Revisiting the local star-forming galaxies observed in the HETDEX Pilot Survey

I have reanalysed 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 similar to 3500-5800 angstrom at similar to 5-angstrom 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 colour excess E(B - V) can be overestimated by as much as 2 sigma or more, although the metallicity can recover the input value to within 1 sigma. 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 similar to 2 sigma. 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 5. 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 H beta-H gamma or H alpha-H beta line pair is ideal for the analysis of forthcoming HETDEX data, but it requires additional cost.