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The initial mass function and star formation law in the outer disc of NGC 2915

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 HI disc. These observations reveal an elliptical distribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the HI 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 HII 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$_\rm{u}$ = 60 M$_\odot$. However, if we assume that all Hα emission is confined to HII regions then the upper-mass limit is restricted to M$_\rm{u}$ $\le$20 M$_\odot$. 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 HI imaging we find the SFL has a power law index $N=1.53 \pm 0.21$. Applying these results to the entire outer HI 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.