Circumnuclear structure and kinematics in the active galaxy NGC 6951

A study is presented of the central structure and kinematics of the galaxy NGC 6951, by means of broad band B'IJK images and high resolution high disp ersion longslit spectroscopy, together with archival HST WFPC2 V and NICMOS 2 J and H images. We find that there is little ongoing star formation insid e the bar dominated region of the galaxy, except for the circumnuclear ring at 5 arcsec radius. There is some evidence that this star formation occurs in two modes, in bursts and continuously, along the ring and inwards, towa rds the nucleus. The equivalent width of the Call triplet absorption lines show that, in the metal rich central region, the continuum is dominated by a population of red supergiants, while red giants dominate outside. The gas eous kinematics along three slit position angles, and the comparison with t he stellar kinematics, suggest the existence of a hierarchy of disks within disks, whose dynamics are decoupled at the two inner Linblad resonances (I LR), that we find to be located at 180 pc and at 1100 pc. This is supported by the structure seen in the high resolution HST images. The nucleus is sp atially resolved in the emission line ratio [NII]/H alpha, and in the FWHM of the emission lines, within a radius of 1.5", just inside the innermost I LR. Outside the iILR, the stellar CaT velocity profile is resolved into two different components, associated with the bar and the disk. Several result s indicate that this is a dynamically old system: the little ongoing star f ormation inside the bar dominated part of the galaxy, the very large relati ve amount of molecular to total mass within the inner 6 arcsec radius, simi lar to 25%, and the geometry of the circumnuclear ring that leads the stell ar bar at a position angle greater than 90 degrees. It is thus possible tha t a nuclear bar has existed in NGC 6951 that drove the gas towards the nucl eus, as in the bars within bars scenario, but that this bar has already dis solved by the gas accumulated within the circumnuclear region. We discuss t he possibility that the kinematical component inside the ilLR could be due to a nuclear outflow produced by the combined effects of SN and SN remnants , or to a nuclear disk, as in the disk within disk scenario that we propose fur the fueling of the AGN in NGC 6951.