We present optical photometry of Hubble Space Telescope (HST ) ACS/WFC data of the resolved stellar populations in the outer disc of the dwarf irregular galaxy DDO 154. The photometry reveals that young main sequence stars are almost absent from the outermost Hi disc. Instead, most are clustered near the main stellar component of the galaxy. We constrain the stellar initial mass function (IMF) by comparing the luminosity function of the main sequence stars to simulated stellar populations assuming a constant star formation rate over the dynamical timescale. The best-fitting IMF is deficient in high mass stars compared to a canonical Kroupa IMF, with a best-fit slope α = −2.45 and upper mass limit M U = 16 M ⊙ . This top-light IMF is consistent with predictions of the Integrated Galaxy-wide IMF theory. Combining the HST images with Hi data from The Hi Nearby Galaxy Survey Treasury (THINGS) we determine the star formation law (SFL) in the outer disc. The fit has a power law exponent N = 2.92 ± 0.22 and zero point A = 4.47 ± 0.65 × 10 −7 M ⊙ yr −1 kpc −2 . This is depressed compared to the Kennicutt-Schmidt Star Formation Law, but consistent with weak star formation observed in diffuse Hi environments. Extrapolating the SFL over the outer disc implies that there could be significant star formation occurring that is not detectable in Hα. Last, we determine the Toomre stability parameter Q of the outer disc of DDO 154 using the THINGS Hi rotation curve and velocity dispersion map. 72% of the Hi in our field has Q ≤ 4 and this incorporates 96% of the observed MS stars. Hence 28% of the Hi in the field is largely dormant.

Using Hubble Space Telescope (HST ) ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations in the outskirts of NGC 2915, a blue compact dwarf with an extended H i disc. These observations reveal an elliptical distribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the H i gas distribution. We constrain the upper-end initial mass function (IMF) and determine the star formation law (SFL) in this field, using the observed main-sequence stars and an assumed constant star formation rate. Previously published Hα observations of the field, which show one faint H ii region, are used to provide further constraints on the IMF. We find that the main-sequence luminosity function analysis alone results in a best-fitting IMF with a power-law slope α = −2.85 and upper-mass limit M u = 60 M . However, if we assume that all Hα emission is confined to H ii regions then the upper-mass limit is restricted to M u 20 M . For the luminosity function fit to be correct we have to discount the Hα observations implying significant diffuse ionized gas or escaping ionizing photons. Combining the HST photometry with H i imaging we find the SFL has a power law index N = 1.53 ± 0.21. Applying these results to the entire outer H i disc indicates that it contributes 11-28% of the total recent star formation in NGC 2915, depending on whether the IMF is constant within the disc or varies from the centre to the outer region.

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.