Interpretation of the CO observations and their relation with stellar and gaseous tracers (K, optical, H , HI and radiocontinuum maps) are made in the light of a numerical model of the clouds hydrodynamics. Gas flow simulations analyse the gas response to a gravitational potential derived from the K-band plate, including the two nested bars. We develop two families of models: first, a single pattern speed solution shared by the outer bar+spiral and by the nuclear bar, and secondly, a two independent bars solution, where the nuclear bar is dynamically decoupled and rotates faster than the primary bar.
We found the best fit solution consisting of a fast pattern ( =160 kms kpc ) for the nuclear bar (with corotation at R =1.2 kpc) decoupled from the slow pattern of the outer bar+spiral ( =23 kms kpc ) (with corotation at R =8-9 kpc). As required by non-linear coupling of spirals (Tagger et al 1987), the corotation of the fast pattern falls in the ILR region of the slow pattern, allowing an efficient transfer of molecular gas towards the nuclear region. Solutions based on a single pattern hypothesis for the whole disk cannot fit the observed molecular gas response and fail to account for the relation between other stellar and gaseous tracers. In the two-bar solution, the gas morphology and kinematics are strongly varying in the rotating frame of the slow large-scale bar, and fit the data periodically during a short fraction (about 20%) of the relative nuclear bar period of 46 Myr (Fig. 10).
Figure 10: We display the particle orbits for molecular clouds in the region where the bar instability develops, as they are seen, firstly, a(top): from the frame rotating at =23kms kpc , for the solution of a single slow pattern and b(bottom): from the frame rotating at =23kms kpc for the best-fit solution of a double pattern ( =160kms kpc (from r=0 to 10 ), =23kms kpc (for r>10 ). The length of the arrows is proportional to the particle speed in the rotating frame. These simulations illustrate the efficiency of fast nuclear bars, dynamically decoupled form the outer bar+spiral structure, in driving the gas towards the nucleus, hence accounting for the observations.