Isophote Twists in the Nuclear Regions of Barred Spiral Galaxies

Details Isophote Twists in the Nuclear Regions of Barred Spiral Galaxies Isophote Twists in the Nuclear Regions of Barred Spiral Galaxies

Jun 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...273...31s&link_type=abstract

Astronomy and Astrophysics, Vol.273, NO. 1/JUNI, P. 31, 1993

Astronomy and Astrophysics

Astrophysics

112

Scientific paper

We present near-IR (1.2-2.2 micron) surface photometry within the central regions of the intermediate-late type barred spiral galaxies NGC 1097, NGC 4736 and NGC 5728. The observed isophotes show striking (60^deg^-90^deg^) isophote twists with respect to the position angles of the principal bar/lens components. In NGC 1097 and NGC 5728 such twists are immediately interior to circumnuclear rings evident in both our, and in previously published, data. Several possible mechanisms for the formation 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 stellar 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. With the specific aims of modelling this isophotal structure, we have conducted N-body simulations appropriate for the inferred mass distributions in these three galaxies. We find that, in barred galaxies with a sufficient mass concentration, the presence of two inner Lindblad resonances (ILR's) leads to the accumulation of gas in a circumnuclear ring. This ring is phase-shifted with respect to the main stellar bar, via the stable (x2) periodic orbits aligned perpendicular to the bar. Collisions between gas clouds reduce the orthogonality of this phase shift between the gas wave and the stellar bar. We propose that the gas accumulated between the ILR's and onto the circumnuclear ring has sufficient mass to perturb the central stellar component. The latter responds by becoming misaligned with respect to the large-scale stellar bar, thus leading to the observed isophote twists. Since these phase- shifted stellar and gaseous components have the same angular speed as the main bar wave in the rest of the disc, the central isophote twists cannot be attributed to a "bar within a bar". A result of our hypothesis is to identify inner rings and isophote twists as being manifestations of the same physical cause - namely the influence of a barred potential coupled with a high nuclear gas concentration. Given that ~80% of SBab to SBc galaxies are thought to possess inner rings, several more examples of this isophotal structure are anticipated.

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