The relationship between warm and hot gas-phase metallicity in massive elliptical galaxies and the influence of active galactic nucleus feedback

¹ Departamento de Física, Universidad de Santiago de Chile Av. Victor Jara 3659 Santiago 9170124, Chile
² Center for Interdisciplinary Research in Astrophysics and Space Exploration (CIRAS), Universidad de Santiago de Chile Santiago 9170124, Chile
³ Department of Physics and Astronomy, University of Kentucky 505 Rose Street Lexington KY 40506, USA
⁴ Astrophysics Branch, NASA-Ames Research Center, MS 245-6 Moffett Field CA 94035, USA
⁵ School of Physics & Astronomy, University of Nottingham, University Park Nottingham NG7 2RD, UK
⁶ Department of Physics, Informatics & Mathematics, University of Modena & Reggio Emilia 41125 Modena, Italy
⁷ LERMA, Observatoire de Paris, Collège de France, PSL University, CNRS, Sorbonne University Paris, France
⁸ Department of Physics and Astronomy, University of Alabama in Huntsville Huntsville AL 35899, USA
⁹ Instituto de Alta Investigación, Universidad de Tarapacá Casilla 7D Arica, Chile
¹⁰ European Southern Observatory Alonso de Córdova 3107 Vitacura Región Metropolitana, Chile
¹¹ Institute of Astrophysics, Facultad de Ciencias Exactas, Universidad Andrés Bello, Sede Concepción Talcahuano, Chile

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 April 2025
Accepted: 7 November 2025

Abstract

Context. Warm ionized gas is ubiquitous at the centers of X-ray bright elliptical galaxies. While it is believed to play a key role in the feeding and feedback processes of supermassive black holes, its origins remain under debate. Existing studies have primarily focused on the morphology and kinematics of warm ionized gas.

Aims. This work aims to provide a new perspective on warm (∼10 000 K) ionized gas and its connection to X-ray-emitting hot gas (> 10⁶ K) by measuring and comparing their metallicities.

Methods. We conducted a joint analysis of 13 massive elliptical galaxies using MUSE/VLT and Chandra observations. Emission-line ratios, including [OIII]/Hβ, [NII]/Hα, were measured using MUSE observations to infer the ionization mechanisms. We derive metallicities of the warm ionized gas using HII, and LINER calibrations. We also computed the warm phase metallicity using X-ray/EUV, and pAGB star models. For two sources at higher redshifts, the direct T_e method was also used to measure warm gas metallicities. The metallicity of the hot gas was measured using Chandra X-ray observations.

Results. Our observations reveal that most sources exhibit composite ionization, with contributions from both star formation and LINER-like emission. The four sources with the lowest star formation rates in our sample – Centaurus, M87, M84, and Abell 496 – are dominated by LINER emission. A positive linear correlation was found between the gas-phase metallicities of the warm and hot phases, ranging from 0.3 to 1.5 Z_⊙. In some sources the warm gas metallicity shows a central drop. A similar radial trend has been reported for the hot gas metallicity in some galaxy clusters.

Conclusions. The ionization mechanisms of cooling flow elliptical galaxies are diverse, suggesting multiple channels for powering the warm ionized gas. The positive correlation found in warm and hot gas metallicities suggest the intimate connection between the two gas phases, likely driven by gas cooling and/or mixing. The large variation in the warm gas metallicity further suggests that cold gas mass derived under the assumption of solar metallicity for the CO-to-H₂ conversion factor needs to be revised by approximately an order of magnitude.