  The Spitzer Space Telescope First Look Survey Infrared Array Camera (IRAC)
near and mid-infrared imaging data partially overlaps the Sloan Digital Sky
Survey (SDSS), with 313 visually selected (r<17.6 mag) SDSS Main Sample
galaxies in the overlap region. The 3.5 and 7.8 um properties of the galaxies
are investigated in the context of their visual properties, where the IRAC
[3.5] magnitude primarily measures starlight, and the [7.8] magnitude primarily
measures PAH emission from the interstellar medium. As expected, we find a
strong inverse correlation between [3.5]-[7.8] and visual color; galaxies red
in visual colors (`red galaxies') tend to show very little dust and molecular
emission (low `PAH-to-star' ratios), and galaxies blue in visual colors (`blue
galaxies,' ie, star-forming galaxies) tend to show large PAH-to-star ratios.
Red galaxies with high PAH-to-star ratios tend to be edge-on disks reddened by
dust lanes. Simple, visually inferred attenuation corrections bring the visual
colors of these galaxies in line with those of face-on disks; ie, PAH emission
is closely related to attenuation-corrected, optically inferred star-formation
rates. Blue galaxies with anomalously low PAH-to-star ratios are all
low-luminosity star-forming galaxies. There is some weak evidence in this
sample that the deficiency in PAH emission for these low-luminosity galaxies
may be related to emission-line metallicity.
