We investigate the star formation history of both the bright star clusters and the diffuse ‘ field star ’ population in the dwarf starburst galaxy NGC 5253 using longslit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph ( STIS ) .
The slit covers a physical area of 370 \times 1.6 pc and includes 8 apparent clusters and several inter-cluster regions of diffuse light which we take to be the field .
The diffuse light spectrum lacks the strong O-star wind features which are clearly visible in spectra of the brightest clusters .
This discrepancy provides compelling evidence that the diffuse light is not reflected light from nearby clusters , but originates in a UV-bright field star population , and it raises the issue of whether the star formation process may be operating differently in the field than in clusters .
We compare our spectra to STARBURST99 evolutionary synthesis models which incorporate a new low metallicity ( \sim 1/4 Z _ { \sun } ) atlas of O-star spectra .
The clusters are well fit by instantaneous burst models with a Salpeter initial mass function ( IMF ) extending up to 100 M _ { \sun } , and we derive ages for them ranging from 1 to 8 Myrs .
Reasonable fits to the field spectrum are obtained by continuous star formation models with either an upper mass cut-off of \sim 30 M _ { \sun } or an IMF slope steeper than Salpeter ( \alpha \sim 3.5 ) .
We favor a scenario which accounts for the paucity of O-stars in the field without requiring the field to have a different IMF than the clusters : stellar clusters form continuously and then dissolve on \sim 10 Myr timescales and disperse their remaining stars into the field .
We consider the probable contribution of an O-star deficient field population to the spatially unresolved spectra of high redshift galaxies .