Nonthermal Pressures: Key to Energy Balance and Structure Formation near Sgr A* in the Milky Way

Mazoochi, F, Tabatabaei, FS, Barnes, AT, Colzi, L, Garcia, P, Henkel, C, Hu, Y, Longmore, SN ORCID: 0000-0001-6353-0170, Martin, S, Sanchez-Monge, A, Rivilla, VM, Schmiedeke, A, Ott, J, Walker, DL, Wang, QD, Williams, GM and Zhang, S (2026) Nonthermal Pressures: Key to Energy Balance and Structure Formation near Sgr A* in the Milky Way. The Astrophysical Journal, 997 (1). ISSN 0004-637X

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Abstract

The circumnuclear region of the Galactic center offers a unique laboratory to study energy balance and structure formation around Sgr A⋆. This work investigates thermal and nonthermal processes within a 7 pc distance from Sgr A⋆. Using MeerKAT 1.3 GHz radio continuum data and Atacama Large Millimeter/submillimeter Array H40α radio recombination line emission from the ACES survey, we separate free–free and synchrotron components at ∼0.2 pc resolution. With a thermal fraction of ≃13%, the 1.3 GHz emission shows tight correlations with the Herschel PACS IR data. The correlation between the equipartition magnetic field and molecular gas traced by JCMT 12CO (J = 3 → 2) observations reveals a balance between the magnetic field, cosmic rays, and molecular gas pressures south of the circumnuclear disk on ∼0.7 pc scales. Unlike the magnetic field and ionized gas, the molecular gas density declines in the cavity (R ≤ 2 pc) toward the center, likely due to feedback from Sgr A⋆. We find that nonthermal pressure from turbulent gas nearly balances magnetic and cosmic-ray pressures and exceeds the thermal pressure by 2 orders of magnitude. The medium surrounding Sgr A⋆ is filled by a low-β (thermal-to-magnetic energy ratio), supersonic plasma, with an Alfvén Mach number ≃ 4 (assuming equipartition). Analysis of the mass-to-magnetic flux ratio suggests that the circumnuclear region is mostly subcritical and, therefore, the magnetic field can help stabilize gas clouds against gravitational collapse.

Item Type:	Article
Uncontrolled Keywords:	5101 Astronomical Sciences; 51 Physical Sciences; 5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics; 0306 Physical Chemistry (incl. Structural); Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	American Astronomical Society
Date of acceptance:	1 November 2025
Date of first compliant Open Access:	5 June 2026
Date Deposited:	05 Jun 2026 15:34
Last Modified:	05 Jun 2026 15:34
DOI or ID number:	10.3847/1538-4357/ae1b93
URI:	https://researchonline.ljmu.ac.uk/id/eprint/28759

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