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 field of the outer bars. We analyze the flow pattern in such nested-bar systems under the conditions of negligible self-gravity and find that secondary bars differ fundamentally from their large-scale counterparts in gas flow pattern and other dynamical properties. In particular, the gas flow 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 flow 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 find 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 flow as well. Therefore, no large-scale shocks form in the nuclear bars, and consequently, no offset dust lanes are expected there. We find that offset 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.