Far-ultraviolet to Far-infrared Spectral Energy Distribution Modeling of the Star Formation History across M31

Our neighboring galaxy M31 has been recently surveyed at the far- and near-ultraviolet (FUV and NUV) with the UVIT telescope on AstroSat, which provides unprecedented sensitivity to young stellar populations. Here the UVIT data are supplemented with optical data, near-infrared (IR) data (Spitzer), and mid- and far-IR data (Herschel). The observations are processed to obtain the spectral energy distributions (SEDs) for 73 regions covering M31. The SEDs are modeled using the Cigale SED fitting code with old and young stellar populations. The old stellar population has an age of 12 Gyr across M31 but has longer formation times at further distances from the center. Significant dependences on the position of dust extinction, dust emission, and young stellar population properties are found. Across M31, there are regions with a low-age (less than or similar to 100 Myr) young population and regions with an intermediate-age (similar to 1 Gyr) young population. The mass in the young population has declined by a factor of similar to 10 for ages 800-100 Myr ago but has increased again for ages less than or similar to 100 Myr. This indicates that cold gas available for star formation has been changing over the past Gyr, whether it is caused by a changing merger rate, changing gas infall, or changes in the gas reservoir in M31. We find that the dust luminosity, based on far-IR observations, is driven by the youngest stars, which are primarily measured in the FUV and NUV bands.