SDSS-IV MaNGA: Data-Model Discrepancy in Temperature-sensitive Line Ratios for Star-forming Galaxies

Abstract: Gas-phase metallicity is a fundamental parameter that helps constrain the star-forming history and chemical evolution of a galaxy. Measuring electron temperature through auroral-to-strong line ratios is a direct approach to deriving metallicity. However, there is a longstanding discrepancy between metallicity measured through the direct method and that based on the photoionization models. This paper aims to verify and understand the discrepancies. We bin ~ 1.5 million spaxels from SDSS-IV MaNGA according to metallicity and ionization parameters derived from theoretical strong-line calibrations. We stack the spectra of spaxels within each bin and measure the flux of strong lines and faint auroral lines. Auroral lines for [OII], [SII], [OIII], and [SIII] are detected in the stacked spectra of most bins, and the [NII] auroral line is detected in fewer bins. We apply an empirical method to correct dust attenuation, which makes more realistic corrections for low ionization lines. We derive electron temperatures for these five ionic species and measure the oxygen and sulfur abundances using the direct method. We present the resulting abundance measurements and compare them with those model-calibrated strong-line abundances. The chemical abundances measured with the direct method are lower than those derived from the photoionization model, with a median of 0.09 dex. This discrepancy is smaller compared to the results based on other metallicity calibrations previously reported. However, we notice that the direct method could not account for the variation in ionization parameters, indicating that the precise calibration of metallicity using the direct method has yet to be fully realized. We report significant discrepancies between data and the photoionization model, which illustrates that the one-dimensional photoionization model is incapable of representing the complexity of real situations.