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

Abstract

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 Hα imaging, we constructed a catalog of FUV-selected objects in the outer disk of M 83. We counted Hα-bright clusters and clusters that are blue in FUV−NUV in the catalog, measured the maximum flux ratio FHα/fλFUV among the clusters, and measured the total flux ratio ΣFHα/ΣfλFUV 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 (α > 3.1) simultaneously reproduce the observed cluster counts, the maximum observed FHα/fλFUV, and the observed ΣFHα/ΣfλFUV. Conclusions. Our results support a non-universal IMF that is deficient in high-mass stars in low-density environments.