Modeling Tracers of Young Stellar Population Age in Star-Forming
  Galaxies
  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 Mo instantaneous burst and a continuous star
formation rate of 1 Mo/yr), and two different treatments of initial rotation
rate (v_rot = 0.0v_crit and 0.4v_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.
