4 Distribution of the ionized gas

In Fig. 3 we show the map of the line intensity in H$\alpha$ which brings out the brightest H II regions. Previous observations in the visible (Márquez & Moles 1993; Wozniak et al. 1995; Rozas et al. 1996a, b; González-Delgado et al. 1997) and in radio continuum emission at 6 cm. Vila et al. (1990) and Saikia et al. (1994) show that the emission is mainly concentrated in the central zone of the galaxy, within a few hundred pc of the nucleus. All these observations indicate vigorous star formation in a zone of some 6 $^{\prime \prime }$$\times$ 9 $^{\prime \prime }$ around the nucleus (Barth et al. 1995; Buta & Crocker 1993). This zone contains giant complexes of H II regions, which are much more luminous than the majority of regions. Outside this zone the H$\alpha$ emission is symmetrically distributed principally in the spiral arms. A more detailed map of the H$\alpha$ emission in the circumnuclear zone is shown in Fig. 4.

We have used our calibrated photometric H$\alpha$ image, previously presented in Rozas et al. (1996a), together with an estimated extinction, extrapolated from the visual value A_v = 3.4 mag (Barth et al. 1995), to estimate the luminosity in H$\alpha$ of the circumnuclear zone at $4.3 \times 10^{41}$ erg s^-1. Of this luminosity only a tiny fraction (some 1%) comes from the nucleus. The star formation rate can be determined, assuming a standard IMF, using the relation: L(H$\alpha$ )/ $1.12\times10^{41}$ erg s^-1(see Kennicutt 1983), which gives $3.9~M_\odot$ yr^-1 for this zone. This is a value typical of nearby starburst galaxies.

In Rozas et al. (1996a,b) we analyzed the physical and statistical properties of the H II regions of NGC 6951, cataloguing 674 regions with luminosities above our detection limit, of which 603 are in the spiral arms, and 71 in the interarm zone, and measuring their sizes and H$\alpha$ luminosities. The luminosity function follows a power law: d $N(L) = A\, L^{\alpha}$ dLwhere $\alpha = -2.21 ({\pm} 0.1)$, coinciding with the values found for other galaxies of comparable morphological type (Kennicutt et al. 1989; Rozas et al. 1996a,b; 1998). The radial dependence of the mean H$\alpha$ surface brightness, found by dividing the total emission in rings of equal radial interval by the area of each respective ring, is shown in Fig. 6, which brings out the dominance of the circumnuclear emission.

In Fig. 5 we show a set of emission line profiles corresponding to the innermost zone of the galaxy. These data, taken from the high resolution cube, are plotted on a grid with 3 arcsec spacings in both RA and Dec. These grid points are marked with crosses in the H$\alpha$ intensity map of Fig. 4. This zone contains two principal emission intensity peaks, which lie on an oval annulus of enhanced emission, centred on the nucleus with semiaxes of 6 arcsec and 4 arcsec. These peaks show up also in the emission by the CO molecule, as demonstrated by Kenney et al. (1992), who postulated that these peaks might correspond to crowded orbits in the gas near the multiple Lindblad resonances, whose presence can be inferred from the behaviour of the rotation curve in the range of nucleocentric radii concerned. The presence of twin intensity peaks in the molecular gas was confirmed by Kohno et al. (1999), who made detections in CO and also in HCN. In fact our H$\alpha$ peaks coincide in position with Kohno et al.'s (1999) HCN peaks rather than with the peaks in the CO. The presence of strong HCN emission in regions with strong star formation has been shown in characteristic nuclear starburst galaxies such as M82 (Shen & Lo 1995) and NGC 253 (Paglione et al. 1995b)