Recent analyses of extended rotation curves (RCs) of late-type spiral galaxies (Persic, Salucci & Stel 1996) have confirmed that in spirals of all luminosities a substantial dark matter (DM) component is detectable already in the optical region. The effect is stronger at lower luminosities: the dark-to-visible mass ratio at the optical radius 1 scales with luminosity .
For early-type spirals the status of our knowledge is different. The RCs presently available for these objects are fragmentary (in particular in the nuclear regions), and only extend to (see Rubin et al. 1985). Consequently, detailed mass decomposition have so far not been possible for these systems. In particular, it is not known whether dark halos are unambiguously present also in early-type spirals.
It may be conjectured that for a given the DM fraction within the optical size is smaller in early than in late-type spirals: this, because in an early spiral the conspicuous stellar bulge, with , can supply a mass compact enough to make the rotation velocity higher than (see Rubin et al. 1985), and the velocity profile different from, that of a late spiral of similar luminosity. In this case, the derivation of the halo parameters would be more uncertain for early than for late types: in fact, at small radii not two mass components (disk + halo, like in Sc-Sd galaxies), but three mass components (bulge + disk + halo) will have locally similar (solid-body like) behaviors. So for non-extended RC data the mass solution of an Sa galaxy would be degenerate even within the maximum-disk solution.
In this paper we present the velocity and velocity-dispersion profiles of the stars and the ionized gas, measured along the major axis, for six early-type spirals. The six selected galaxies (Table 1) were already known to show emission lines and their photometric properties were known. Of these, 5 had already been observed spectroscopically by Rubin et al. (1985), who obtained the RCs of the ionized gas, and photometrically by Kent (1988). NGC 2179 was the only galaxy in our sample still lacking spectroscopical and photometrical observations. To the originally observed sample belonged the early-type spiral NGC 3593 too. Its stellar and the gaseous kinematics, found to exhibit a star vs. star counterrotation, is presented and discussed by Bertola et al. (1996). Three-component models (bulge + disk + halo) based on observed photometry and kinematics are obtained for two galaxies of the sample: NGC 2179 and NGC 2775.
Table 1. Parameters of the sample galaxies.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999