Summary and conclusions

We have obtained broad band BIJK44#44 images and high resolution high dispersion longslit spectroscopy for the ionized gas (around H5#5 to [Oiii] and H1#1 to [Sii]), and for the stellar populations (in the Caii triplet lines around 8500 Å) of the galaxy NGC 6951. Together with archival HST VJH images, we analyse these data to study the central structure and kinematics and find that:

• There is little star formation ongoing inside 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 towards the nucleus.
• The nuclear spectrum shows both very high excitation and very strong [Nii] and [Sii] lines, making the classification of the AGN somewhat uncertain between a high excitation LINER and a possibly high nitrogen abundant Seyfert 2, depending mainly on the uncertain correction for underlying absorption in H5#5. The electron density varies between 300 cm^-3 and 1000 cm^-3.
• The equivalent width of the Caii triplet absorption lines show that in the metal rich central region of this galaxy, within 5 radius, the continuum light is dominated by a population of red supergiant stars, while outside the circumnuclear ring the stellar population is that of giants.
• We suggest that the gaseous and stellar kinematics along the three slit position angles can be interpreted as the existence of a hierachy of disks within disks, with dynamics decoupled at the two ILRs, that we find to be located at 180 pc and at 1100 pc. This would be supported by the structure seen in the high resolution HST images.
• The nucleus is partly resolved within a radius of 1.5 (just inside the iILR) both in the emission line ratio [Nii]/H1#1, and in the FWHM of the emission lines.
• Outside the iILR the stellar CaT velocity profile can be partly resolved into two different components that seem to be associated to the bar and to a disk.
• We discuss the possibility that the kinematic component inside the iILR could be due to a nuclear disk, as in the disk within disk scenario suggested above, or to a nuclear outflow produced by the combined effects of SN and SN remnants.
• Several clues indicate that this is a dynamically old system: (i) there is little star formation ongoing inside the bar dominated part of the galaxy (except for the circumnuclear ring), (ii) the relative amount of molecular to total mass within the inner 6 arcsec radius is very large 2#2%, and (iii) the geometry of the circumnuclear ring leading at a position angle greater than 90 from the stellar bar. It is thus possible that a nuclear bar has existed in NGC 6951 that drove the gas towards the nucleus, as in the bars within bars scenario, but that this bar has already dissolved by the gas accumulated within the circumnuclear region.

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