Ionized gas in quiescent galaxies: Temperature measurement and constraint on the ionization source

¹ Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China
² Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁴ Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington KY 40506, USA
⁵ UC Observatories, MS: UCO Lick, UC Santa Cruz, 1156 High St, Santa Cruz CA 95064, USA
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I50125 Florence, Italy

Received: 1 November 2023
Accepted: 1 July 2024

Abstract

In non-star-forming, passively evolving galaxies, regions with emission lines dominated by low-ionization species are classified as low-ionization emission regions (LIERs). The ionization mechanism behind such regions has long been a mystery. Active galactic nuclei (AGNs), which were once believed to be the source, have been found not to be the dominant mechanism, especially in regions distant from the galaxy nuclei. The remaining candidates, photoionization by post-asymptotic giant branch (pAGB) stars and interstellar shocks can only be distinguished with in-depth analysis. As the temperature predictions of these two models differ, temperature measurements can provide strong constraints on this puzzle. We selected a sample of 2795 quiescent red-sequence galaxies from the Sloan Digital Sky Survey IV (SDSS-IV) Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. We divided the sample spectra into three groups based on the [N II]/Hα flux ratio, and utilized stacking techniques to improve the signal-to-noise ratio of the observed spectra. We determined the temperature of [O III], [N II], [S II], and [O II] through their temperature-sensitive emission line ratios. Subsequently, we compared the measured temperatures with predictions from different models. The results demonstrate consistency with the interstellar shock model with pre-shock density n = 1 cm⁻³ and solar metallicity, thus supporting shocks as the dominant ionization source of LIERs. Additionally, we also find that the interstellar dust extinction value measured through the Balmer decrement appears to be larger than that implied by the forbidden line ratios of low-ionization lines.