Constraining the top-light initial mass function in the extended ultraviolet disk of M 83

Context: The universality or non-universality of the initial mass function (IMF) has significant implications for determining star formation rates and star formation histories from photometric properties of stellar populations.
Aims: We reexamine whether the IMF is deficient in high-mass stars (top-light) in the low-density environment of the outer disk of M 83 and constrain the shape of the IMF therein.
Methods: Using archival Galaxy Evolution Explorer (GALEX) far ultraviolet (FUV) and near ultraviolet (NUV) data and new deep OmegaCAM narrowband H alpha imaging, we constructed a catalog of FUV-selected objects in the outer disk of M 83. We counted H alpha-bright clusters and clusters that are blue in FUV-NUV in the catalog, measured the maximum flux ratio F-H alpha/f(lambda FUV) among the clusters, and measured the total flux ratio Sigma F-H alpha/Sigma f(lambda FUV) over the catalog. We then compared these measurements to predictions from stellar population synthesis models made with a standard Salpeter IMF, truncated IMFs, and steep IMFs. We also investigated the effect of varying the assumed internal extinction on our results.
Results: We are not able to reproduce our observations with models using the standard Salpeter IMF or the truncated IMFs. It is only when assuming an average internal extinction of 0.10 < AV < 0.15 in the outer disk stellar clusters that models with steep IMFs (alpha > 3.1) simultaneously reproduce the observed cluster counts, the maximum observed F-H alpha/f lambda(FUV), and the observed Sigma F-H alpha/Sigma f(lambda FUV).
Conclusions: Our results support a non-universal IMF that is deficient in high-mass stars in low-density environments.