Magnetic field morphology and evolution in the Central Molecular Zone and its effect on gas dynamics

¹ Institute of Physics, Laboratory for Galaxy Evolution and Spectral Modelling, EPFL, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
³ Department of Physics, University of Surrey, Guildford GU2 7XH, UK
⁴ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁵ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, D-69120 Heidelberg, Germany
⁶ SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, Fife, KY16 9SS, UK
⁷ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁸ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, Merate, 23807, Italy
⁹ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
¹⁰ ESO Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
¹¹ University of Connecticut, Department of Physics, 196A Auditorium Road, Unit 3046, Storrs, CT 06269 USA
¹² Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹³ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁴ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
¹⁵ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China
¹⁶ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta 1270709, Chile
¹⁷ Department of Astronomy, University of Florida, PO Box 112055, Gainesville, FL 32611-2055, USA
¹⁸ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA
¹⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²⁰ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France
²¹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²² Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
²³ INAF Astronomical Observatory of Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
²⁴ Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
²⁵ MIT Haystack Observatory, 99 Millstone Road, Westford, MA, 01886, USA
²⁶ Technical University of Munich, School of Engineering and Design, Department of Aerospace and Geodesy, Chair of Remote Sensing Technology, Arcisstr. 21, 80333 Munich, Germany
²⁷ Cosmic Origins Of Life (COOL) Research DAO, Munich, Germany ;
²⁸ Department of Physics and Astronomy, University of California, Los Angeles, Box 951547, CA 90095-1547 USA
²⁹ Space, Earth and Environment Department, Chalmers University of Technology, 412 96 Gothenburg, Sweden
³⁰ Institut de Ciències de l’Espai (ICE, CSIC), Can Magrans s/n, 08193, Bellaterra, Barcelona, Spain
³¹ Institut d’Estudis Espacials de Catalunya (IEEC), 08034, Barcelona, Spain
³² Istituto di Astrofisica e Planetologia Spaziale (IAPS). INAF. Via Fosso del Cavaliere 100, 00133 Roma, Italy
³³ Instituto de Radioastronomía y Astrofísica, UNAM. Apdo. Postal 72-3 (Xangari), Morelia, Michocán 58089, México
³⁴ Center for Astrophysics, Harvard & Smithsonian, MS-42, 60 Garden Street, Cambridge, MA 02138, USA

^★ Corresponding author; robin.tress@epfl.ch; mattiacarlo.sormani@gmail.com

Received: 19 March 2024
Accepted: 8 September 2024

Abstract

The interstellar medium in the Milky Way’s Central Molecular Zone (CMZ) is known to be strongly magnetised, but its large-scale morphology and impact on the gas dynamics are not well understood. We explore the impact and properties of magnetic fields in the CMZ using three-dimensional non-self gravitating magnetohydrodynamical simulations of gas flow in an external Milky Way barred potential. We find that: (1) The magnetic field is conveniently decomposed into a regular time-averaged component and an irregular turbulent component. The regular component aligns well with the velocity vectors of the gas everywhere, including within the bar lanes. (2) The field geometry transitions from parallel to the Galactic plane near ɀ = 0 to poloidal away from the plane. (3) The magneto-rotational instability (MRI) causes an in-plane inflow of matter from the CMZ gas ring towards the central few parsecs of 0.01−0.1 M_⊙ yr⁻¹ that is absent in the unmagnetised simulations. However, the magnetic fields have no significant effect on the larger-scale bar-driven inflow that brings the gas from the Galactic disc into the CMZ. (4) A combination of bar inflow and MRI-driven turbulence can sustain a turbulent vertical velocity dispersion of σ_ɀ = 5 km s⁻¹ on scales of 20 pc in the CMZ ring. The MRI alone sustains a velocity dispersion of σ_ɀ ≃ 3 km s⁻¹. Both these numbers are lower than the observed velocity dispersion of gas in the CMZ, suggesting that other processes such as stellar feedback are necessary to explain the observations. (5) Dynamo action driven by differential rotation and the MRI amplifies the magnetic fields in the CMZ ring until they saturate at a value that scales with the average local density as B ≃ 102 (n/10³ cm⁻³)^0.33 µG. Finally, we discuss the implications of our results within the observational context in the CMZ.