MOLECULAR GAS IN THE BARRED SPIRAL M-100 - II - (CO)-C-12(1-0) INTERFEROMETER OBSERVATIONS AND NUMERICAL SIMULATIONS

Using the IRAM interferometer we have mapped at high resolution (2.'' 2 x 1.'' 2) the (CO)-C-12(1-0) emission in the nucleus of the doubled barred SABbc spiral M 100. Our synthesized map includes the zero spaci ng flux of the single-dish 30m map (Sempere & Garcia-Burillo, 1997, Pa per I). Molecular gas is distributed in a two spiral arm structure sta rting from the end points of the nuclear bar (r = 600 pc) up to r = 1. 2 kpc, and a central source (r similar to 100 pc). The kinematics of t he gas indicates the existence of a steep rotation curve (v(rot) = 180 km s(-1) at r similar to 100 pc) and strong streaming motions charact eristic of a trailing spiral wave inside corotation. Interpretation of the CO observations and their relation with stellar and gaseous trace rs (K, optical, H alpha, H I and radiocontinuum maps) are made in the light of a numerical model of the clouds hydrodynamics. Gas flow simul ations analyse the gas response to a gravitational potential derived f rom 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 i ndependent bars solution, where the nuclear bar is dynamically decoupl ed and rotates faster than the primary bar. We found the best fit solu tion consisting of a fast pattern (Omega(f) = 160 kms(-1)kpc(-1)) for the nuclear bar (with corotation at R-COR(F)=12 kpc) decoupled from th e slow pattern of the outer bar+spiral (Omega(f)=23 kms(-1)kpc(-1)) (w ith corotation at R-COR(S)=8-9 kpc). As required by non-linear couplin g of spirals (Tagger et al 1987), the corotation of the East pattern f alls in the ILR region of the slow pattern, allowing an efficient tran sfer 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 ot her stellar and gaseous tracers. In the two-bar solution, the gas morp hology and kinematics are strongly varying in the rotating frame of th e slow large-scale bar, and fit the data periodically during a short f raction (about 20%) of the relative nuclear bar period of 46 Myr.