2 NGC 6951

NGC 6951 is a spiral galaxy, classified as SAB(rs)bc (de Vaucouleurs et al. 1991), which has a large stellar bar revealed by near-infrared photometry (Mulchaey et al. 1997; Friedli et al. 1996; Elmegreen et al. 1996) and distorted spiral arms (see Fig. 1). Its central zone is compact and highly ionized, and surrounded by an annulus formed by bright H II regions (Buta & Crocker 1993). Measured line ratios in the nuclear zone are pointers to activity; the nucleus has been classified as Seyfert 1, Seyfert 2, and Liner (Filippenko & Sargent 1985; de Robertis & Osterbrock 1986; Depree et al. 1991). Filippenko & Sargent (1985) pointing out that the ratio of [NII]/H$\alpha$ reaches values of 3 at the nucleus, falling to values less than 1 in the H II regions in the ring. Values significantly greater than unity are characteristic of active nuclei, while values less than 1 characterize H II regions in general. The radial decline in the [NII]/H$\alpha$ ratio was confirmed by Muñoz-Tuñón et al. (1989), who found values close to 4 in the central arcsecond, falling to 0.2 at 20 arcsec from the centre. In fact the nucleus has been classified as a Seyfert 1 (de Robertis & Osterbrock 1986) or a Seyfert 2 (Depree et al. 1991), and this second option seems now to be confirmed by Ho et al. (1997) with the most accurate study to date performed, with high data quality and good correction for underlying absorption. Finaly, Pérez et al. (2000) suggest that the nucleus in NGC 6951 can be considered as a transition object between a very high excitation Liner and a possible nitrogen overabundant Seyfert 2. Its general optical and kinematic properties have been covered in papers by Márquez & Moles (1993), Boer & Schulz (1993) and Rozas et al. (1996). The CO and H$\alpha$ emission within 10 arcsec of the nucleus show a twin-peaked structure (Kenney et al. 1992) which suggests the presence of a two armed spiral or a broken ring. Radio frequency maps by Saikia et al. (1989) and by Vila-Vilaró et al. (1990) also show a two-component structure, but with extended emission extending from 5 arcsec SE to 5 arcsec NW of the nucleus, with a compact central peak.

Near infrared imaging has brought out clearly the presence of a stellar bar with a semimajor axis of 44 arcsec (Friedli et al. 1996; Elmegreen et al. 1996; Mulchaey et al. 1997), which can in fact be picked out in optical images. In Fig. 1 we show a V-band image of the galaxy, in which some of its main large scale features can be seen: two non-symmetric spiral arms emerge from the ends of a major EW bar structure. The southern arm is less well defined than its northern counterpart. The internal isophotes show distorsions related to the dust lanes parallel to the bar, but which cut it within the circumnuclear zone (Kenney et al. 1992; Márquez & Moles 1993; Wozniak et al. 1995). These dust lanes trace the shock front in the gas towards the edges of the bar, predicted by theoretical models (Roberts et al. 1979; Athanassoula 1992).

Figure 4: Integrated intensity map of H$\alpha$ emission in the central region of NGC 6951. The contour levels are 5, 6, 7... 15$\sigma$. The grid positions in the line profile map (Fig. 5) are indicated here by crosses.

Figure 5: Line profile map of the H$\alpha$ emission derived from the full resolution data cube of the central region of NGC 6951. The line profiles were measured at $6 \times 6$ points on the 3'' spacing $\alpha$ - $\delta$ grid. The width of a velocity bin is 15.69 km s-1 .

It is worth noting that NGC 6951 is remarkably isolated. It is conventionally claimed that galaxies with active star formation are interacting with others (Adams 1977; Dahari 1985; Joseph et al. 1984; Joseph & Wright 1985; Laurikainen & Moles 1988, 1989). However there is no theoretical reason why an isolated galaxy should not show such activity. We have searched the CfA catalogue, and found that there are no detected objects within 1 Mpc of NGC 6951, nor within a redshift of 500 km s^-1 of it, so that we can infer that no significant gravitational interaction with a neighbour galaxy has occurred for the past 10⁹ years. We have summarized the key observational parameters for the galaxy in Table 1.

 

 

Table 1: NGC 6951: basic parameters.

Parameter	Value	Reference
Type	SAB(rs)bc	1
Nuclear activity	Seyfert 2	2
RA (1950)	$20^{\rm h}3\rm 6^m36\hbox{$.\!\!^{\rm s}$ }59$	3
Dec (1950)	$65^{\circ}55^{'}46^{''}$	3
D25 $\times$ d25	3.9 $\times$ 3.2	1
Inclination angle	42$^\circ$	4,5
Position angle	137$^\circ$	4
Accepted distance	24.1 Mpc	6

(1): de Vaucouleurs et al. (1991); (2): Ho et al. (1995); (3): Saikia et al. (1994); (4): this work; (5): Márquez & Moles (1993); (6): Tully (1988).