HETDEX [O III] Emitters. I. A Spectroscopically Selected Low-redshift Population of Low-mass, Low-metallicity Galaxies

We assemble a sample of 17 low-metallicity (7.45 < log(O/H)+12 < 8.12) galaxies with z ≲ 0.1 found spectroscopically, without photometric preselection, in early data from the Hobby-Eberly Telescope Dark Energy Experiment. Star-forming galaxies that occupy the lowest-mass and lowest-metallicity end of the mass-metallicity relation tend to be undersampled in continuum-based surveys as their spectra are typically dominated by emission from newly forming stars. We search for galaxies with high [O III]λ5007/[O II]λ3727, implying highly ionized nebular emission often indicative of low-metallicity systems. With the Second Generation Low Resolution Spectrograph on the Hobby-Eberly Telescope we acquired follow-up spectra, with higher resolution and broader wavelength coverage, of each low-metallicity candidate in order to confirm the redshift, measure the Hα and [N II] line strengths, and, in many cases, obtain deeper spectra of the blue lines. We find our galaxies are consistent with the mass-metallicity relation of typical low-mass galaxies. However, galaxies in our sample tend to have similar specific star formation rates as the incredibly rare "blueberry" galaxies found by Yang et al. We illustrate the power of spectroscopic surveys for finding low-mass and low-metallicity galaxies and reveal that we find a sample of galaxies that are a hybrid between the properties of typical dwarf galaxies and the more extreme blueberry galaxies.