The Initial Mass Function in the Extended Ultraviolet Disk of M83

TL;DR

This study uses Hubble data to analyze the initial mass function in M83's extended ultraviolet disk, finding it similar to the standard IMF but with potential deficiencies in high-mass stars, and demonstrating the robustness of these results.

Contribution

It provides the first detailed measurement of the IMF in the XUV disk of M83 using resolved stellar populations and compares it with Hα emission constraints.

Findings

IMF slope consistent with Salpeter/Kroupa IMF

Upper mass limit around 25 solar masses

XUV disks may have fewer high-mass stars than expected

Abstract

Using Hubble Space Telescope ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations of four fields within the extended ultraviolet disk (XUV disk) of M83. These observations show a clumpy distribution of main-sequence stars and a mostly smooth distribution of red giant branch stars. We constrain the upper-end of the initial mass function (IMF) in the outer disk using the detected population of main-sequence stars and an assumed constant star formation rate (SFR) over the last 300 Myr. By comparing the observed main-sequence luminosity function to simulations, we determine the best-fitting IMF to have a power law slope $\alpha=-2.35 \pm 0.3$ and an upper-mass limit $\rm M_{u}=25_{-3}^{+17} \, M_\odot$. This IMF is consistent with the observed H$\alpha$ emission, which we use to provide additional constraints on the IMF. We explore the influence of deviations from the constant SFR assumption, finding that our IMF conclusions are robust against all but strong recent variations in SFR, but these are excluded by causality arguments. These results, along with our similar studies of other nearby galaxies, indicate that some XUV disks are deficient in high-mass stars compared to a Kroupa IMF. There are over one hundred galaxies within 5 Mpc, many already observed with HST, thus allowing a more comprehensive investigation of the IMF, and how it varies, using the techniques developed here.