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

The Spitzer Space Telescope First Look Survey Infrared Array Camera (IRAC)near and mid-infrared imaging data partially overlaps the Sloan Digital SkySurvey (SDSS), with 313 visually selected (r<17.6 mag) SDSS Main Samplegalaxies in the overlap region. The 3.5 and 7.8 um properties of the galaxiesare investigated in the context of their visual properties, where the IRAC[3.5] magnitude primarily measures starlight, and the [7.8] magnitude primarilymeasures PAH emission from the interstellar medium. As expected, we find astrong inverse correlation between [3.5]-[7.8] and visual color; galaxies redin visual colors (`red galaxies') tend to show very little dust and molecularemission (low `PAH-to-star' ratios), and galaxies blue in visual colors (`bluegalaxies,' 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 bydust lanes. Simple, visually inferred attenuation corrections bring the visualcolors of these galaxies in line with those of face-on disks; ie, PAH emissionis closely related to attenuation-corrected, optically inferred star-formationrates. Blue galaxies with anomalously low PAH-to-star ratios are alllow-luminosity star-forming galaxies. There is some weak evidence in thissample that the deficiency in PAH emission for these low-luminosity galaxiesmay be related to emission-line metallicity.Comment: submitted to ApJ. Because of some obscure arXiv bug, the RGB figure may appear correctly only in the PDF versio