Mid-Infrared Galaxy Morphology along the Hubble Sequence

The mid-infrared emission from 18 nearby galaxies were imaged with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope, sampling the spatial distributions of the reddening-free stellar photospheric emission and the warm dust in the interstellar medium. These two components provide a new framework for galaxy morphology classification in which the presence of spiral arms and their emission strength relative to the starlight can be measured directly and with high contrast. Four mid-infrared classification methods are explored, three of which are based on quantitative global parameters (colors and bulge-to-disk ratio) that are similar to those used in the past for optical studies; in this limited sample, all correlate well with traditional B-band classification. We suggest reasons why infrared classification may be superior to optical classification.