1 Introduction

The possible relation between active galactic nuclei (AGN) and the starburst phenomenon in galactic circumnuclear zones has been widely treated in recent years. In this context there is a special interest in studying those galaxies in which nuclear activity and violent star formation are both observed. Arsenault (1989) showed a correlation between the presence of circumnuclear rings of star formation and nuclear activity, though the physical connection between them was not clear. It is also clear that one should not try to interpret nuclear activity without considering the framework of the host galaxy (see e.g. Moles et al. 1986). We have chosen to observe NGC 6951, a barred spiral with active star formation and also nuclear activity, with these problems in mind. It is noteworthy that in those spirals which contain bright circumnuclear hot spots, such as NGC 6951, the presence of these and other distinctive features shows a strong tendency to be related to the presence of a bar. In their pioneering study of this phenomenon, Sersic & Pastoriza (1967) found that in a quite sizeable sample of 174 bright galaxies, all the cases of striking circumnuclear morphology occur in galaxies which are either barred, SB, or mixed SAB. In a comprehensive follow up to this programme, Buta & Crocker (1993) put together an extensive catalogue of galaxies with "nuclear rings", in which many of the cases of circumnuclear star formation are located in localized hot spots which collectively form a kiloparsec scale ring or spiral pattern around the nucleus. Theoretical modelling studies suggest that star-forming annular zones should develop in those barred galaxies where gas, plus dust, flow inwards from an inner Lindblad resonance (ILR), or between two ILR's (Combes & Gerin 1985; Knapen et al. 1995; Ho et al. 1997). Multi-wavelength studies of these objects have shown that these rings are sites of intense star formation, with high supernova rates (Hummel et al. 1987; Wilson et al. 1991; Forbes et al. 1994b). A review of observations of nuclear hot spots can be found in Kennicutt (1994).

The kinematics of the circumnuclear zone is often more complex than the large-scale disc kinematics, due to a combination of factors: (a) gas flows induced by non-axisymmetric potentials, e.g. due to bars, which alter the mass distribution around the centre far more than in other zones; (b) the formation of multiple resonances; (c) possible nuclear activity, induced by the mass concentration. However in order to understand fully the circumnuclear kinematics, one must consider it in the context of the dynamics of the whole galaxy. There is observational evidence, supported by theoretical models, that significant mass redistribution can occur within galaxies on timescales short compared with the Hubble time (Friedli & Martinet 1993; Berentzen et al. 1998). This can occur via processes internal to the galaxy, associated with non-axisymmetric components of the gravitational potential, and more readily if there is an external torque, which can exert a powerful influence, above all on the gas components of discs, but also on their stellar components (Shlosman 1990; Zhang 1996; Combes 1988). Barred galaxies show frequent evidence of secular dynamical evolution; they often show "starburst" activity in their central zones, with well marked circumnuclear regions of star formation (Kennicutt 1994; Buta & Combes 1996), on $\sim$1 kpc scales. High resolution imaging in emission lines notably in H$\alpha$ reveal these inner structures, which can also be detected via interferometry in their molecular emission via CO, and where adequate resolution can be applied, even at 21 cm in H I.

The evolution of these circumnuclear regions can be followed by combined observations of a number of key parameters, which allow us to study the rate of inflow of gas along the bar, the star formation efficiency in the circumnuclear zone, and the dynamical effects of the gas and stars accumulating around the nucleus. These parameters include the surface densities of the gas components, the surface emissivity of the ionized gas, and the velocity fields both in gas and stars (the former shows the direct gas response to all the gravitational influences, and the latter can provide the underlying potential). The best way to proceed is in fact to measure the relevant parameters not only with the inner 1 kpc but also out along the bar and into the disc. With these ends in view, we planned observations of the ionized gas in NGC 6951 via H$\alpha$ emission, using Fabry-Perot interferometry. This technique permits us to obtain kinematic information across the whole face of a galaxy at once; it offers a three-dimensional "data cube" of intensity versus position in each of a set of discretized wavelength channels, from which moment maps of integrated intensity, radial velocity, and velocity dispersion along the line of sight can be extracted. From this data set we can use the ionized hydrogen as a tracer of star formation, and of velocity fields within and perpendicular to the plane of the galaxy, as well as dynamical processes within individual star forming zones. The types of velocity fields anticipated include "streaming motions" across spiral arms, or gas flows in the direction of the major bar axis; measurements of these fields allow us to infer the response of the gas to the underlying potentials involved (cf. Knapen et al. 1997; Rozas et al. 2000). The technique for this is to assume symmetry in the underlying rotation curve, and first order reflection symmetry in the rest of the velocity pattern; the rotational curve is then subtracted from the observed field, and the residual field enables the non-circular components to be fairly well estimated. It is also possible to use the data cube to extract the point by point velocity dispersion map, from which the internal motions of dynamically active regions, and any velocity components perpendicular to the plane of the galaxy may be derived (see Combes & Becquaret 1997; Rozas et al. 1998).

In the present article we apply this technique to study kinematically the barred active spiral NGC 6951 from a "TAURUS" Fabry-Perot map of NGC 6951. In Sect. 2 we give a descriptive summary of the previously measured properties of the galaxy, in Sect. 3 we detail the observations, their reduction, and the preliminary analysis, in Sect. 4 we analyze the brightness distribution of the emitting ionized gas, in Sect. 5 we analyze the velocity distribution, in Sect. 6 we go into more depth in a treatment of the circumnuclear region, and in Sect. 7 we set out our conclusions.

Figure 1: Optical image of NGC 6951.

Figure 2: Planes of the H$\alpha$ high resolution (1.3 $^{\prime \prime }$) data cube for NGC 6951 (before "cleaning'' the data cube; see text for details) at a series of velocities about the central systemic value of 1422 km s-1.

Figure 2: continued.

Figure 3: Intensity map (zeroth moment) of the H$\alpha$ emission in NGC 6951 obtained with the high resolution (1.3 $^{\prime \prime }$) H$\alpha$ data cube.