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Figure: Full frame B (a) and Ks (b) broad band images of NGC 6951. The images show clearly two spiral arms, the bar, dust lanes along the bar and the circumnuclear ring at 5 arcsec from the nucleus. The orientation in all the images is North up and East to the left. The scale is 92 pc arcsec^-1. The contour levels are (a) in B 24.0 22.5 21.5 20.5 19.5 19.0, (b) in Ks 21.0 20.0 19.5 19.0 18.5 18.0 17.5 17.0 16.5 16 15.5 15.0.

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Figure: Broad band photometric parameters as a function of the root square of the distance to the nucleus. Surface brigthness (10#10): upper pannels; ellipticity (48#48): middle pannels; and position angle (PA): bottom pannels. The parameters are derived by fitting isophotal profiles to the images: B (left pannels); I (middle left pannels); J (middle right pannels) and Ks (right pannels).

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Figure: Full frame colour images. (a) B-I and (b) J-Ks. B-I traces very well the spiral arms and the circumnuclear ring, where recent star formation is taking place. Dust lanes along the bar are also seen as dark features. Grayscales are displayed between B-I=3.1 and 1.9, and J-Ks=-1.0 and -2.0.

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Figure: colour images of the bar and inner regions of NGC 6951: (a) B-K and (b) I-K. The inner part of the ring is dominated by blue colours, except for the nucleus that appears as a red central knot. Grayscales are displayed between B-K=0.06 and 0.16, and I-K=0.55 and 1.30.

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Figure: J-H colour image of the central 127#712. These images were observed by HST+NICMOS. The spatial scale is 0.075 arcsec pixel^-1, and the grayscale has been displayed between J-H=0.55 and 0.31 mag. The circumnuclear ring is mainly delinated as two short spiral arms. In the inner part of the ring, the dust delineates a multi-spiral structure that ends in the nucleus.

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Figure: B-K colour image of the central 207#720 arcsec. H1#1 contours (from the data of González Delgado et al. 1997) are also plotted. Note that there is not a perfect correspondence between the blue knots in the B-K image and the nebular emission. The three slit positions for the spectroscopy data are also plotted. Grayscale is displayed between B-K=0.05 and 0.16.

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Figure: H1#1/H5#5 emission line ratios (dots) measured along the three slit PA. This ratio has been corrected for underlying stellar absorption as explained in the text. The theoretical ratio predicted by the case B recombination is plotted as a horizontal dashed line. The H1#1 flux along the slit is also shown as a full line.

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Figure: H1#1 (open circles + dashed line) and [Nii] (squares + dashed) fluxes and their ratio [Nii]/H1#1 (filled circles + full line) measured along the three slit PA. Note that the large [Nii]/H1#1 ratio in the inner 17#172 arcsec is due to a strong increase in the [Nii] flux.

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Figure: H5#5 (filled circles + dashed line) and [Oiii] (open points + full line) fluxes along PA=138. The [Oiii] emission is mainly concentrated in the inner 17#172 arcsec, where the H5#5 emission is very weak.

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Figure: One dimensional spectrum, from 4750 Å to 5150 Å, of the nucleus and of the two regions on either side of the nucleus along PA=48. Note that these regions have low excitation, consistent with the gas being photoionized by stars.

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Figure: Nuclear spectrum (the central 0.72 arcsec) in the Caii triplet region. The most relevant lines are labelled. Mgi 34#348807 is very strong, indicating that the metallicity is solar. Paschen 14 is very weak, thus Pa13 and Pa15 must contribute very little to the Caii 34#348542 and 34#348662, respectively.

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Figure: Equivalent width of the Caii 34#348542 + 34#348662 lines, measured using the line and continuum windows defined by Díaz et al. (1989). The equivalent width is 49#49 7 Å in the nucleus and out to the circumnuclear ring, indicating the presence of red supergiant stars in the central 57#75 arcsec of the galaxy.

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Figure: Velocity curve (dots) obtained from the pixel to pixel measurements of the H1#1 flux (full line), derived fitting a gaussian to the profile of the emission line. Curves are plotted for the three slit positions. At PA=138 (photometric major axis of the galaxy) the curve shows the maximum velocity amplitud. At PA=48 the curve shows more structure than expected for the minor axis of the galaxy. The inset within each figure shows the circumnuclear 50#50 arcsec expanded; the three insets have the same scales in both distance and velocity axes, respectively.

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Figure: FWHM of the emission lines H1#1 (filled circles) and [Nii] (empty squares). The velocity dispersion of the gas is maximum in the inner 17#171.2 arcsec, and minimum at the position of the Hii regions in the ring. The H1#1 flux is plotted as a dashed line.

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Figure: Velocity curves from the two strongest CaT absorption lines ( 51#518542, 8662), measured by cross-correlation of the galaxy frames with spectra of K giant stars. The velocity curve of the ionized gas (dots) is shown for comparison with the stellar velocity curve (circles). In the central region the CaT absorption lines are resolved in two components, plotted as filled and open circles. The insets show the central 17#1710 arcsec; the three have the same radial and velocity scales respectively, and the gas velocity has been plotted as a full line. Notice how in the inner 17#172 arcsec, the stars and the ionized gas follow the same velocity pattern at PA=84 and 138, but not along the minor axis, where the H1#1 curve presents more structure.

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Figure: HST WFPC2 V (a) and NICMOS H (b) images of the circumnuclear region. To show more clearly the morphological structure of the cicumnuclear region, the original images have been sharp enhanced by dividing them by the corresponding median filtered image. Note the spiralling structure delineated by dust in the V image down into the nucleus.

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