Orbital Support of Fast and Slow Inner Bars in Double Barred Galaxies

TL;DR

This study investigates how the orbital support of the inner bar in double-barred galaxies varies with its pattern speed, revealing limits for stable support and morphological changes linked to orbital dynamics.

Contribution

It provides a comprehensive analysis of the relationship between inner bar pattern speed and orbital support, connecting orbital dynamics to observed galaxy morphologies.

Findings

Orbital support disappears below a minimum pattern speed.

High pattern speeds produce more round, less distinct inner bars.

Lower pattern speeds result in more eccentric, pulsating, and less coherent bars.

Abstract

We analyze how the orbital support of the inner bar in a double-barred galaxy (nested bars) depends on the angular velocity (i.e. pattern speed) of this bar. We study orbits in seven models of double bars using the method of invariant loops. The range of pattern speed is covered exhaustively. We find that not all pattern speeds are allowed when the inner bar rotates in the same direction as the outer bar. Below a certain minimum pattern speed orbital support for the inner bar abruptly disappears, while at high values of this speed the orbits indicate an increasingly round bar that looks more like a twist in the nuclear isophotes than a dynamically independent component. For values between these two extremes, orbits supporting the inner bar extend further out as the bar's pattern speed decreases, their corresponding loops become more eccentric, pulsate more, and their rotation becomes increasingly non-uniform, as they speed up and slow down in their motion. Lower pattern speeds also lead to a less coherent bar, as the pulsation and acceleration increasingly varies among the loops supporting the inner bar. The morphologies of fast and slow inner bars expected from the orbital structure studied here are recently recovered observationally by decomposition of double barred galaxies. Our findings allow us to link the observed morphology to the dynamics of the inner bar.