The centers of spiral galaxies are frequent sites of activity, marked by the presence of intense star formation (SF) and/or an active galactic nucleus (AGN). This activity needs to be fueled with a supply of gas, whose reservoir can be provided by the disk of spiral galaxies. An efficient way for this gas to lose its angular momentum is provided by the existence of non-axisymmetric components of the galactic potential, together with a strong central gravitational potential. The latter is necessary to explain that AGNs, irrespective of the presence of morphological perturbations, appear preferentially in early type spirals, with the frequency peak at Sb. On the other hand, galaxies dominated by disk star formation occur preferentially in later types with a distribution peaking at Scd (Moles et al. 1995; Ho et al. 1997a and references therein).
The existence of gas to fuel the circumnuclear activity is necessary but not sufficient (e.g., Moles et al. 1995; Maoiolino et al. 1997; Mulchaey & Regan 1997). The right dynamical and physical conditions must exist for this gas to be used effectively in either infalling to the nucleus proper, and feeding the AGN or nuclear starburst, or collapsing by self-gravity in the circumnuclear region in the form of intense SF. There are examples of galaxies where the gas is known to be available in the central regions, but where the onset of SF has yet to occur. For example, NGC 4151, which harbours a Seyfert type 1 nucleus, has a circumnuclear ring of material discovered in the form of dust extinction, and has a large column of neutral hydrogen towards the nucleus, but there is no significant star formation occurring at present (Robinson et al. 1994; Vila-Vilaró et al. 1995; Mundell et al. 1995). However, in the circumnuclear ring of star formation in the target galaxy of this paper, NGC 6951, Kohno et al. (1999) find that although the dynamical shocks provide the mechanism for the accumulation of molecular gas along the ring, this may not be the mechanism responsible for the star formation, but rather the gravitational instability of the clouds thus formed is the driving force for the onset of star formation.
To understand what are the conditions and mechanisms for the onset of nuclear activity in spiral galaxies, a detailed characterisation of morphological and kinematical components in galaxies of different morphology and activity level is needed. The DEGAS collaboration was established to address such a detailed study in a small sample of spirals with an AGN, selected as isolated to avoid the possible external contribution to the assymmetries in the potential. We plan to procede with a parallel analysis of a sample of isolated non-AGN spirals, to look for differences and/or similarities and their implications. IR imaging data of the complete sample are presented and analysed in Márquez et al. (1999a, 1999b). Other recent IR imaging studies of active and control samples include McLeod & Rieke (1995), Mulchaey et al. (1997), and Peletier et al. (1998). In this paper we present the first results for the SAB(rs)bc galaxy NGC 6951. There are a number of studies of this galaxy that include broad and narrow band optical and infrared images (Buta & Crocker 1993; Márquez & Moles 1993, hereafter MM93; Barth et al. 1995; Wozniak et al. 1995; Elmegreen et al. 1996; Friedli et al. 1996; Rozas et al. 1996a,b; González Delgado et al. 1997; González Delgado & Pérez 1997; Mulchaey et al. 1997), spectroscopy of the nuclear region (e.g., Boer & Schulz 1993; Filippenko & Sargent 1985; Muñoz-Tuñón et al. 1989; MM93; Ho et al. 1995, 1997b; Ho et al. 1997c), radio interferometric maps (Vila et al. 1990; Saikia et al. 1994), and high resolution spectroscopic molecular maps (Kenney et al. 1992; Kohno et al. 1999). To better characterize the detailed kinematical components and their relation to the morphological structures, we have obtained high resolution, high dispersion spectroscopic observations of the gaseous and of the stellar components, and direct imaging in optical and infrared bands.
This paper is structured as follows. In section 2 we present the imaging and spectroscopic data. Sections 3 and 4 detail the results from the analysis of the images and from the spectroscopy respectively, while in section 5 we discuss these results into a common picture of NGC 6951. Section 6 gives the summary and our main conclusions.