Circumnuclear structure and kinematics in the active galaxy NGC 6951

Abstract

A study is presented of the central structure and kinematics of the galaxy NGC 6951, by means of broad band B{\arcminIJK} images and high resolution high dispersion longslit spectroscopy, together with archival HST WFPC2 V and NICMOS2 J and H images. We find that there is little ongoing star formation 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 equivalent width of the Ca Ii 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 gaseous kinematics along three slit position angles, and the comparison with the stellar kinematics, suggest the existence of a hierarchy of disks within disks, whose dynamics are decoupled at the two inner Linblad resonances (ILR), 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 spatially resolved in the emission line ratio [N Ii]/Hα , and in the FWHM of the emission lines, within a radius of 1.5{\arcsec}, just inside the innermost ILR. Outside the iILR, the stellar CaT velocity profile is resolved into two different components, associated with the bar and the disk. Several results indicate that this is a dynamically old system: the little ongoing star formation inside the bar dominated part of the galaxy, the very large relative amount of molecular to total mass within the inner 6 arcsec radius, ~ 25%, and the geometry of the circumnuclear ring that leads the stellar bar at a position angle greater than 90°. 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. We discuss the possibility that the kinematical component inside the iILR 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 for the fueling of the AGN in NGC 6951. Based on observations made with the 4.2m William Herschel Telescope operated by the Isaac Newton Group, and the 2.6m Nordic Optical Telescope operated jointly by Denmark, Finland, Iceland, Norway, and Sweden, on the island of La Palma in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Also based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the ESA Space Telescope European Coordinating Facility.