The young stellar population of a star-forming galaxy is the primary engine driving its radiative properties .
As a result , the age of a galaxy ’ s youngest generation of stars is critical for a detailed understanding of its star formation history , stellar content , and evolutionary state .
Here we present predicted equivalent widths for the H \beta , H \alpha and Br \gamma recombination lines as a function of stellar population age .
The equivalent widths are produced by the latest generations of stellar evolutionary tracks and the Starburst99 stellar population synthesis code , and are the first to fully account for the combined effects of both nebular emission and continuum absorption produced by the synthetic stellar population .
Our grid of model stellar populations spans six metallicities ( 0.001 < Z < 0.04 ) , two treatments of star formation history ( a 10 ^ { 6 } M _ { \odot } instantaneous burst and a continuous star formation rate of 1 M _ { \odot } yr ^ { -1 } ) , and two different treatments of initial rotation rate ( v _ { rot } = 0.0 v _ { crit } and 0.4 v _ { crit } ) .
We also investigate the effects of varying the initial mass function .
Given constraints on galaxy metallicity , our predicted equivalent widths can be applied to observations of star-forming galaxies to approximate the age of their young stellar populations .