Nested Bars in Disk Galaxies: No Offset Dust Lanes in Secondary Nuclear Bars

Abstract

Under certain conditions, subkiloparsec nuclear bars form inside large-scale stellar bars of disk galaxies. These secondary bars spend a fraction of their lifetime in a dynamically decoupled state, tumbling in the gravitational Ðeld of the outer bars. We analyze the Ñow pattern in such nested-bar systems under the conditions of negligible self-gravity and Ðnd that secondary bars di †er fundamentally from their large-scale counterparts in gas Ñow pattern and other dynamical properties. In particular, the gas Ñow across the bar-bar interface in these systems can be more chaotic or more regular in nature and, contrary to predictions, has no difficulty in penetrating the secondary bars along the primary large-scale shocks. The outer parts of both short and long nuclear bars (with respect to their corotation) appear to be depopulated of gas, while deep inside them the Ñow exhibits low Mach numbers and follows oval-shaped orbits with little dissipation. Long nuclear bars remain gas-rich longer and for this relatively short period of time are largely of a rectangular shape, again with a small dissipation. We Ðnd that gas-dominated and star-dominated nuclear bars avoid the bar-bar interface, making both types of bars short relative to their corotation. Furthermore, our earlier work has shown that dynamically coupled secondary bars exhibit a similarly relaxed low-dissipation Ñow as well. Therefore, no large-scale shocks form in the nuclear bars, and consequently, no o †set dust lanes are expected there. We Ðnd that o †set dust lanes cannot be used in the search for secondary (nuclear) bars. Finally, we discuss the importance of gas self-gravity in the further evolution of these systems.