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A.R. Thakar (The Johns Hopkins University), B.S. Ryden (The Ohio State University)
We investigate, via numerical simulation, the formation of kinematically decoupled bulges (KDBs) and cores (KDCs), including counterrotating bulges/cores, in spiral galaxies. KDBs arise when the entire bulge follows the decoupled kinematics, whereas KDCs are inner regions of bulges that are decoupled from the kinematics of the rest of the bulge and the disk. The existence of KDCs in ellipticals has been known for some time, and major and minor merger models have been invoked to explain most of these. In the case of spirals, however, there are stronger constraints on the viable models because the primary disks must be able to survive the process(es) responsible for producing KDBs/KDCs. A minor merger with a dense or massive satellite produces an unacceptable level of heating in the disk even when it survives the interaction. A gas-rich satellite mitigates this effect, but not by much unless the satellite is predominantly gaseous. A less dense satellite that gets tidally stripped prior to merging is less damaging to the disk, but it is also less likely to form a KDB or KDC.
Motivated by these difficulties with the merger model, we also consider an alternative, inside-out model in which a rapidly rotating faint elliptical, or an early-type spiral with a prominent bulge, acquires a gas disk on an orbit inclined with respect to the plane of rotation of the elliptical/bulge, thereby resulting in a bulge that is kinematically decoupled with respect to the newly acquired disk. This model can presumably only produce KDBs, not KDCs. Special cases of this scenario are polar and retrograde orbits, with which we attempt to produce orthogonal (e.g. NGC 4698) and counterrotating (e.g. NGC 7331) disk/bulge systems respectively. The accretion of the gas disk is by means of gas infall in our current models, although we intend to try inclined-orbit mergers with gas-rich dwarfs also in the future.
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