The three slit orientations used are depicted in Fig. ![[*]](/home/soft/latex2html/icons.gif/cross_ref_motif.gif){kind=icon}
,
that shows the H1#1
contours (from the data of González Delgado et al. 1997)
on a B/K grayscale colour map. The three position angles
correspond to the major axis (PA=138), the minor axis (PA=48),
and an intermediate angle (PA=84). The slits cross the circumnuclear
ring, but only one of the bright H1#1
knots.
We have measured the emission line intensities at every spatial increment
along the slit, by means of fitting gaussian components with LONGSLIT
(Wilkins & Axon 1991). Fig. shows the spatial variations
of the H1#1
line intensity and the H1#1/H5#5
ratio for
position angles (a) 48, (b) 84 and (c) the major axis at 138.
The Balmer lines are affected by underlying absorption
components in the nucleus and in the ring Hii regions. A detailed
analysis of the stellar population contributing to the Balmer absorption is
beyond the scope of this paper, but a comparison of the spectra with
evolutionary population synthesis profiles of the Balmer absorption line
profiles (González Delgado et al. 1999b), indicates that the Balmer
emission lines should be corrected by a line core equivalent width of 1 Å in
the nucleus and 0.5 Å in the ring; this result is found through the
comparison of the absorption wings detected in the H1#1
and H5#5lines and in the nearby metal absorptions with the predictions of the models.
The H1#1/H5#5
ratio shown in Fig. has been corrected in
this manner. The Balmer emission line ratio maps the effect of extinction by
the dust lanes; it is high in the nucleus (H1#1/H
15#15
at the center),
then becomes smaller in the transition region between the nucleus and the ring
of Hii regions (H1#1/H
16#16
at a distance of 17#171.5 arcsec),
and increases again across the ring to reach values of H1#1/H
18#18
in the outer side of the ring. This reddening across the ring is
clearly produced by the dust lanes, as seen in Fig. .
The extinction that corresponds to the above values of the
Balmer ratio is c(H5#5)=1.6, 0.4, and 1.6 mag for the nucleus, the
transition region and the ring respectively. Thus, just outside the partially
resolved nucleus, at 17#171.5 arcsec, the extinction reaches a minimum and the
Balmer ratio takes the case B value. The HST image in Fig. shows
indeed little dust in that intermediate region. The Galactic extinction towards
NGC 6951 quoted in the NED IPAC database is 0.88 mag in B, corresponding
to E(B-V)=0.22 or c(H5#5)=0.31, i.e. most of the extinction that we
measure in the intermediate region between the nucleus and the circumnuclear ring
is of Galactic origin and not intrinsic to NGC 6951.
Fig. shows the line ratio [Nii]6583/H1#1
together
with the H1#1
and [Nii] fluxes. For all three PA the ratio becomes
larger than unity within the inner 17#172 arcsec, reaching values of 5 in the
nucleus. This large ratio is due to a strong increase in the [Nii] flux
and not to underlying absorption in H1#1.
Indeed, the [Nii] takes
normal Hii region values in the ring, but increases sharply inwards.
This region of high [Nii]6583/H1#1
is significantly more extended than the nuclear point spread function, and
thus it is spatially resolved. The [Sii]/H1#1
ratio (not
shown) also shows a qualitatively similar behaviour, with an
increase from [Sii]/H1#1=0.18 in the ring Hii regions to
[Sii]/H
19#19
in the nucleus. The electron density, computed
from the line ratio [Sii]6717/6731, increases from low values,
20#20,
in the ring Hii regions to values larger
than 1000 cm-3 within the inner region and the nucleus (cf. Table
). These [Nii] and [Sii] line ratios seem to
indicate the presence of a shocked component, also supported by kinematical
evidence, as explained below.
| PA | distance | density |
| (arcsec) | cm-3 | |
| 138 SE | -4.5 | 530 |
| 138 SE | -1.1 | 1440 |
| 138 nuc | 0.0 | 1340 |
| 138 NW | 1.0 | 990 |
| 138 NW | 4.5 | 290 |
Fig. shows the H5#5
and [Oiii] 5007 line fluxes along
PA 138. The nucleus has a high excitation. The [Oiii] flux
decreases sharply to reach a ratio [Oiii]5007/H21#21
outside
22#22
arcsec from the nucleus. Thus the ring Hii regions have very
low excitation; in fact, we cannot measure
[Oiii] 5007 pixel by pixel outside the nuclear component at any of the
three position angles. In order to compute this excitation ratio for the
ring Hii regions, we have extracted three one-dimensional spectra at
each position angle, integrating the nucleus and the two sides where the slit
crosses the ring. Fig. shows these for PA=48.
The measured H5#5
and [Oiii] 5007 line fluxes and the ratio
[Oiii]5007/H5#5
are given in Table , as well as
the ratio with the H5#5
flux corrected for underlying absorption
(an absorption equivalent width of 1 Å in the nucleus and 0.5 Å
in the Hii regions).
| PA | orientation | extraction | F(H5#5) | F(5007)/F(H5#5) | F(5007)/F(H5#5) |
| arcsec | 23#23 | correcteda | |||
| 138 | SE | -5.69,-2.82 | 1.13 | 0.08 | 0.07 |
| 138 | nuc | -1.40, 1.11 | 0.20 | 16.26 | 5.51 |
| 138 | NW | 2.54, 5.40 | 1.17 | 0.03 | 0.03 |
| 84 | E | -4.00,-1.80 | 0.38 | 0.36 | 0.29 |
| 84 | nuc | -1.10, 1.40 | 0.17 | 24.82 | 7.03 |
| 84 | W | 2.70, 4.90 | 0.52 | 0.14 | 0.12 |
| 48 | NE | -4.11,-1.61 | 0.56 | 0.30 | 0.25 |
| 48 | nuc | -0.89, 1.25 | 0.37 | 16.57 | 7.96 |
| 48 | SW | 1.97, 4.47 | 0.23 | 0.53 | 0.39 |
a H5#5 corrected for an absorption equivalent width of 1 Å in the nucleus and 0.5 Å in the Hii regions.