A SPITZER STUDY OF ASYMPTOTIC GIANT BRANCH STARS. III. DUST PRODUCTION AND GAS RETURN IN LOCAL GROUP DWARF IRREGULAR GALAXIES

We present the third and final part of a census of asymptotic giant branch (AGB) stars in Local Group dwarf irregular (dIrr) galaxies. Papers I and II presented the results for WLM and IC 1613. Included here are Phoenix, LGS 3, DDO 210, Leo A, Pegasus dIrr, and Sextans A. Spitzer photometry at 3.6, 4.5, 5.8, and 8 μm are presented, along with a more thorough treatment of background galaxy contamination than was presented in Papers I and II. We find that at least a small population of completely optically obscured AGB stars exists in each galaxy, regardless of the galaxy's metallicity, but that higher metallicity galaxies tend to harbor more stars with slight infrared excesses. The optical incompleteness increases for the redder AGB stars, in line with the expectation that some AGB stars are not detected in the optical due to large amounts of extinction associated with in situ dust production. Overall, there is an underrepresentation of 30%-40% in the optical AGB within the 1σ errors for all of the galaxies in our sample. This undetected population is large enough to affect star formation histories derived from optical color-magnitude diagrams. As measured from the [3.6] - [4.5] color excesses, we find average stellar mass-loss rates (MLRs) ranging from 3.1 x 10-7to6.6 x 10-6 M sun yr-1, and integrated galaxy MLRs ranging from 4.4 x 10-5to1.4 x 10-3 M sun yr-1. The integrated MLR is sufficient to sustain the current star formation rate in only LGS 3 and DDO 210, requiring either significant nondusty mass loss or gas accretion in Phoenix, Leo A, Pegasus dIrr, Sextans A, WLM, and IC 1613 if they are to maintain their status as gas-rich galaxies.