The SMUGGLE-Ring project: Bar and bulge effects on nuclear disk and ring formation

¹ Leibniz-Institüt für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, Via Piero Gobetti 93/2, I-40129 Bologna, Italy
³ INAF, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, I-40129 Bologna, Italy
⁴ Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany
⁵ Université Côte d’Azur, Observatoire de la Côte d’Azur, Laboratoire Lagrange, CNRS, Blvd de l’Observatoire, 06304 Nice, France
⁶ Department of Physics & Astronomy, Seoul National University, Seoul 08826, Republic of Korea
⁷ SNU Astronomy Research Center, Seoul National University, Seoul 08826, Republic of Korea
⁸ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁹ University of California, Riverside, 900 University Ave., Riverside, CA 92521, USA
¹⁰ Department of Astronomy, Tsinghua University, Haidian DS, 100084 Beijing, China

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Received: 12 March 2026
Accepted: 14 May 2026

Abstract

We present the first results from the SMUGGLE-Ring project, a suite of simulations employing the Stars and MUltiphase Gas in GaLaxiEs (SMUGGLE) interstellar medium (ISM) and stellar feedback model to explore nuclear structures in Milky Way-mass galaxies. We discuss the results from three simulations evolved for 5 Gyr in isolation, in which we vary the classical bulge mass while keeping the disk and halo structures identical. Nuclear stellar disks and rings emerge exclusively in our bulge models, with more massive bulges associated with earlier formation and more extended initial gas reservoirs shortly after bar formation. After gas depletion via active star formation, the nuclear stellar disks bifurcate into pressure-supported nuclear star clusters (NSCs; v_ϕ/σ_R < 0.7) and rotationally supported nuclear stellar rings (NSRs; v_ϕ/σ_R = 1.2–1.7, radii 0.64–0.76 kpc). In our bulgeless model, stellar feedback more effectively disrupts and delays the buildup of stable nuclear gas disks. The enclosed stellar mass of NSCs (∼ 10⁹ M_⊙) dominates over that of NSRs (∼ 10⁸ M_⊙). The star formation rates decline over time due to gas depletion (NSCs 0.1–1 M_⊙ yr⁻¹, NSRs 0.01–0.1 M_⊙ yr⁻¹). Kinematics reveal outward-shifting rotation peaks with σ-drops in NSRs, while a fraction of stars in NSCs exhibits radial shift after 3 Gyr. These findings support inside-out NSD formation via secular bar evolution, with NSRs tracing the star-forming outer edge of the nuclear gas disk and NSCs forming the kinematically hotter inner component. The range of nuclear stellar disk sizes (0.25–0.76 kpc) falls within the observationally inferred ranges, but the existence of larger rings would require external gas flow and/or a longer period of evolution. Future SMUGGLE-Ring extensions will incorporate varying gas fractions, tidal–merger effects, and the circumgalactic medium to further investigate nuclear diversity and outliers.