Revisiting the Local Star-Forming Galaxies Observed in the HETDEX Pilot Survey

Abstract

I have reanalyzed the data obtained for local (z < 0.15) star-forming galaxies during the pilot survey for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX)-called the HETDEX Pilot Survey (HPS)-which uses an integral-field-unit spectrograph and covers ∼ 3500 − 5800 Å at ∼ 5 Å resolution. I have newly determined the gas metallicities, 12 + log (O/H), following the Bayesian analysis scheme of the previous study, but dealing carefully with the uncertainty of strong-line calibration, performing reproducibility tests with mock data, and monitoring the convergence of the Markov-Chain Monte Carlo (MCMC) sampling. From the mock-data tests, I found that the nebular emission-line color excess E(B − V) can be overestimated by as much as 2-σ or more, although the metallicity can recover the input value to within 1-σ. The new metallicity estimates on the HPS data are from well-converged MCMC samples (effective sample sizes > 2000), and they are higher than the previous estimates by ∼2-σ. Using the HPS data, I also showed that the MCMC sampling can have the statistical accuracy as poor as the one near the iteration start if done without convergence monitoring. The overestimation of E(B − V) indicates the overestimation of the star-formation rates (SFRs) in the previous study, which can be as much as a factor of five. This finding undermines the previous suggestion of a hitherto-unknown galaxy population based on the locations of galaxies in the mass-SFR plane. I found that the independent determination of E(B − V) using either Hβ-Hγ or Hα-Hβ line pair is ideal for the analysis of forthcoming HETDEX data, but it requires additional cost.