Mid-Infrared Images of Stars and Dust in Irregular Galaxies

We present mid-infrared to optical properties of 22 representative irregulargalaxies: 18 Im, 3 BCDs, and one Sm. The mid-IR is based on images from theSpitzer Space Telescope archives. The 3.6 and 4.5 micron bands and the UBVJHKimages are used to examine disk morphology and the integrated and azimuthallyaveraged magnitudes and colors of stars. The non-stellar contribution to the4.5 micron images is used to trace hot dust. The 5.8 and 8.0 micron imagesreveal emission from hot dust and PAHs, and both may contribute to thesepassbands, although we refer to the non-stellar emission as PAH emission. Wecompare the 8.0 micron images to Halpha. Im galaxies have no hidden bars, andthose with double-exponential optical light profiles have the same at mid-IR.Most galaxies have similar optical mid-IR scale lengths. Four galaxies havesuper star clusters that are not visible at optical bands. Galaxies with higherarea-normalized star formation rates have more dust and PAH emission relativeto starlight. Hot dust and PAH emission is found mostly in high surfacebrightness HII regions, implying that massive stars are the primary source ofheating. Galaxies with intense, wide-spread star formation have more extendedPAH emssion. The ratio of PAH to Halpha emission is not constant on smallscales. PAHs are associated with shell and giant filaments, so they are notdestroyed during shell formation.Comment: To be published in AJ Available from ftp.lowell.edu/pub/dah/papers/ira