In this section we study the monochromatic surface brightness
distribution
at 
Å, 
Å, 
Å, 
Å and 
Å (see Table 1 (click here)), whose
images are shown in Fig. 11 (click here).
After masking disturbing field stars, ellipses were fitted to
the isophotes following the algorithm described in
Jedrzejewski (1987) and implemented in the ELLIPSE task of the
IRAF.STSDAS package. The results for Å
image is shown in Fig. 12 (click here).
In Fig. 13 (click here) we plot the ellipticities and position
angles (PA) of the fitted ellipses against its semi-major axis for
the Å image. It presents the same trends already
noted by Varela et al. (1990) for B, V, R, I and Z
images and by MMG for near-infrared images.
Also in our case the other continua images show essentially the same
behaviour as in Å.
At 
, where PA 
, the ellipticity has its
first maximum 
, suggesting an inner bar-like
structure. It follows a transition zone at 
,
where the ellipses are nearly round 
. Then,
increases again, accompanied by a strong variation in PA
from 25'' to 35''. With increasing radius, the ellipticity
increases from 0.12 to 0.18 at 
, the edge of our frames.
MMG have found that
increases further to 0.25 at 
and to 
at larger
radii. The local maximum in the ellipticity at 50'' corresponds
to the external border of the inner ring, at 45''. As mentioned by
Kormendy (1993), these nested elliptical regions of different
indicate that at most one of these zones can be axially
symmetric and part of the disk must be oval, since warps can
be excluded at this high surface brightness. Assuming axial
symmetry for the zone between r=50'' and 70'', we get an
inclination angle of 
. This behaviour of and PA
is what one might expect of an oval or
bar-like structure, more marked in young stars than in old
populations, and with dust obscuration near the young component
(Varela et al. 1990).
The mean surface brightness of the fitted ellipses are plotted
against the ellipses major axis in Fig. 14 (click here).
A least square test, using a downhill simplex minimization algorithm
("amoeba") implemented in the task NFIT1D of the IRAF package, shows
that the best fit to these profiles are attained
with a de Vaucouleurs' law, representing a small bulge, plus two
Freeman's (1970) disks, a thick and a thin one.
The de Vaucouleurs' law is given by
where 
is the effective radius (the radius of the isophote that
contains a half of the total bulge luminosity) and 
is the
surface brightness at 
.
The Freeman's exponential disk law is
where 
is the disk scale length and I0 is the luminosity
density at r=0.
We express 
and 
in Oke & Gunn's (1983)
magnitude scale, where the parameters 
and 
are given by
The respective parameters of each component are also given in Fig. 14 (click here). Sanders & Tubbs (1980) also required the superposition of two disks plus a bulge, to explain the gas kinematics in NGC5383 galaxy.
Figure 16: Gray scale representation of two dimensional color maps
,
, 
and
. The limits are indicated in each image,
white represents bluer colors. (1pixel = 0.303arcsec)
The young stellar population underlying to the ring of HII regions
produces a bump in the observed continua profiles, which is more
prominent in the Å profile (Fig. 14 (click here)),
where it extends itself between 
. In order to
search for other substructures in the central regions, we generated
a synthetic image from the profile fitted to Å image,
addopting for each brightness level the ellipse shown in
Fig. 12 (click here). This procedure is performed with the IRAF task
BMODEL. The result of the subtraction of the model from the data
for the central 20'' is shown in
Fig. 15 (click here). An oval ring of 
is detected,
elongated in the same direction as the small central bar studied by
MMG. It coincides with the isophote of highest ellipticity (Fig.
13 (click here)) and settles the beginning of the
strong variation in the isophotes
position angle. At 
a smaller circular ring is found
(Fig. 15 (click here)), which coincides with the innermost ILR as
calculated by Shaw et al. (1993). The central
features inside r < 2'' are artifacts of images subtraction.
We present in Fig. 16 (click here) the two-dimensional distribution of
the color indexes
),
),
)
and 
).
In these pictures, young (blue) and disk (red) populations appear as
white and grey colors respectively. The population of the HII\
regions ring detaches clearly as white, in color indexes
and , becomes blured in
and disappears in .
Inside the ring appear small arcs of dust, which present
spiral-like structure. A comparison with Fig. 1 (click here) reveals
that HII regions 55, 56 and 72 are located in the outer part
of these dusty features
to the northwest, probably indicating star formation activity
induced by a spiral perturbation in the inner disk.
Figure 17: Comparison of the continuum distribution with combinations of
Bica's templates. We quote the contribution of each age template
at 5270 Å. Bica's templates include star cluster spectra with
different age and metallicity. Globular clusters: 
);
); 
); 
);
).
Intermediate age: I1(109 yr); I2(5
yr). Young clusters:
Y1(107 yr); Y2(5
yr); Y3(108 yr);
Y4(5
yr)
We obtained the continuum brightness distribution for five positions across the disk by integrating azimuthal profiles. These results are compared in Fig. 17 (click here) to combinations of Bica's (1988) templates, through a simple stellar population synthesis. From the bulge outwards it is possible to see a small increase of younger stellar population contribution, which appears massively on the ring, at 46''. In the outer disk, at 60'', there is a clear presence of a younger population than in the inner disk.
At the very center 
the population is an old one (Bica's
template G1, 
Gyr); at r=10'' we need to introduce intermediate age
population of 
yr (template I1) in a similar amount
than the old one; at r=20'' we need to add an even younger stellar population
of 500 Myr and at
r=46'', the ring of HII regions, we need a substantial contribution of
107 years old stellar population, which corresponds to the continuum of
ionizing clusters. The spectrum
of the disk outside the ring of HII regions (r=60''), shows the presence
of younger stellar population than the inner disk, showing that
the star formation activity took place not only on the ring
but also in the external disk.