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

TL;DR

This study interrogates how the innermost kiloparsec gas morphology in barred galaxies forms by disentangling orbital dynamics from hydrodynamics. Using RAMSES, it simulates isothermal gas responses to a Ferrers-bar potential across five main models and two very weak-bar analogs, varying the sound speed $c_s$ to probe pressure effects. The results show that potential parameters alone do not predict gas structures; the gas morphology is critically shaped by hydrodynamics, especially $c_s$, with ILRs promoting nuclear rings and x2-driven spirals, while higher $c_s$ suppresses leading spirals and favors trailing, grand-design patterns. The work highlights the pivotal role of pressure support in shaping central gas features and provides a framework for interpreting observed nuclear rings and spirals under different bar strengths and central mass concentrations, noting that best matches to local galaxies occur at $c_s\approx 20$ km s$^{-1}$ within this isothermal, non-self-gravitating approximation.

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 $c_s=20\,\rm{km\,s^{-1}}$.

Figures (23)

• Figure 1: Rotation curve of model R001. The blue curve corresponds to the axisymmetric part of the potential, while the orange one to the full potential.
• Figure 2: (a) Characteristic diagrams for the x1, x2 and x4 families in model R001. The zero velocity curve is depicted with a green color and indicated with "ZVC." (b) Hénon index, $\alpha$, for the same families. We find no changes in the stability of any of these families.
• Figure 3: Typical surface of section for $E_J < E_J(\text{iILR})$ of model R001, at $E_J=-240000$. The x4 periodic orbit is located at the center of the left stability island, while x1 is at the center of the right one.
• Figure 4: Orbits of the x1 and x2 families in model R001. The flow in our response model follows either the black x1, or the magenta x2 orbits. The gray x1 orbits exist for $E_J$'s, where only x2 orbits are populated. The labels from "1" to "6" indicate successively the x2 orbits in descending $E_J$ order.
• Figure 5: Model R001: Central regions of three models with different sound speeds, (a) $c_s=2$ km s$^{-1}$, (b) $c_s=10$ km s$^{-1}$, and (c) $c_s=20$ km s$^{-1}$. A leading spiral is apparent in (a) and (b), while in (c) we have the formation of a smaller pseudo-ring.