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 H{\alpha}
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
{\alpha}=-2.85 and upper-mass limit M$_\rm{u}$ = 60 M$_\odot$. However, if we
assume that all H{\alpha} 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 H{\alpha}
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.
