Interstellar medium phases and abundances in the central parsec

A JWST MIRI/MRS view of the Galactic center

¹ LIRA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
² Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095-1547, USA
³ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
⁴ Université Paris-Cité, Paris, France
⁵ Department of Physics, 601 University Dr., Texas State University, San Marcos, TX 78666, USA
⁶ Department of Astronomy, University of California, Berkeley, USA

^★ Corresponding author: pierre.vermot@obspm.f

Received: 18 July 2025
Accepted: 9 October 2025

Abstract

Context. The Galactic center (GC) is a unique and extreme astrophysical laboratory for studying the interplay between gas, stars, and a supermassive black hole (SMBH). In particular, the circumnuclear disk (CND) and its central cavity (CC) present two contrasting environments in terms of gas content, density, and stellar activity, making them ideal regions in which to study the multiphase structure and chemical composition of the interstellar medium (ISM).

Aims. We aim to determine the properties (temperature, density, abundances, and spatial distribution) of the various phases of the ISM in the central parsec of the GC, with particular attention in this paper to the ionized medium.

Methods. We used newly obtained observations from the Mid-Infrared Instrument (MIRI) equipped with the Medium Resolution Spectrometer (MRS) aboard the James Webb Space Telescope (JWST) to extract spectra covering the entire spectral range from 5 to 27 µm in the CND and in the CC. We used the photoionization code CLOUDY to generate synthetic spectra with the same spectral range and resolution, simulating a wide range of gas phases and abundances. We then determined the contribution of each phase to the spectra. Once the abundances and contribution from each phase of the gas were determined, we identified four dominant phases and performed a spatial analysis to determine their contribution to each spaxel of the datacubes.

Results. We find that in both the CND and the CC, the bulk of the emission originates from warm ionized gas with temperatures of between 10⁴ and 10^4.8 K. In the CND, molecular gas contributes significantly to the flux and is spatially structured, while the CC shows minimal molecular gas content, as is expected from these regions. Coronal gas is detected in both regions at the interface between molecular and warm ionized gas. The hottest coronal phase appears faint and patchy in the CC, and has an elongated morphology in the CND. Abundance fitting (in solar-normalized logarithmic units) is primarily constrained by abundances: we measure a robust depletion of Fe relative to α elements with log(Fe/α) = −0.78 ± 0.20 (CC) and −0.84 ± 0.26 (CND), while CNO is only mildly enhanced relative to α, log(CNO/α) = 0.27 ± 0.20 (CC) and 0.05 ± 0.26 (CND). Absolute abundances are supersolar but more degenerate; the best-fitting models yield (log α, log CNO, log Fe) = (1.4, 1.4, 0.4) in the CC and (2.0, 1.8, 1.2) in the CND.

Conclusions. The observed abundance pattern (enhanced CNO and α elements with suppressed Fe) indicates a chemically young environment, recently enriched by core-collapse supernovae and stellar winds, with a limited contribution from older Type Ia supernovae. This favors a scenario of massive, recent star formation rather than cumulative long-term enrichment. Additionally, the projected orientation of the newly identified CND elongated hot coronal feature, perpendicular to the direction toward the SMBH, suggests the action of a large-scale shock possibly resulting from past energetic outflows.