Mid‐Infrared and Visible Photometry of Galaxies: Anomalously Low Polycyclic Aromatic Hydrocarbon Emission from Low‐Luminosity Galaxies

Abstract

The Spitzer Space Telescope First Look Survey Infrared Array Camera (IRAC) near- and mid-infrared imaging data partially overlap the Sloan Digital Sky Survey (SDSS), with 313 visible-selected (r < 17.6 mag) SDSS main sample galaxies in the overlap region. The 3.5 and 7.8 μm properties of the galaxies are investigated in the context of their visible properties, where the IRAC [3.5] magnitude primarily measures starlight and the [7.8] magnitude primarily measures polycyclic aromatic hydrocarbon (PAH) emission from the interstellar medium. As expected, we find a strong inverse correlation between [3.5]-[7.8] and visible color; galaxies red in visible colors ("red galaxies") tend to show very little dust and molecular emission (low PAH-to-star ratios), and galaxies blue in visible colors ("blue galaxies," i.e., 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 attenuation corrections inferred in the visible bring the visible colors of these galaxies in line with those of face-on disks; i.e., PAH emission is closely related to attenuation-corrected star formation rates inferred in the visible. 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.