infrared Nearby Galaxies Survey
(SINGS) and 8 galaxies that were serendipitously observed by SINGS. The
morphological parameters demonstrate strong variations along the Hubble
sequence, including statistically significant differences between S0/a-Sab and
Sc-Sd galaxies. Early-type galaxies are generally found to be compact,
centralized, symmetric sources in the 24 micron band, while late-type galaxies
are generally found to be extended, asymmetric sources. These results suggest
that the processes that increase the real or apparent sizes of galaxies' bulges
also lead to more centralized 24 micron dust emission. Several phenomena, such
as strong nuclear star formation, Seyfert activity, or outer ring structures,
may cause galaxies to deviate from the general morphological trends observed at
24 microns. We also note that the 24 micron morphologies of Sdm-Im galaxies are
quite varied, with some objects appearing very compact and symmetric while
others appear diffuse and asymmetric. These variations reflect the wide
variation in star formation in irregular galaxies as observed at other
wavelengths. The variations in the 24 micron morphological parameters across
the Hubble sequence mirror many of the morphological trends seen in other
tracers of the ISM and in stellar emission. However, the 24 micron
morphological parameters for the galaxies in this sample do not match the
morphological parameters measured in the stellar wave bands. This implies that
the distribution of dust emission is related to but not equivalent to the
distribution of stellar emission.
