Closing the gap: secular evolution of bar-induced dark gaps in presence of thick discs

TL;DR

This study uses N-body simulations to explore how dark gaps form and evolve in barred galaxies, revealing their properties are linked to bar strength and are not universally associated with bar resonances.

Contribution

It provides a systematic analysis of dark gap formation in both thin and thick disc galaxy models, challenging previous assumptions about their relation to bar resonances.

Findings

Dark gaps result from star trapping by the bar.

Dark gap properties correlate with bar strength.

Dark gaps are not universally linked to bar resonances.

Abstract

The presence of dark gaps, a preferential light deficit along the bar minor axis, is observationally well known. The properties of dark gaps are thought to be associated with the properties of bars, and their spatial locations are often associated with bar resonances. However, a systematic study, testing the robustness and universality of these assumptions, is still largely missing. Here, we investigate the formation and evolution of bar-induced dark gaps using a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs with varying thick disc mass fraction, and different thin-to-thick disc geometry. We find that dark gaps are a natural consequence of the trapping of disc stars by the bar. The properties of dark gaps (such as strength and extent) are well correlated with the properties of bars. For stronger dark gaps, the fractional mass loss along the bar minor axis can reach up to ~60-80 percent of the initial mass contained, which is redistributed within the bar. These trends hold true irrespective of the mass fraction in the thick disc and the assumed disc geometry. In all our models harbouring slow bars, none of the resonances (corotation, Inner Lindblad resonance, and 4:1 ultra-harmonic resonance) associated with the bar correspond to the location of dark gaps, thereby suggesting that the location of dark gaps is not a universal proxy for these bar resonances, in contrast with earlier studies.