Mid-Infrared Images of Stars and Dust in Irregular Galaxies

Abstract

We present mid-infrared to optical properties of 22 representative irregular galaxies: 18 Im, 3 BCDs, and one Sm. The mid-IR is based on images from the Spitzer Space Telescope archives. The 3.6 and 4.5 micron bands and the UBVJHK images are used to examine disk morphology and the integrated and azimuthally averaged magnitudes and colors of stars. The non-stellar contribution to the 4.5 micron images is used to trace hot dust. The 5.8 and 8.0 micron images reveal emission from hot dust and PAHs, and both may contribute to these passbands, although we refer to the non-stellar emission as PAH emission. We compare the 8.0 micron images to Halpha. Im galaxies have no hidden bars, and those with double-exponential optical light profiles have the same at mid-IR. Most galaxies have similar optical mid-IR scale lengths. Four galaxies have super star clusters that are not visible at optical bands. Galaxies with higher area-normalized star formation rates have more dust and PAH emission relative to starlight. Hot dust and PAH emission is found mostly in high surface brightness HII regions, implying that massive stars are the primary source of heating. Galaxies with intense, wide-spread star formation have more extended PAH emssion. The ratio of PAH to Halpha emission is not constant on small scales. PAHs are associated with shell and giant filaments, so they are not destroyed during shell formation.