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Astron. Astrophys. 360, 439-446 (2000)
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.
![[FIGURE]](img83.gif) |
Fig. 5. The stellar (filled symbols ) and ionized-gas (open circles ) kinematics measured along the disk major axis (i.e. , the bulge minor axis at ) of NGC 4672. The systemic velocity is . All rotation velocities are plotted as observed without applying any inclination correction. The filled circles and the filled squares represent data obtained in August 1998 and July 1999, respectively.
|
![[FIGURE]](img87.gif) |
Fig. 6. The stellar (filled circles ) and ionized-gas (open circles ) kinematics measured along the disk minor axis (i.e. , the bulge major axis at ) of NGC 4672.
|
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
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