Molecular Gas in NUclei of GAlaxies (NUGA). I. The counter-rotating LINER NGC 4826

We present new high-resolution observations of the nucleus of the counter-rotating LINER NGC 4826, made in the J=1-0 and J=2-1 lines of 12CO with the IRAM Plateau de Bure mm-interferometer(PdBI).The CO maps, which achieve 0.8arcsec (16 pc) resolution in the 2-1 line, fully resolve an inner molecular gas disk which is truncated at an outer radius of 700 pc. The total molecular gas mass (3.1*E8 Msun) is distributed in a lopsided nuclear disk of 40 pc radius, containing 15% of the total gas mass, and two one-arm spirals, which develop at different radii in the disk. The distribution and kinematics of molecular gas in the inner 1 kpc of NGC 4826 show the prevalence of different types of m=1 perturbations in the gas. Although dominated by rotation, the gas kinematics are perturbed by streaming motions related to the m=1 instabilities. The non-circular motions associated with the inner m=1 perturbations (lopsided instability and inner one-arm spiral) agree qualitatively with the pattern expected for a trailing wave developed outside corotation (``fast'' wave). In contrast, the streaming motions in the outer m=1 spiral are better explained by a ``slow'' wave. A paradoxical consequence is that the inner m=1 perturbations would not favour AGN feeding. An independent confirmation that the AGN is not being generously fueled at present is found in the low values of the gravitational torques exerted by the stellar potential for R<530 pc. The distribution of star formation in the disk of NGC 4826 is also strongly asymmetrical. The observed asymmetries, revealed by HST images of the inner disk, follow the scales of the various m=1 perturbations identified in the molecular gas disk. Massive star formation is still vigorous, fed by the significant molecular gas reservoir at R<700 pc. There is supporting evidence for a recent large mass inflow episode in NGC 4826. The onset of m=1 instabilities of the type observed in NGC 4826 may be a consequence of secular evolution of disks with high gas mass contents.