Mid-Infrared Images of Stars and Dust in Irregular Galaxies

Abstract

We present mid-IR to optical properties of 22 representative irregular galaxies: 18 irregular (Im) galaxies, 3 blue compact dwarfs, and 1 Magellanic-type spiral galaxy. The mid-IR is based on images from the Spitzer Space Telescope archives. The 3.6 and 4.5 μm bands and the UBVJHK images are used to examine disk morphology and the integrated and azimuthally averaged magnitudes and colors of stars. The nonstellar contribution to the 4.5 μm images is used to trace hot dust. The 5.8 and 8.0 μm images reveal emission from hot dust and polycyclic aromatic hydrocarbons (PAHs), and both may contribute to these passbands, although we refer to the nonstellar emission as PAH emission. We compare the 8.0 μm images to Hα. 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 and 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 H II regions, implying that massive stars are the primary source of heating. Galaxies with intense, widespread star formation have more extended PAH emission. The ratio of PAH to Hα emission is not constant on small scales. PAHs are associated with shells and giant filaments, so they are not destroyed during shell formation. This work is based in part on archival data obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.