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 H \alpha 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 \alpha = -2.85 and upper-mass limit M _ { u } = 60 M _ { \sun } .
However , if we assume that all H \alpha emission is confined to H ii regions then the upper-mass limit is restricted to M _ { u } \la 20 M _ { \sun } .
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 H i imaging we find the SFL has a power law index N = 1.53 \pm 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 .