Molecular gas and star formation in central rings across nearby galaxies

¹ Max-Planck-Institut für Astronomie Königstuhl 17 D-69117 Heidelberg, Germany
² National Astronomical Observatory of Japan 2-21-1 Osawa Mitaka Tokyo 181-8588, Japan
³ Sterrenkundig Observatorium, Universiteit Gent Krijgslaan 281 S9 B-9000 Gent, Belgium
⁴ Observatorio Astronómico Nacional (IGN), C/Alfonso XII 3 E-28014 Madrid, Spain
⁵ European Southern Observatory Karl-Schwarzschild Straße 2 D-85748 Garching bei München, Germany
⁶ Univ Lyon, Univ Lyon 1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574 F-69230 Saint-Genis-Laval, France
⁷ Department of Astronomy, The Ohio State University 140 West 18th Avenue Columbus Ohio 43210, USA
⁸ Argelander-Institut für Astronomie, Universität Bonn Auf dem Hügel 71 53121 Bonn, Germany
⁹ Department of Physics, University of Alberta Edmonton AB T6G 2E1, Canada
¹⁰ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik Albert-Ueberle-Str 2 D-69120 Heidelberg, Germany
¹¹ Cosmic Origins Of Life (COOL) Research DAO, https://coolresearch.io
¹² Department of Physics and Astronomy, University of Wyoming Laramie WY 82071, USA
¹³ Research School of Astronomy and Astrophysics, Australian National University Canberra ACT 2611, Australia
¹⁴ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen Im Neuenheimer Feld 205 D-69120 Heidelberg, Germany
¹⁵ Harvard-Smithsonian Center for Astrophysics 60 Garden Street Cambridge MA 02138, USA
¹⁶ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University 10 Garden Street Cambridge MA 02138, USA
¹⁷ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA
¹⁸ Department of Physics, Tamkang University, No.151 Yingzhuan Rd. Tamsui Dist. New Taipei City 251301, Taiwan
¹⁹ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid E-28040, Spain
²⁰ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria Via Valleggio 11 22100 Como, Italy
²¹ Department of Astrophysical Sciences, Princeton University 4 Ivy Ln. Princeton NJ 08544, USA
²² Department of Astronomy, University of Maryland 4296 Stadium Drive College Park MD 20742, USA
²³ Sub-department of Astrophysics, Department of Physics, University of Oxford Keble Road Oxford OX1 3RH, UK

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

Received: 26 September 2025
Accepted: 29 December 2025

Abstract

Context. Nearby galaxies exhibit a variety of structures, including so-called central or (circum-)nuclear rings that are similar to the Milky Way (MW) Central Molecular Zone (CMZ). These rings are common in barred galaxies and can be gas-rich and highly star-forming.

Aims. We aim to study the molecular gas content and star formation rate of central rings within nearby galaxies and link them to global galaxy properties, especially the bar morphology.

Methods. We utilized 1″(≲100 pc) resolution CO(2–1) observations from the PHANGS-ALMA survey, visually identifying 20 central rings and determine their properties. For 14 of these rings, MUSE observations tracing star formation rate (SFR) surface density were available. We derived the rings’ geometry, integrated molecular gas masses, SFRs, depletion times, and compared them to host galaxy and bar properties from the literature.

Results. Molecular gas is an effective tracer for central rings. Previous studies have used ionized gas and dust tracers to identify central rings in galaxies of similar morphological types as the PHANGS galaxies (numerical Hubble type T ∼ −3 to T ∼ 9). In comparison, molecular gas yields similar fractions of galaxies hosting central rings and similar radii distributions. The gaseous central rings have typical radii of ∼ 400⁺²⁵⁰₋₁₅₀pc, molecular gas masses of log(M/M_⊙) ∼ 8.1^+0.17_−0.23, and SFRs of ∼ 0.21^+0.15_−0.16 M_⊙/yr. As a result, they contribute 5.6^+4.5_−2.1% and 13⁺¹⁰₋₅% to their host galaxies’ molecular gas mass and SFR, respectively. While the MW CMZ sits at the lower end of the radius, molecular gas mass, and SFR distribution, it matches well in terms of ring molecular gas mass and SFR fraction, and depletion time. Longer bars contain more massive molecular central rings, but there is no correlation between the classical bar strength parameters (Q_b, ε_bar, A₂^max) and the ring’s molecular gas content.

Conclusions. Although absolute central ring properties (ring radius, molecular gas mass, SFR) likely depend on host galaxy properties, the similarities between the MW CMZ and PHANGS central rings in relative parameters (molecular gas and SFR fraction, depletion time) suggest that the processes of gas inflow and star formation are similar for central rings across nearby galaxies.