A monochromatic image was constructed from the Perot-Fabry
observational data in order to see the morphology of the inner part of
NGC 6181. The H
map of the central part of NGC 6181 (Fig. 1 (click here))
reveals two bright central sources none of which coincides with the
center of the isophotes in the continuum. Two faint tails of
H
emission embrace the central continuum source which is
located in the area of very weak emission. Bearing in mind the absence
of radio emission from the NGC 6181 nucleus, one may conclude that the
nucleus of this galaxy is very quiescent.
The direct images obtained with the 1m telescope were used first of
all to find the precise position of the center of the galaxy in the
continuum. It was determined with respect to five nearby stars.
Location of the center was compared with the H
distribution
and with the dynamical center position (see the next section). In
addition we tried to derive some surface brightness distribution
characteristics. Fig. 2 (click here) demonstrates rather smooth image in I,
with a weak bar-like disk elongation in the inner part, and a more
clumpy image in B; prominent spiral arms extend up to the outermost
radii in all passbands, confirming the grand-design classification of
arms made by Elmegreen & Elmegreen (1984).
Figure 4: Radial distributions of azimuthally averaged surface brightness
in the B and V passbands. The straight lines represent the disk exponential
laws fitted to the radius range of 27
to 46
Figure 4 (click here) presents B and V azimuthally averaged radial
surface brightness profiles
assuming PA (line of nodes)=
and inclination
in accordance with the velocity field analysis (see below). It
shows that this galaxy possesses a very compact bulge which does not
affect light distributions beyond the radius 7
. In the range
a brightness excess is noticeble over the simple
exponential law extrapolated from the outer parts; this excess is
reproduced in all four filters being the largest (0.15 mag)
in the B passband. It seems that the radius of 25
is a
boundary between two disk subsystems. The disk scale measured in the
range 25
-45
for all four passbands slightly
decreases from blue to red (Fig. 5 (click here)) being in general accordance with
earlier results of Elmegreen & Elmegreen (1984) and Roth (1994).
Figure 5: Disk scale variation with spectral range. Previous
published data are also plotted as comparison
Figure 6: Color radial profiles averaged over azimuth in the galaxy plane.
The rms error of a single point is less than 0.05 mag
The azimuthally averaged color tends to be bluer up to 
and then some reddening occurs, but the slopes of these
trends are quite different for different colors (Figs. 6 (click here) and 7 (click here)). In
Fig. 7 (click here) we try to compare radial color variations in NGC 6181 with
models for old stellar populations showing pure metallicity trend
(Worthey 1994) and with a mean observational sequence of galactic
colors (Buta & Williams 1995), which is known to be defined mainly
by different present-time star formation rate. Color excesses expected
due to interstellar reddening in the Galaxy are also shown.
The reddening in B-V at 
has an azimuthally
non-homogeneous character: it is a distinct, very red spot to the west
from the nucleus obviously related to a local dust concentration;
the optical characteristics of the dust may be unusual because
the spot is absent in V-R and V-I colors. The other color
variations seem to be rather azimuthally homogeneous. Comparison
of the observed and the expected color trends shows that between the
radii 7
and 25
, i.e. in the inner disk distinguished
by some brightness excess, the observed color variations may be
satisfactorily explained by variations of star formation intensity,
because the point grid is roughly parallel to the observational
sequence of integrated galactic colors. However the observed color
variations in the outer disk are more complicated and rather unusual -
especially for 
, where the reddening of B-V and V-R occurs
under the constant V-I. This looks quite inexplicable in the frames of
simple effects which influence the color.
Figure 7: (V-R, B-V) a) and (V-I, B-V) b) diagrams
for the radial color
variations in NGC 6181. Points are plotted through one arcsecond step.
Estimates of V-R and V-I have systematic shift (see the text)
Figure 8: Variations of the orientation of the photometric major axis
along the radius. For 
measurements taken for all passbands
were averaged, beyond this radius only I isophotes were used.
The long-dashed line indicates the orientation of the outermost isophotes
according to our measurement of the SKYVIEW picture, the short-dashed line
shows the kinematical line of nodes
The isophote form analysis (pure-ellipse fitting) was
carried out to check a possible deviation from axial symmetry. The
ellipticity between 
and 
from the center gradually
increases from 0.10 (bulge) to 0.35 - a behavior which is quite
normal for a galaxy whose inclination is about 
. The radial
dependence of 
is presented in Fig. 8 (click here). We see an
unambiguous turn of isophotes in the very center of NGC 6181.
Measurements in all passbands show that at the radius of
the position angle of the major axis is
with an uncertainty less than 
, which differs by
from the orientation of the outermost isophotes (
,
Nilson 1973;
, our measurement of the SKYVIEW isophote at the
). In the radius range 
isophotes are
also twisted by 
, but in the opposite sense with respect to
the innermost region. Only beyond 
, where the
radial brightness distribution follows a pure exponential law, the
isophote major axis becomes aligned with the line of nodes.