We investigate the integrated properties of massive ( > 10 ~ { } { M _ { \odot } } ) , rotating , single-star stellar populations for a variety of initial rotation rates ( v / v _ { crit } = 0.0 , 0.2 , 0.4 , 0.5 , and 0.6 ) .
We couple the new MESA Isochrone and Stellar Tracks ( MIST ) models to the Flexible Stellar Population Synthesis ( FSPS ) package , extending the stellar population synthesis models to include the contributions from very massive stars ( > 100 ~ { } { M _ { \odot } } ) , which can be significant in the first \sim 4 Myr after a starburst .
These models predict ionizing luminosities that are consistent with recent observations of young nuclear star clusters .
We also construct composite stellar populations assuming a distribution of initial rotation rates .
Even in low-metallicity environments where rotation has a significant effect on the evolution of massive stars , we find that stellar population models require a significant contribution from fast-rotating ( v / v _ { crit } > 0.4 ) stars in order to sustain the production of ionizing photons beyond a few Myr following a starburst .
These results have potentially important implications for cosmic reionization by massive stars and the interpretation of nebular emission lines in high-redshift star-forming galaxies .