Mid‐Infrared Galaxy Morphology along the Hubble Sequence

The mid-infrared emission from 18 nearby galaxies imaged with the IRACinstrument on Spitzer Space Telescope samples the spatial distributions of thereddening-free stellar photospheric emission and the warm dust in the ISM.These two components provide a new framework for galaxy morphologicalclassification, in which the presence of spiral arms and their emissionstrength relative to the starlight can be measured directly and with highcontrast. Four mid-infrared classification methods are explored, three of whichare based on quantitative global parameters (colors, bulge-to-disk ratio)similar to those used in the past for optical studies; in this limited sample,all correlate well with traditional B-band classification. We suggest reasonswhy infrared classification may be superior to optical classification.