Morphologies arising from the gas flow in the innermost kiloparsec of barred galaxy models

¹ Max-Planck-Institut für Extraterrestrische Physik (MPE) Gießenbachstr. 1 D-85748 Garching, Germany
² Max-Planck-Institut für Astrophysik (MPA) Karl-Schwarzschild-Str. 1 D-85748 Garching, Germany
³ Research Center for Astronomy and Applied Mathematics, Academy of Athens Soranou Efessiou 4 11527 Athens, Greece
⁴ Aix Marseille Université, CNRS, LAM (Laboratoire d’ Astrophysique de Marseille), UMR 7326 13388 Marseille 13, France

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Received: 4 August 2025
Accepted: 23 November 2025

Abstract

Context. We study a series of response models to investigate the formation of specific morphological features in the central 1 kpc region of the gas component in barred spiral galaxies.

Aims. We aim to understand how structures, such as nuclear rings and spirals, form by varying the parameters of a general gravitational potential and gas properties. Our goal is to determine how much the shape of these structures is driven by the orbital dynamics of the models compared to the influence of the hydrodynamics of the gas. In particular, we examine the effects of the bar strength, bar shape, pattern speed, and central density, as well as their mutual interdependence.

Methods. We modeled the gas flow using hydrodynamical simulations run with the Eulerian RAMSES code. The underlying gravitational potential was a two-dimensional Ferrers bar and the gas was considered to be isothermal. Alongside analyzing the gas response to the imposed gravitational potentials, we carried out orbital studies for all models. This involved assessing the shapes and stability of periodic orbits and analyzing the distribution of regular versus chaotic regions within the systems.

Results. The parameters of the gravitational potential alone are insufficient to accurately predict the gas dynamics in a system. The morphology of the gaseous response varies substantially with changes in sound speed, emphasizing the fundamental role of hydrodynamic processes in determining the structure of the gas within the central region. We identify the factors that affect the morphology of nuclear rings and trailing and leading nuclear spirals. The best alignment between our models and structures observed in local barred galaxies is achieved by assuming a sound speed of c_s = 20 km s⁻¹.