4. The kinematics of NGC 4672
4.1. Heliocentric systemic velocity
We derived the heliocentric systemic velocity of NGC 4672 by fitting the major-axis rotation curves with a suitable odd function. In Fig. 4 we compare our heliocentric systemic velocity with previous determinations based on optical and radio measurements. It is in agreement within the error with the values given by Dawe et al. (1977, ), Dickens et al. (1986, ), Lauberts & Valentijn (1989, ), Aaronson et al. (1989, ), de Vaucouleurs et al. (1991, ), Schommer et al. (1993, ), Richter et al. (1994, ). Only the systemic velocity given by Garcia (1993, ) does not fall in this range.
The position-velocity curves and velocity dispersion profiles we measured for the stellar and gaseous components along the major and minor axis of NGC 4672 are presented in Fig. 5 and Fig. 6, respectively.
At each radius the plotted velocities V of ionized gas and stars are the observed ones after subtracting the value of the systemic heliocentric velocity without correcting for the galaxy inclination.
4.2. Stellar kinematics
The major-axis stellar kinematics is measured out to about on each side of the nucleus (Fig. 5). The velocity curve is characterized by a central plateau showing no rotation for . At larger radii we observe an asymmetrical increasing of the rotation velocity on both sides of the major axis. The rotation velocity at the farthest measured point is about 90 and about 200 on the receding and approaching side, respectively. The velocity dispersion profile displays a central dip of about 60 . In fact the velocity dispersion is at and about 210 at . Outwards it decreases reaching an almost constant value of 100 for .
The minor-axis stellar kinematics extends to on the NW side and to on the SE side, respectively (Fig. 6). The velocity curve shows a steep gradient in the nucleus () rising to maximum rotation of about 85 ( ). At larger radii it tends to drop to a zero value on the SE side, while it remains almost constant at about 40 on the NW side. Along the minor axis the stellar velocity dispersion profile has a more uncertain behaviour. It seems to fall from a central value of to about 80 further out.
4.3. Ionized-gas kinematics
The major-axis ionized gas kinematics is measured to about from the center on both sides of the galaxy (Fig. 5). The gas velocity curve is radially asymmetric. On the SW side it exhibits a sharp gradient reaching about 110 at . Further out it shows bumps and wiggles and the velocity oscillates between and in the radial range between and . Then it rises to about 170 at . Along the NE side the rotation velocity increases almost linearly with radius from 0 to about 170 between and , then it remains approximately constant up to . Outside it declines to about 140 at . The gas velocity dispersion is lower than 70 at all radii.
Along the minor axis the measurements extend between and (Fig. 6). No rotation is detected and the corresponding velocity dispersion declines from about 60 to 0 moving outwards from the center.
The fact that the same velocity dispersion corresponds to different rotation velocities on both sides of the galaxy along its major axis, shows that the ionized gas is an unreliable tracer of the circular rotation in the observed radial range of NGC 4672. Besides, this kind of disturbed rotation curves are common in disk galaxies, as shown in a recent study of the optical velocity curves of a large number of Virgo S0's and spirals by Rubin et al. (1999).
The analysis of the interplay between the ionized gas and stellar kinematics along both the major and minor axis shows a decoupling between the rotation of the gas and that of the inner () stellar component. Similarly, the stellar velocity curves indicate that a kinematical decoupling between the inner and outer stellar components is also present as well.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000