  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. We
reexamine whether the IMF is deficient in high-mass stars (top-light) in the
low-density environment of the outer disk of M83 and constrain the shape of the
IMF therein. 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 M83. We counted H$\alpha$-bright clusters and clusters that are
blue in FUV$-$NUV in the catalog, measured the maximum flux ratio
$F_{\mathrm{H}\alpha}/f_{\lambda \mathrm{FUV}}$ among the clusters, and
measured the total flux ratio $\Sigma F_{\mathrm{H}\alpha}/\Sigma f_{\lambda
\mathrm{FUV}}$ 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. 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
$0.10 < A_{\mathrm{V}} < 0.15$ in the outer disk stellar clusters that models
with steep IMFs ($\alpha > 3.1$) simultaneously reproduce the observed cluster
counts, the maximum observed $F_{\mathrm{H}\alpha}/f_{\lambda \mathrm{FUV}}$,
and the observed $\Sigma F_{\mathrm{H}\alpha}/\Sigma f_{\lambda \mathrm{FUV}}$.
Our results support a non-universal IMF that is deficient in high-mass stars in
low-density environments.
