Circumstellar Dust around AGB Stars and Implications for Infrared Emission from Galaxies

Stellar population synthesis (SPS) models are used to infer many galactic properties including star formation histories, metallicities, and stellar and dust masses. However, most SPS models neglect the effect of circumstellar dust shells around evolved stars and it is unclear to what extent they impact the analysis of spectral energy distributions (SEDs). To overcome this shortcoming we have created a new set of circumstellar dust models, using the radiative transfer code DUSTY, for asymptotic giant branch (AGB) stars and incorporated them into the Flexible Stellar Population Synthesis code. The circumstellar dust models provide a good fit to individual AGB stars as well as the IR color-magnitude diagrams of the Large and Small Magellanic Clouds. IR luminosity functions from the Large and Small Magellanic Clouds are not well-fit by the 2008 Padova isochrones when coupled to our circumstellar dust models and so we adjusted the lifetimes of AGB stars in the models to provide a match to the data. We show, in agreement with previous work, that circumstellar dust from AGB stars can make a significant contribution to the IR (≳ 4 μ m) emission from galaxies that contain relatively little diffuse dust, including low-metallicity and/or non-star-forming galaxies. Our models provide a good fit to the mid-IR spectra of early-type galaxies. Circumstellar dust around AGB stars appears to have a small effect on the IR SEDs of metal-rich star-forming galaxies (i.e., when A_V ≳ 0.1). Stellar population models that include circumstellar dust will be needed to accurately interpret data from the James Webb Space Telescope and other IR facilities.