Kinematics of the Ionized Gas in the Inner Regions of Disk Galaxies

We have studied the kinematics of the ionized gas in the inner regions of disk galaxies in order to investigate the presence of circumnuclear Keplerian disks (CNKDs) of ionized gas and kinematically decoupled components in a sample of 25 disk galaxies. We obtained long-slit spectra at intermediate or high spectral resolution along the major axis of all the sample galaxies, whereas minor-axis spectra, broadband, and narrowband imaging have been obtained for only a subsample of them. We have measured the ionized-gas kinematics in the inner regions of 23 galaxies, whose morphological types range from S0 to Sc. The velocity gradients, velocity dispersions, and integrated fluxes of the emission lines have been measured at and . We can distinguish three classes of ′′ ′′ r 1 r 4 galaxy spectra taking into account the two-dimensional shape of the emission lines, the comparison of the velocity gradients, and the ratio of the fluxes at the different radii where we measured them. We interpret the variety of position-velocity diagrams by applying a modeling technique which assumes that the gas resides in an infinitesimally thin disk whose mean motion is characterized by circular orbits in the plane of the galaxy. Under these assumptions, two parameters play an important role in giving rise to the observed shape of the emission line: the value of the central mass concentration and the cuspiness of the intrinsic gaseous disk surface brightness. This modeling technique takes into account the instrumental effects (slit width and pixel size of the detector) and the seeing. This kind of analysis allows the identification of spiral galaxies that are good candidates to harbor central supermassive black holes and to be followed up with higher spatial resolution. We find that 13% of sample galaxies exhibit features in the position-velocity diagrams that are consistent with the presence of a CNKD (J. G. Funes et al. 2001, in preparation). We also demonstrate the possibility of inferring the presence of gaseous CNKDs using optical ground-based telescopes properly equipped. The peculiar bidimensional shape of the emission lines of five early-type disk galaxies have been modeled as due to the motion of a gaseous disk rotating in the combined potential of a central pointlike mass and of an extended stellar disk. An upper limit of about 10 M, has been given for the central mass concentration of NGC 2179, NGC 4343, NGC 4435, and NGC 4459 (F. Bertola et al. 1998, ApJ, 509, L93). The possibility of the detection of low-mass black holes is also discussed. The study of kinematically decoupled components in disk galaxies focused on three Sa galaxies, namely, NGC 3595, NGC 4698, and NGC 4672. The galaxy NGC 3593 is characterized by the presence of two counterrotating stellar disks of different size and luminosity. We present the results of narrowband Ha and near-infrared [N ii] imaging of the early-type spiral NGC 3593 in combination with a study of the flux radial profiles of the (ll6548, 6583), [N ii] Ha, and (ll6717, 6731) emission lines along its major [S ii] axis. We found that the Ha emission mainly derives from a small central region of . It consists of a filamentary pattern ′′ ′′ 57 # 25 with a central ring. The ring, which hosts an ongoing process of star formation, is interpreted as the result of the interaction between the acquired retrograde gas which later formed the smaller counterrotating stellar disk and the preexisting prograde gas of the galaxy (E. M. Corsini et al. 1998, AA this is also true for the outer parts of the bulge if a parametric photometric decomposition is adopted (F. Bertola et al. 1999, ApJ, 519, L127). The morphological features of NGC 4672, as well as its velocity curves and velocity dispersion profiles of stars and ionized gas along both its major and minor axes, are similar to those we observed in NGC 4698. We conclude that also NGC 4672 has a bulge elongated perpendicularly to the disk and a stellar core rotating perpendicularly to the disk. The presence of the isolated core suggests that the disk component is the end result of the acquisition of external material in polar orbits around a preexisting oblate spheroid as in the case of the ring component of AM 2020 504, the prototype of polar ring elliptical galaxies (M. Sarzi et al. 2000, A&A, 360, 439).