1 Introduction

It is now commonly accepted that supermassive black holes (SMBHs hereafter) are nearly ubiquitous in elliptical galaxies and bulges. According to the standard paradigm, they should constitute the relics of the intense quasar activity that occurred in the early phase of galaxy evolution (see Ho 1999 for a review). The study of SMBHs has greatly benefited from the high resolution capabilities of the spectrographs onboard the Hubble Space Telescope (HST). Nowadays about 40 galaxies, belonging to different morphological types are known to harbor a SMBH. Their masses, $M_\bullet$, range from 10⁶to 10¹⁰ $M_{\odot }$ and have been derived from the analysis of the stellar orbital structure or from the dynamics of gaseous circumnuclear Keplerian disks (CNKD), using both optical and radio observations (see Kormendy & Gebhardt 2001 and Merritt & Ferrarese 2001b for both the object list and a discussion of the accuracy of $M_\bullet$ determinations).

The census of SMBHs is now large enough to probe the links between $M_\bullet$ and the global properties of the host galaxies. $M_\bullet$ correlates with the luminosity, $L_{\rm sph}$, and velocity dispersion, $\sigma$, of the spheroidal component of the host galaxy (Kormendy & Richstone 1995; Magorrian et al. 1998; Ferrarese & Merritt 2000; Gebhardt et al. 2000a). A lively debate is ongoing about the slope of the $M_\bullet-\sigma$ relation (Merritt & Ferrarese 2001a), although with its neglegible scatter it is a tighter correlation than the $M_\bullet-L_{\rm sph}$ relation. The consistency of $M_\bullet$ in active and quiescent galaxies has been discussed by different authors. Gebhardt et al. (2000b) and Ferrarese et al. (2001) showed that SMBH masses from reverberation mapping agree with the $M_\bullet-\sigma$ relation. Similarly, McLure & Dunlop (2001, 2002) found that the values of $M_\bullet$ inferred from $L_{\rm sph}$ for a large sample of quasars and Seyfert galaxies agree with those inferred from $\sigma$ measurements. Elliptical and disk galaxies seem to follow the same $M_\bullet-L_{\rm sph}$ and $M_\bullet-\sigma$ correlations, suggesting a close connection between the processes leading to the growth of central SMBHs and the formation of galaxy spheroids, whether they are ellipticals, classical bulges or pseudobulges (Kormendy 2001). Morever, $M_\bullet$ does not correlate with disks as it does with spheroids. To date, however, dynamical SMBH detections are available for only a dozen disk galaxies, and, therefore, the addition of new $M_\bullet$ determinations for S0's and spirals is highly desirable.

Over the course of the last few years, we have undertaken a vast program aimed at detecting CNKDs in disk galaxies using ground-based spectroscopic observations. Our goal is to measure upper limits for SMBH masses by using HST spectra to constrain them further (Bertola et al. 1998). Here we present a survey of the ionized-gas kinematics of the inner regions of 23 disk galaxies. The study complements the recent results regarding rapidly rotating gaseous nuclear disks in Rubin et al. (1997) and Sofue et al. (1998). The paper is organized as follows: in Sect. 2 we give an overview of the global properties of the sample galaxies and discuss observations and data reduction. In Sect. 3 we derive the position-velocity (PV) diagrams of the emission lines for each sample galaxy, suggesting a scheme for their classification. In Sect. 4 we present our conclusions. Relevant properties and the PV diagram of individual galaxies are discussed in the appendix.