Appendix A: Notes on individual galaxies

The PV diagrams of the sample galaxies are plotted in Fig. 2 and their classifications according to the criteria proposed in Sect. 3.3 are given in Table 4. In this section, we describe the most important properties of the sample galaxies and discuss the classification of their PV diagrams. The values of the velocities and the velocity dispersions include no corrections for inclination.

NGC 470. This intermediate-type spiral hosts either two nested bars (Wozniak et al. 1995) or a bar with a triaxial bulge (Friedli et al. 1996). The velocity gradient and velocity dispersion of the [O  III] $~\lambda5007$ line do not significantly change moving from the nuclear to the outer regions, while its integrated flux increases toward the center. We classify the PV diagram of NGC 470 as Type III.

NGC 772. According to the dynamical modelling of Pignatelli et al. (2001), the velocity of the ionized gas traces the circular speed in gravitational equilibrium. The PV diagram of NGC 772 derived from the [O  III] $~\lambda5007$ emission line is classified as Type III since the integrated flux of the emission line in the nucleus is greater than that measured in the outer regions.

NGC 949. As in the previous case, also the Type III classification of the PV diagram of NGC 949 results from the increase of the integrated flux of the [O  III] $~\lambda5007$ line toward the center.

NGC 980. In the PV diagram of this S0 galaxy, a tilted and bright component appears to be superimposed on a normal rotation curve. The increase of both the velocity gradient and the velocity dispersion toward the center are indicative of the kinematics of a CNKD and give this PV diagram its Type I classification. This is also the case for NGC 2179 and NGC 7782.

NGC 1160. The PV diagram of this Scd spiral is characterized by a constant velocity gradient and a constant integrated flux for the [O  III] $~\lambda5007$ line. This makes its Type II classification straightforward.

NGC 2179. The PV diagram of NGC 2179 is the prototype of the Type I class. The two-dimensional shapes of the emission lines are such that they gives the erroneous impression of two distinct velocity components. One apparent velocity component has the form of a highly tilted straight line rising from zero velocity in the galaxy center (i.e., a faster-rising rotation curve). The other component is a less-tilted straight line (i.e., a slower-rising rotation curve) superimposed on the first. Both lines naively appear to imply solid-body rotation in the inner parts of this galaxy; both lines culminate as the radius increases to almost the same maximum velocity; the slower-rising rotation curve shows a flat portion in its outer regions. Rather than being of two different physical origins, we have shown that properly accounting for the seeing, slit width, and pixel size effects, these two apparently solid-body rotation curves can be modelled as the velocity field of a thin gaseous disk rotating in the combined gravitational potential of central point-like mass and an extended stellar disk (see Bertola et al. 1998 for details).

NGC 2541. The bright central component in this PV diagram is characterized by the same velocity dispersion measured in the outer parts of the disk. This is typical of Type III diagrams.

NGC 2683. According to Merrifield & Kuijken (1999), the PV diagram of NGC 2683 has a "figure-of-eight'' shape produced by the presence of two kinematically distinct gaseous components. This feature is barely visible in our PV diagram because of the lower S/N ratio of the spectrum. Although the properties of the PV diagram of NGC 2683 are similar to those of NGC 980, NGC 2179 and NGC 7782, it does not warrant a Type I classification. Indeed in NGC 2683 we are observing two gas components which are spatially distinct and superimposed along the line of sight because of the high inclination of the galaxy ( $i=78^\circ$). They are generated by the presence of a non-axisymmetric potential (Kuijken & Merrifield 1995; Bureau & Athanassoula 1999).

This is not the case for NGC 980, NGC 2179 and NGC 7782, which are less inclined ( $i=58^\circ,47^\circ$ and $58^\circ$, respectively) and unbarred, exhibiting unique gaseous components.

NGC 2768. The presence of a definite outer envelope with subtle dust patches surrounding the bulge (see panels 38 and 53 in CAG) supports the S0 classification of this galaxy, which appears as an E6$\ast$ in RC3. The kinematical decoupling between the ionized gas and the stars, detected by Bertola et al. (1992), has been interpreted as a result of the presence of gas orbiting in a polar ring (Möllenhoff et al. 1992; Fried & Illinghworth 1994). The inner velocity gradient is higher than the outer one which is one of the lowest we measured ( $(\Delta V / \Delta r)_{\rm out}$= 0.09  $\rm km~s^{-1}~pc^{-1}$). The constant velocity dispersion and the steep increase of [N  II] $~\lambda6583$ flux in the center imply a Type III classification for this PV diagram.

NGC 2815. The presence of the spectrum of broad emission lines in the nuclear region of NGC 2815 with wings which are very close each to other makes the subtraction of the galaxy continuum critical. Indeed, residual continuum is still visible in the PV diagram derived from the H$\alpha$ line; it gives the erroneous impression that the PVdiagram shape is similar to that of NGC 2179. We classify this PVdiagram as Type III class because of its constant velocity gradient and large central-to-outer integrated-flux ratio.

NGC 2841. According to Sil'Chenko et al. (1997) the ionized gas is rotating orthogonally with respect to the galaxy plane in the inner 5''. Alternatively, Sofue et al. (1998) reported that the central portion of the PV diagram derived from the H$\alpha$ and [N  II] $~\lambda6583$ lines is slightly tilted in the direction of the galactic rotation, suggesting the presence of a rapidly rotating nuclear disk. Our PV diagram exhibits a complex and asymmetric shape that could be related to these different kinematic components. However, we do not measure a significant variation of the velocity gradient or the velocity dispersion with radius. The line flux increases slightly toward the center. These features are similar to those of PV diagrams included in the Type III class, and make it difficult to associate with the central component to a fast-rotating disk as by indicated Sofue et al. (1998).

NGC 3031. HST H$\alpha$ imaging reveals the presence of a nuclear gaseous disk (Dereveux et al. 1997) similar in size and shape to the CNKD of M 87 (see Macchetto et al. 1997 and references therein). The disk is rotating around a SMBH with $M_{\bullet} = 3\times10^6$ $M_{\odot }$, according to determinations based on stellar kinematics (Bower et al. 1996) and broad-line emission (Ho et al. 1996). The spatial and spectral resolution of our spectrum allow us only to detect the presence of a broad and bright central component in the PVdiagram. In fact, it exhibits the highest central-to-outer integrated-flux ratio of our sample, which warrants a Type III classification. From the available spectrum, it is difficult to claim that NGC 3031 is hosting a CNKD even though we measure a remarkably large $(\Delta V / \Delta r)_{\rm in}$ (=3.4  $\rm km~s^{-1}~pc^{-1}$) and a large inner-to-outer velocity-gradient ratio ( $\Gamma=3.0$).

NGC 3281. The ionized-gas kinematics measured by Rubin et al. (1985) and Corsini et al. (1999) extends out to about 50'' from the nucleus but in our spectrum the emission is confined in the innermost 5''. The inner velocity gradient is steeper than the outer one and this early-type spiral has one of the highest central-to-outer flux ratios of the whole sample. Even if the emission is not extended we consider the PV diagram of NGC 3281 to be of Type III because of its intense nuclear emission.

NGC 3368. From NIR photometry, Jungwiert et al. (1997) identified a possible double-barred structure within this Sab spiral. Although the PV diagram seems to have a two-component structure, the constant velocity gradient and steep increase of the integrated flux of the [O  III] $~\lambda5007$ line toward the center suggest a Type III classification.

NGC 3521. The PV diagram of this intermediate-type spiral has been recently measured by Sofue et al. (1998) from the H$\alpha$ and the [N  II] $~\lambda6583$ emission lines. They interpreted the central component observed in the [N  II] $~\lambda6583$ line as an indication of the presence of a fast rotating gaseous disk in the nucleus. We suggest that this feature, which is clearly visible also in the PV diagram we derived from the [O  III] $~\lambda5007$ emission line, results from the increase of the line flux rather than the velocity gradient. Morever, the velocity dispersion does not change with radius. Therefore it is Type III diagram.

NGC 3705. The Type III classification of this PV diagram mostly results from the centrally-peaked radial profile of the integrated flux of the [O  III] $~\lambda5007$ line, which gives the impression of a steep central component superimposed on a slowly-rotating component.

NGC 3898. The ionized-gas distribution and kinematics of this Sa galaxy have recently been studied in detail by Pignatelli et al. (2001). They found that in the innermost region ($\vert r\vert\la8''$) of NGC 3898, the ionized gas is rotating more slowly than the circular velocity predicted by dynamical modelling based on stellar kinematics and photometry. The fingerprint of such a "slowly-rising'' rotation curve (according to Kent 1988 definition) can be recognized in the decrease of the velocity gradient at smaller radii. The two-component shape of the PV diagram results from the bright nuclear emission and not the increase of velocity gradient or the velocity dispersion. This is a Type III PV diagram and its similarity to the PV diagram of NGC 4419 is remarkable.

NGC 4419. The spectrum and consequently the PV diagram of NGC 4419 are similar to those of NGC 3898. The spectra show the same strong and broad H$\alpha$ absorption and the PV diagrams are characterized by the same bright central component. They have also similar inner-to-outer velocity gradient and dispersion ratios and both belong to the Type III class. As NGC 3898 also NGC 4419 is one of the bulge-dominated spirals displaying a slowly-rising rotation curve of the ionized gas discussed by Kent (1988).

NGC 4698. This Sa galaxy shows a remarkable orthogonal geometrical and kinematical decoupling between the inner portion of the bulge and galaxy disk (Bertola et al. 1999). The asymmetric shapes of the H$\alpha$ and [N  II] lines are seen at a simple visual inspection of the spectrum, and they are more evident in the PV diagram obtained from the [N  II] $~\lambda6583$ line. Although we measured an increase of the velocity gradient toward the center, we note that NGC 4698 has the shallowest outer gradient of all the sample galaxies ( $(\Delta V / \Delta r)_{\rm out}$= 0.05  $\rm km~s^{-1}~pc^{-1}$). This gradient corresponds to the central plateau measured in the ionized-gas rotation curve by Bertola & Corsini (2000). The Type III classification has been assigned to this PVdiagram on the basis of its high central-to-outer integrated-flux ratio.

NGC 5064. The PV diagram of NGC 5064 is the prototype of the Type II class. It is useful to compare the emission-line spectrum of NGC 5064 to that of NGC 2179, because both spectra have been obtained with same setup and observing conditions. In contrast with NGC 2179, the emission-line spectrum of NGC 5064 does not show any peculiar features; there is only one component in the central region. The velocity increases linearly with radius until it reaches about 200  $\rm km~s^{-1}$, 4'' from the center. The velocity dispersion and the integrated flux of the H$\alpha$ line remain almost constant in this radial range.

NGC 7320. The spectrum we obtained for this late-type spiral, which belongs to Stephan's Quintet is of poor quality. The [O  III] $~\lambda5007$ line shows a bright knot at about 5'' from the center resulting in the observed $F_{0}/F_{\rm out}=0.3$. The Type II classification of the PV diagram is based on the constant inner-to-outer velocity gradient and velocity dispersion.

NGC 7331. The presence of a SMBH ( $M_\bullet \sim 10^8$ $M_{\odot }$) in the center of NGC 7331 has been debated by different authors (Afanasiev et al. 1989; Bower et al. 1993; Mediavilla et al. 1997; Sil'Chenko 1999). The debate centers on observations of the distribution and kinematics of ionized gas. Our PV diagram is similar to that of NGC 772. We measure an increase of the integrated flux of the [O  III] $~\lambda5007$ line at smaller radii, along with constant velocity gradient and to slight increase of the velocity dispersion. The PV diagram of NGC 7331 is of Type III.

NGC 7782. The inner region of this PV diagram is characterized by a sharp increase of the velocity gradient, as confirmed by the large inner-to-outer velocity-gradient ratio we measure. The [O  III] $~\lambda5007$ line exhibits a bright nuclear component and it velocity dispersion rapidly decreases with radius. The properties of the PV diagram of NGC 7782 are close to those of NGC 2179 leading to the Type I classification. The nuclear ionized-gas kinematics of NGC 7782 is indicative of a CNKD.