HYDRODYNAMICAL SIMULATIONS OF THE BARRED SPIRAL GALAXY NGC-1365 - DYNAMICAL INTERPRETATION OF OBSERVATIONS

We perform two-dimensional, time dependent, hydrodynamical simulations of the gas flow in a potential representing the barred spiral galaxy NGC 1365 using the FS2 code originally written by G.D. van Albada. Non -circular motions present in NGC 1365 prevent us from using an observe d angle-averaged rotation curve as representative of the axisymmetric forces in the bar region. Comparing observed kinematics and gas featur es in NGC 1365 with corresponding quantities in the models enables us to estimate a possible representation of the true axisymmetric forces. We compute models perturbed by a pure bar potential and models pertur bed by a bar + spiral potential. A J-band image is used to estimate th e bar potential under the assumption of a constant M/L(J) ratio. The s piral potential is determined from the same image and has the same pat tern speed as the bar, but is not assumed to have necessarily the same M/L(J) ratio. We find good agreement between observations and models, both concerning the H I and CO density distribution and the velocity field. Using just the shape of the observed bar potential we drive the main spiral features out to the required radius. Our best fitting bar model has a pattern speed of Omega(p) = 20 km s(-1) kpc(-1) placing c orotation ata distance R(CR) similar to 145'' = 1.21R(bar), where R(ba r) is the optical semimajor axis of the bar. The offset dust lanes obs erved in optical images are reproduced by the models provided the latt er have an inner Lindblad resonance at a radius of similar to 27''. St eep velocity gradients across the dust lanes and spiral arms are seen in both model and observations. Most observed features are reproduced in the models, and the general streaming motions are in qualitative ag reement with linear gas orbit theory. Contrary to observations, the pu re bar perturbed models cannot drive an inner arm across corotation. M odels having both a bar and spiral perturbing potential reduce this pr oblem, thus suggesting the existence of massive spiral arms in NGC 136 5. Our best fitting bar + spiral model has a pattern speed of Omega(p) = 18 km s(-1) kpc(-1) placing corotation at a distance R(CR) similar to 157'' = 1.31 R(bar).