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 $\sim 3500-5800$ $\unicode{x212B}$ at $\sim 5$ $\unicode{x212B}$ resolution. I have newly determined the gas metallicities, $12+\text{log(O/H)}$, 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-$\sigma$ 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 $\sim$2-$\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 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 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.

19 pages, 15 figures, MNRAS in press, online supplementary data included in the source file

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