The HETDEX Pilot Survey. IV. The Evolution of [O II] Emitting Galaxies from z ~ 0.5 to z ~ 0

TL;DR

This study analyzes the evolution of [O II] emitting galaxies from redshift 0.5 to 0, showing a decline in luminosity, equivalent widths, and overall star formation rate density over the last 5 billion years.

Contribution

It provides the first [O II]-based measurements of star formation rate density evolution in the z ~ 0.5 to 0 range, using data from the HETDEX pilot survey.

Findings

Luminosity function L* decreases by ~0.6-0.9 dex over 5 Gyr.

Fraction of high equivalent-width emitters declines with time.

Star formation rate density decreases by a factor of ~2.5 from z ~ 0.5 to 0.

Abstract

We present an analysis of the luminosities and equivalent widths of the 284 z < 0.56 [O II]-emitting galaxies found in the 169 square arcmin 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 de-reddened [O II] 3727 luminosity and calculate its total star formation rate. We show that over the last ~5 Gyr of cosmic time there has been substantial evolution in the [O II] emission-line luminosity function, with L* decreasing by ~0.6 +/-0.2 dex in the observed function, and by ~0.9 +/-0.2 dex in the de-reddened 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 ~5 Gyr, and that the star formation rate density of the universe has declined by a factor of ~2.5 between z ~ 0.5 and z ~ 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.