1998A&A...333..864G

Using the IRAM interferometer we have mapped at high resolution (2.2"x1.2") the ¹²CO(1-0) emission in the nucleus of the doubled barred SABbc spiral M 100. Our synthesized map includes the zero spacing flux of the single-dish 30m map (Sempere & Garcia-Burillo, 1997A&A...325..769S, Paper I). Molecular gas is distributed in a two spiral arm structure starting from the end points of the nuclear bar (r=600pc) up to r=1.2kpc, and a central source (r∼100pc). The kinematics of the gas indicates the existence of a steep rotation curve (v_rot=180km/s at r∼100pc) and strong streaming motions characteristic of a trailing spiral wave inside corotation. Interpretation of the CO observations and their relation with stellar and gaseous tracers (K, optical, Hα, 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 R and K-band plates, 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 ({OMEGA}_f=160km/s/kpc) for the nuclear bar (with corotation at R^F_COR=1.2kpc) decoupled from the slow pattern of the outer bar+spiral ({OMEGA}_f=23km/s/kpc) (with corotation at R^S_COR=8-9kpc). 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 46Myr.