4 Kinematical results

The kinematical profiles obtained for the four galaxies are displayed in Figs. 2-4 and 6. In each figure, the lengths of the sketched slits overimposed on the image correspond to the presented profiles.

For the interpretation of the velocity profiles, it is important to keep in mind that the spectra have been taken along slits parallel and perpendicular to the nuclear bars (respectively named Slit1 and Slit2 hereafter). The major and minor axis of the galaxies do not coincide with that of the bars, so that we do expect some rotation along both axis, due to the inclination effect. The various position angles and inclinations are displayed in Table 1.

   
4.1 Individual descriptions

NGC 1097, Seyfert 2, $1\hbox{$^{\prime\prime}$ }\sim 81$ pc:

Figure 2: Kinematical profiles of NGC 1097. From top to bottom: NIR image of the galaxy centre (North is up, East is left), indicating the positions and lengths of the two slits, parallel and perpendicular to the nuclear bar (the labels 1 and 2 overimposed on the image indicate positive abscissa); luminosity profiles (in log) along the two slits; velocity and dispersion profiles with error bars representing $3\cdot S_V$ and $3\cdot S_{\sigma }$ values (see Sect. 3.3)

The luminosity profile along Slit1 falls down to a plateau near the end of the nuclear bar, and then decreases longward $\sim$ $10\hbox{$^{\prime\prime}$ }$with a roughly exponential law characteristic of a disc population. This is fully consistent with the $10.3\hbox{$^{\prime\prime}$ }$ semi-major axis extent of the nuclear bar mesured by Friedli et al. (1996). The luminosity along Slit2 follows the same behaviour, but with a bump just after the plateau due to its well-known circumnuclear clumpy ring (actually a tightly wound spiral structure in the NIR; e.g. Kotilainen et al. 2000).

The velocity profile along Slit1 is quite flat, reflecting the fact that it is nearly perpendicular to the kinematical line of nodes (which we assume to be given by the major-axis photometric position angle of the outer disc of the galaxy). The global shape of the rotation curve along Slit2 roughly resembles the H$\alpha$ velocity curve derived (along a position angle of $130\hbox{$^\circ$ }$) by Storchi-Bergmann et al. (1996). The maximum stellar velocity along Slit2 ( $V_{\rm max,2}$ $\sim$ 210 km$\,$s^-1) is reached in the circumnuclear ring, similarly to the ionised gas for which Storchi-Bergmann et al. (1996) measures maxima of $V_{\rm max}$[H$\alpha$] $\sim 225$ km$\,$s^-1. We thus measure a roughly constant stellar velocity gradient of $\sim$290 km$\,$s^-1kpc^-1. Our good spatial resolution however allows to reveal a richer velocity structure. Inside $R = 5 \hbox{$^{\prime\prime}$ }$, the velocity profile along Slit2 exhibits an S-shape with nearly flat ends. Those plateaus in the velocity correspond to maxima in the dispersion profile ( $\sigma_{2} \sim 220$ km$\,$s^-1), whereas the inner part is characterised by a quite surprising drop in the dispersion (down to $\sigma_{2} \sim 145$ km$\,$s^-1 at the centre). Velocities increase almost linearly from a radius of $5\hbox{$^{\prime\prime}$ }$reaching a maximum near the edge of the circumnuclear ring at $\sim$ $9\hbox{$^{\prime\prime}$ }$, where they then starts to decrease. Outside $5\hbox{$^{\prime\prime}$ }$, the dispersion decreases outwards down to $\sim$95 km$\,$s^-1. Note that the dispersion drop and local maxima in the dispersion are also present along Slit1.

NGC 1365, Seyfert 1, $1\hbox{$^{\prime\prime}$ }\sim 90$ pc: The Seyfert 1 nucleus of NGC 1365 dominates the light in the central arcsecond, and thus strongly dilutes the absorption ¹²CO bandhead: this prevented us to derive any meaningful kinematics in this region. We will deal here only with the profiles outward $R \geq 2 \hbox{$^{\prime\prime}$ }$.

Like in NGC 1097, the flatness of the velocity profile along Slit 2 is a consequence of the slit orientation with respect to the line of nodes. The central kpc morphology of this galaxy is disturbed by an intense star formation (see Lindblad 1999 for a review on this object). It is thus difficult to see the signature of the bar in the luminosity profile. Ellipse fitting on H-band isophotes provided by Jungwiert et al. (1997) gave a rough estimate of the extent of the presumed secondary bar: $\sim$ $9\!-\!10 \hbox{$^{\prime\prime}$ }$. However, high resolution near-infrared images of the central region of NGC 1365 recently obtained with NICMOS/HST, and the VLT (ISAAC and FORS1) suggest that the ellipticity of the component detected in the central 10 $^{\prime\prime}$ is solely due to the inclination of the galaxy (the photometric major-axis being thus coincident with the line of nodes). There are therefore no evidence left for the presence of a nuclear bar. We then simply interpret the observed flattened system in the centre as a nuclear disc, well circumvented by a ring-like (and spiral arm) structure at a radius of $\sim$ $7\hbox{$^{\prime\prime}$ }$. Inside this radius, the velocity increases up to its maximum value ( $V_{\rm max,1} \sim 175$ km$\,$s^-1 at $R \sim 5 \hbox{$^{\prime\prime}$ }$) with a steep gradient ($\sim 390$ km$\,$s^-1kpc^-1), and then remains roughly constant until the end of the disc. The dispersion along both axis shows no clear structure: it remains nearly constant inside the nuclear disc with a mean $\sigma_{1} \sim 100$ km$\,$s^-1. There may be a slight increase outwards up to $\sigma_{2} \sim 120-130$ km$\,$s^-1, but this is within the error bars.

Figure 3: Same as Fig. 2 for NGC 1365. The light in the central arcsecond (region marked by the vertical dashed lines) is completely dominated by the non-thermal contribution of the Seyfert 1 nucleus, thus preventing us to derive any meaningful kinematics in this region

NGC 1808, Seyfert 2, $1\hbox{$^{\prime\prime}$ }\sim 53$ pc: The central kpc of this galaxy, disturbed by "hot spots'' of star formation (e.g. Kotilainen et al. 1996), is the brightest of our sample in the K-band, hence providing the nicest kinematic profiles. Again, we estimate the nuclear bar length to be $\sim$ $6 \hbox{$^{\prime\prime}$ }$ from Jungwiert et al. (1997), with an axis ratio around 0.5. The velocity profiles along both axis show an increase up to the end of the nuclear bar and then a decrease. Slit1 velocity profile is significantly asymmetric with respect to the systemic velocity outside $2\hbox{$^{\prime\prime}$ }$, with $V_{\min,1} = -124$ km$\,$s^-1 and $V_{\max,1} = 81$ km$\,$s^-1: this asymmetry is also clearly present in the surface brighness profile (Fig. 5) but does not appear in the dispersion curve.

Figure 4: Same as Fig. 2 for NGC 1808

Figure 5: Velocity (left, absolute values) and surface brightness (right) profiles along Slit1 of NGC 1808: the crosses and solid lines correspond to the north-west side, the circles and dotted lines to the south-east side

Figure 6: Same as Fig. 2 for NGC 5728. Note the different spatial extent of the major and minor axis plots

Along Slit2, the velocity profile is roughly symmetric with local extrema $V_{\min,2} \sim -45$ km$\,$s^-1 and $V_{\max,2} \sim 53$ km$\,$s^-1 at radii of $\pm 2\hbox{$^{\prime\prime}$ }$, followed by a decrease outwards down to systemic velocity. The mean slopes in the rising parts are $\sim$300 km$\,$s^-1kpc^-1 and 260 km$\,$s^-1kpc^-1 along Slit1 and Slit2 respectively. There is however a distinct kink in the velocity profile along Slit 1 at a radius of $-2\hbox{$^{\prime\prime}$ }$, with no symmetric counterpart. The velocity gradient changes inside $0\hbox{$.\!\!^{\prime\prime}$ }4$ of Slit 2, to only $\sim$140 km$\,$s^-1kpc^-1, a real feature considering the final spatial resolution for this slit ($\sim$ $0\hbox{$.\!\!^{\prime\prime}$ }6$ FWHM; see Table 3). The dispersion profiles remain in the range 80-120 km$\,$s^-1and exhibit a trend similar to the one of NGC 1097: an increase toward the centre followed by a significant drop this time inside 1 $^{\prime\prime}$. The magnitude of this drop is low ($\sim$15 km$\,$s^-1) but real.

NGC 5728, Seyfert 2, $1\hbox{$^{\prime\prime}$ }\sim 179$ pc: NGC 5728 is the faintest galaxy in our sample, however the signal-to-noise is enough to have a good measure of the velocity profiles in the central 5 $^{\prime\prime}$. Once again, there are clear signatures of a decoupled dynamical component in the central kinematics. The maximum velocity gradient is observed along Slit2 as expected from the position angle of the line of nodes (see Table 1), similarly to the case of NGC 1097. Slit1 for NGC 5728 is very close to the slit used by Prada & Gutierrez (1999, hereafter PG99; PA $=86\hbox{$^\circ$ }$), our velocity profile being consistent with theirs. The velocity and velocity dispersion profiles are slightly asymmetric along both slits. The K band surface brightness profile also exhibits an asymmetry along Slit1 at the edge of the nuclear bar. There is a dip in the dispersion profiles of NGC 5728, with a central value of 147 km$\,$s^-1, although it is less convincing than in the cases of NGC 1097 and NGC 1808. This value is slightly smaller than but within the error bar of the one derived by PG99. We do not detect any double component in our LOSVDs, in apparent contradiction with the data of PG99. We observe a high excitation [Ca VIII] emission line, burried within the second ¹²CO absorption feature. The emitting region is restricted to the central spectra, and is consistent with an unresolved point-like source, thus certainly linked with the AGN. This point will be examined in details in a forthcoming paper (Paper II).

4.2 Global results

The first striking result of those observations is that, in all 4 observed targets, the rotational velocity reveals a maximum inside the nuclear bar (or disc for NGC 1365) and then decreases, showing that the nuclear region is a well decoupled dynamical component of the galaxies. For the three cases with nuclear bars (NGC 1097, NGC 1808 and NGC 5728), this follows suggestions made from photometric studies as no preferential angle was observed between the two bars (Greusard et al. 2000 and references therein). The existence of such structures could be doubted when dealing with a galaxy like NGC 1808, where there are numerous clumps of star forming systems within the central arcseconds. But even in that case, the NIR photometric elongation embedded within the ring present in the WFPC2/HST (archival) images strongly suggests the presence of a nuclear bar. This point is further examined in the light of dynamical models (Sect. 5). This is therefore the first direct confirmation of the dynamically decoupled nature of nuclear bars.

The second surprising result comes from the dispersion profiles: they exhibit a significant drop at the centre (but again we cannot say anything concerning NGC 1365). This is particularly clear in the cases of NGC 1097 and NGC 1808. We have checked that the dilution of the lines by any featureless continuum component does not affect the dispersion (and velocity) as long as the ¹²CO lines remain strong enough. We do indeed see some dilution and changes in the ¹²CO line strength, but this does not affect our result. We have also checked that the observed central dispersion drop is not due to a template mismatching effect (Paper II).