ISOPHOTE TWISTS IN THE NUCLEAR REGIONS OF BARRED SPIRAL GALAXIES

We present near-IR (1.2-2.2 mum) surface photometry within the central regions of the intermediate-late type barred spiral galaxies NGC1097, NGC4736 and NGC5728. The observed isophotes show striking (60-degrees -90-degrees) isophote twists with respect to the position angles of th e principal bar/lens components. In NGC1097 and NGC5728 such twists ar e immediately interior to circumnuclear rings evident in both our, and in previously published, data. Several possible mechanisms for the fo rmation of such structures within barred potentials are considered. It is argued that the lack of evidence for active star formation within those regions dominated by the isophotal twists implies that they are not due to the presence of M supergiants formed within inner gas flows . Furthermore, from N-body simulations using self-gravitating star and gas components, we think unlikely that such twists are a purely stell ar dynamical phenomenon: stellar isophote twists are unstable as they are quickly destroyed by dynamical heating in the barred potential. A combined stellar and gas dynamical scheme appears most appropriate. Wi th the specific aims of modelling this isophotal structure, we have co nducted N-body simulations appropriate for the inferred mass distribut ions in these three galaxies. We find that, in barred galaxies with a sufficient mass concentration, the presence of two inner Lindblad reso nances (ILR's) leads to the accumulation of gas in a circumnuclear rin g. This ring is phase-shifted with respect to the main stellar bar, vi a the stable (x2) periodic orbits aligned perpendicular to the bar. Co llisions between gas clouds reduce the orthogonality of this phase shi ft between the gas wave and the stellar bar. We propose-that the gas a ccumulated between the ILR's and onto the circumnuclear ring has suffi cient mass to perturb the central stellar component. The latter respon ds by becoming misaligned with respect to the large-scale stellar bar, thus leading to the observed isophote twists. Since these phase-shift ed stellar and gaseous components have the same angular speed as the m ain bar wave in the rest of the disc, the central isophote twists cann ot be attributed to a ''bar within a bar''. A result of our hypothesis is to identify inner rings and isophote twists as being manifestation s of the same physical cause - namely the influence of a barred potent ial coupled with a high nuclear gas concentration. Given that approxim ately 80% of SBab to SBc galaxies are thought to possess inner rings, several more examples of this isophotal structure are anticipated.