SDSS-IV MaNGA: Data-model discrepancy in temperature-sensitive line ratios for star-forming galaxies

¹ Department Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China
² CUHK Shenzhen Research institute, No.10, 2nd Yuexing Road, Nanshan, Shenzhen, China
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁵ Department of Astronomy, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA

^★ Corresponding authors: zmpeng@link.cuhk.edu.hk; rbyan@cuhk.edu.hk

Received: 8 July 2025
Accepted: 21 September 2025

Abstract

Aims. Gas-phase metallicity is a fundamental parameter that helps constrain the star-forming history and chemical evolution of a galaxy. Measuring the 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.

Methods. We binned ~1.5 million spaxels from SDSS-IV MaNGA according to metallicity and ionization parameters derived from theoretical strong-line calibrations. We stacked the spectra of spaxels within each bin and measured the flux of strong lines and faint auroral lines. Auroral lines for [O II], [S II], [O III], and [S III] were detected in the stacked spectra of most bins, and the [N II] auroral line was detected in fewer bins. We applied an empirical method to correct dust attenuation, which makes more realistic corrections for low ionization lines.

Results. We derived electron temperatures for these five ionic species and measured 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 theoretical 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 1D photoionization model is incapable of representing the complexity of real situations, and cannot predict the increase in the auroral-to-strong line ratio of [O II] at high metallicity.