THE HETDEX PILOT SURVEY. IV. THE EVOLUTION OF [O II] EMITTING GALAXIES FROM z ∼ 0.5 TO z ∼ 0

We present an analysis of the luminosities and equivalent widths of the 284 z < 0.56 [O II]-emitting galaxies found in the 169 arcmin(2) pilot survey for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). By combining emission-line fluxes obtained from the Mitchell spectrograph on the McDonald 2.7 m telescope with deep broadband photometry from archival data, we derive each galaxy's dereddened [O II] lambda 3727 luminosity and calculate its total star formation rate. We show that over the last similar to 5Gyr of cosmic time, there has been substantial evolution in the [O II] emission-line luminosity function, with L* decreasing by similar to 0.6 +/- 0.2 dex in the observed function, and by similar to 0.9 +/- 0.2 dex in the dereddened relation. Accompanying this decline is a significant shift in the distribution of [O II] equivalent widths, with the fraction of high equivalent-width emitters declining dramatically with time. Overall, the data imply that the relative intensity of star formation within galaxies has decreased over the past similar to 5Gyr, and that the star formation rate density of the universe has declined by a factor of similar to 2.5 between z similar to 0.5 and z similar to 0. These observations represent the first [O II]-based star formation rate density measurements in this redshift range, and foreshadow the advancements which will be generated by the main HETDEX survey.