Recent Results from the Spitzer Space Telescope: A New View of Galaxy Morphology and Classification
The IRAC instrument on the Spitzer Space Telescope has imaged two dozen nearby galaxies. The images reveal the reddening-free spatial distributions of the stellar photospheric emission and of the warm dust in the ISM. These two components provide a new framework for galaxy morphological 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, 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.
Two lenticular galaxies exhibit warm dust emission emanating from spiral arms—an IR feature that is not apparent even in an HST color map. Spiral arms of dust in lenticular galaxies may provide evidence for an evolutionary link between them and early-type spiral galaxies.
Warm dust emission in M 81 can be traced all the way to the nucleus, unlike the near-ultraviolet emission. Finally, the presence of an AGN in M 81 can be inferred both from nuclear colors redder than the bulge and the presence of a point source at the center.
Keywordsinfrared galaxy classification interstellar dust
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