  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.
