Molecular gas in NUclei of GAlaxies (NUGA) X. The Seyfert 2 galaxy NGC 3147

We present (12)CO(1 - 0) and (12)CO(2 - 1) observations of the SA(rs) bc Seyfert 2 galaxy NGC3147, obtained with the IRAM interferometer at 1 ''.9 x 1 ''.6 and 1 ''.x 6 x 1 ''.x 4 resolutions, respectively. We have also observed the central region of NGC3147 with the IRAM 30m telescope (at resolutions of 22 '' and 12 '' for (12)CO(1 - 0) and (12)CO(2 - 1), respectively), in order to obtain complete sampling at low spatial frequencies. These observations have been made in the context of the NUclei of GAlaxies (NUGA) project, aimed at the study of the different mechanisms for gas fueling of active galactic nuclei (AGN). A central peak seen mainly in (12)CO(2 - 1) and a ring-like structure at r similar or equal to 10 '' similar to 2 kpc dominate the (12)CO maps. In (12)CO(1 - 0) an outer spiral at r similar or equal to 20 '' similar to 4 kpc is also detected, not visible in (12)CO(2 - 1) emission because it falls outside the field-of-view of the primary beam. The average I(21)/I(10) line ratio is similar to 0.7 in temperature units over the region mapped in both lines, consistent with the optically thick emission expected in the nuclei of spiral galaxies. The kinematics of the molecular structures are quite regular, although there is evidence for local non-circular or streaming motions. We show that the molecular gas distribution is similar but not exactly identical to those of star formation tracers, i.e., infrared (Spitzer) and ultraviolet (GALEX) emission. This agreement is consistent with a scenario of steady, ongoing formation of stars from the molecular clouds at a rate of similar to 1 M circle dot yr(-1) within the innermost 4 kpc in radius. Using a near-infrared (NIR) image obtained with adaptive optics at the Canada-France-Hawaii Telescope (CFHT), we identify a weak bar in NGC3147, which is classified as non-barred galaxy in the optical. We then compute the gravity torques exerted by this stellar bar on the gas. The torque is obtained first at each point in the map, and then azimuthally averaged with a weighting determined by the gas surface density traced by the CO emission. We find that the gas inside the inner CO ring is subject to a net negative torque and loses angular momentum. This is expected for gas at the ultra-harmonic resonance (UHR), just inside the corotation resonance of the stellar bar. In contrast, the gas outside corotation, in the spiral arms comprising the outer spiral structure, suffers positive torques and is driven outwards. We conclude that some molecular gas is presently flowing into the central region, since we find negative torques down to the resolution limit of our images.