The initial mass function in the extended ultraviolet disc of M83

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 disc (XUV disc) 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 disc 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 α = −2.35 ± 0.3 and an upper mass limit $M_{\rm u}=25_{-3}^{+17} \, \mathrm{M}_\odot$. This IMF is consistent with the observed H $\rm \alpha$ 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 discs 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.

A study of outer disk stellar populations of face-on star-forming galaxies in SDSS-IV MaNGA: causes of H$α$ deficiency

“…Figure 10 demonstrates that the same basic conclusions reached by Bruzzese et al (2020) in their analysis of the stellar populations of four HST fields in the outer XUV disk of M83 also apply to the outer disks of the galaxies in the subsample selected from MaNGA using optical spectral diagnostics. In each of the three panels in the figure, the black points are individual measurements from stacked spectra in the radial range 2 < R/Re < 3.…”

“…Very rapid recent shutdown of star formation across such a large region violates causality constraints. Bruzzese et al (2020) similarly argue that recent stochastic fluctuations in star formation are highly red: inner disk blue: outer disk unlikely in M83 because the Hα deficiency is present in all four widely separated HST fields that they study.…”

“…These models do a better job of spanning the observed range in Dn( 4000), but still overpredict Hα. It should be noted that Bruzzese et al (2020) found that the main sequence population of the M83 outer disk was well fit assuming constant star formation over the same 0.5 Gyr timescale, but that Hα was similarly overpredicted.…”

“…The most recent work on this topic has made use of star counts derived from HST imaging of main sequence and red giant branch stars in the outer disk of M83 to simultaneously constrain the slope and the upper mass cutoff of the IMF (Bruzzese et al 2020). Averaged over the 4 HST fields that were available for this galaxy, the main sequence luminosity function provides some evidence for an IMF deficient in the most massive stars.…”

A comparison of the UV and HI properties of the extended UV (XUV) disk galaxies NGC 2541, NGC 5832 and ESO406-042

“…Their existence supports models of inside out star formation, where star formation occurs in the extreme (optically) low surface brightness (LSB) zones of galaxy disks resulting in disk building and galaxy growth. XUV disks are also one of the best examples of star formation in low density environemnets (Krumholz & McKee, 2008) and the stellar disk initial mass function (IMF) in such environments is known to be deficient in massive stars (Bruzzese et al, 2020). Hence, XUV disks are interesting to study -both for understanding disk growth as well as for studying star formation in underdense environments.…”