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

TL;DR

The paper presents a joint analysis of warm and hot gas metallicities in 13 massive elliptical galaxies using MUSE/VLT and Chandra observations to probe the interplay between cooling, condensation, and AGN feedback. By applying multiple ionization-specific abundance calibrations and Te-based measurements where possible, the study reveals a strong positive correlation between warm- and hot-gas metallicities and identifies central abundance drops in several systems, likely linked to AGN-driven redistribution and dust processes. The findings imply that the metallicity of the warm phase tracks the hot halo, informing estimates of cold gas masses and CO-to-H2 conversion factors, and highlight the need for multiwavelength diagnostics to robustly measure abundances across gas phases. Overall, the work demonstrates a tight coupling between gas phases in cluster centers and underscores the multifaceted role of AGN feedback in shaping metal distributions and star formation within massive galaxies.

Abstract

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. This work aims to provide a new perspective on warm (10,000 K) ionized gas and its connection to X-ray-emitting hot gas (>10^6 K) by measuring and comparing their metallicities. We conducted a joint analysis of 13 massive elliptical galaxies using MUSE/VLT and Chandra observations. Emission-line ratios were measured for the warm ionized gas using MUSE observation, and used to infer the ionization mechanisms and derive metallicities of the warm ionized gas using HII, and LIN(E)R calibrations. We also computed the warm phase metallicity using X-ray/EUV, and pAGB stars models. For two sources at higher redshift, direct Te method was also used to measure warm gas metallicities. Our observations reveal that most sources exhibit composite ionization, with contributions from both star formation and LINER-like 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 Zsun, and suggest the intimate connection between the two gas phases, likely driven by gas cooling and/or mixing. 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. The ionization mechanisms of cooling flow elliptical galaxies are diverse, suggesting multiple channels for powering the warm ionized gas. 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.

Figures (14)

• Figure 1: Map of warm ionized gas traced by H$\alpha$ emission from MUSE observations of our sample.
• Figure 2: Example of resolved BPT-NII classic diagrams for Centaurus (top panels), and Abell 2597 (bottom panels). Spaxels are color-coded based on their location relative to boundaries between the well-known empirical and theoretical classification schemes kewley01kauffmann03schawinski07 shown in black solid, dashed, and pointed lines, respectively. Yellow, green, blue, and red spaxels correspond to AGN-, LI(N)ER-, composite-, and HII-dominated regions, respectively.
• Figure 3: Comparison of warm and hot-gas phase metallicities for a sample of sources using MUSE and Chandra observations. We derived warm gas metallicity using various calibration methods, depending on the source and their ionization mechanism. For LI(N)ER-like regions and sources (magenta circles), we used the Kumari2019 calibration. For HII-dominated regions, we used the Marino2013 calibration (yellow circles). Composite and LI(N)ER region derived using pAGB models are shown with pink rectangles. The warm-phase metallicity obtained from X-ray/EUV models are shown with green crosses. Three sources, including MACS 1931.8-2635, have warm-gas metallicity measurements using the $\rm T_{e}$-direct method (red inverted triangles). The errors for the warm gas phase abundance we included both the uncertainties associated with the calibration and 1$\sigma$ deviation for the regions. The purple dashed line correspond to the best fit, and the dashed region correspond to 1$\sigma$ uncertainty. The dashed gray line shows the one-to-one relation The best fit correspond to, $\rm Z_{warm} = (0.15\pm0.09) + Z_{hot}^{0.90 \pm 0.10}$.
• Figure 4: Top panel:Example X-ray spectrum of Abell 2597 from one of the two regions used to derive the hot gas-phase metallicity. The observed spectrum, corresponding to one Chandra observation, is shown as gray data points, while the best-fit model is overlaid in pink. Bottom Panel: Example of MUSE spectrum of one spaxel of Abell 2597 shown in gray, and the best fitted model is shown in pink.
• Figure 5: Comparison of O/H abundance profiles derived from the different O/H maps: X-ray/EUV and pAGB models, and HII and LINER calibration for RXJ0821. The median O/H profile is indicated by the pink line. O/H abundances profile derived from composite and LIN(E)R spaxels using X-ray and pAGB models, and LINER calibration are shown as blue, black, and purple points, respectively, while values obtained from H II spaxels are shown as gray points.