Up: New techniques for
Defering the application of the present techniques to such a sample
of galaxies that would be needed for deriving astrophysical results,
a few objects have been selected to test the method and compare its
results to previous similar studies.
Table 1 (click here) lists these objects with relevant information about the
used observations, and the quality of the model. All the frames were
obtained in the R band (Cousin's definition).
- Column 1: NGC number of the object.
- Column 2: Morphological type from Michard & Marchal
- Column 3: Classification of the envelope from the same source.
- Column 4: Telescope, either CFH for the 3.5 m
Canada-France-Hawai telescope; TBL for the 2 m Bernard Lyot Pic du Midi
telescope; OHP for the 120 cm instrument of Observatoire de Haute-Provence.
- Column 5: FWHM of the frame PSF in arcsec.
- Column 6: Major axis of the outermost measured isophote.
- Column 7: Major axis of the valid range of the two-component
- Column 8: Mean value of the model residuals in %.
- Column 9: Standard deviation of the same.
As may be seen from the table, the CFHT frames have a resolution of about
0.6 arcsec but a small field. The low resolution and large field frames
from the OHP were used only to obtain synthetic frames by merging with
the CFHT frames (see below): this is done only if the frames have a large
enough range of good agreement in their respective Carter-like analysis. For
edge-on disk galaxies, the OHP frames are so badly affected by seeing that
the merging of data is to be avoided. The Pic du Midi frames have a
resolution of circa 1 arcsec and a sufficient field.
When a mixed synthetic image was used, the figures in the last
three columns refer to this image.
To represent a galaxy with two systems of concentric and coaxial ellipses,
it is advantageous to get rid of the features in the original image that
cannot comply with such a model: this includes asymmetries resulting from
dust, low contrast bar or spiral structure, and isophotal twists (such
as the kind described by Michard & Marchal 1990). For this
purpose, the image is analysed in terms of the representation first
introduced by Carter (1978): each isophote
is described by a reference mean ellipse and harmonics coefficients
ei, fi describing deviations from this ellipse. Then a modified
synthetic image is built from the results of this analysis, forcing all
isophotes to a common center and orientation, discarding the sine harmonics
fi and the odd cosine harmonics e2i+1.
Such a synthetic image retain the essential features of the real galaxy as
far as our problem is concerned, and has the advantage to be nearly devoid
of noise, making more efficients the algorithms designed for its
Finally if we have two images of complementary qualities for a given
object, for instance a high resolution but small field image, and a lower
resolution but large field one, both can be separately analysed along
Carter's principles, then the resulting files intercompared and
eventually mixed if they show a large enough range of good agreement.
Then a synthetic image can be obtained from the mixed file, keeping
the resolution of the best frame and the field of the other.
In Fig. 1 (click here) are presented the main relevant data and the run of the
bulge and disk models for the test galaxy NGC 2549. Similar figures for
the 8 other test galaxies are displayed with the same captions.
The data and results for each galaxy will now be commented upon.
Figure 1: The adopted abscissae are a1/4 in arcsec. In the
three upper windows are shown the c/a ratio, the e4 and e6
parameters from our analysis according to Carter. The lower window gives
the run of the and parameters. The intermediate one
gives the profiles of the total SuBr (full squares), the bulge SuBr
(circles) and the disk SuBr (spades)
- NGC 2549 The data from one CFHT and one OHP frame were successfully
merged to produce a synthetic "observed frame". The and
e4 coeeficients have 3 maxima, at a circa 7, 33 and 80 arcsec as
regards . Correspondingly our solution shows a disk with two
bright rings and a dip in between. The bulge nearly obeys a a1/4
law. In the outer range, the values decreases while the
values increases to form a thick disk envelope. The residuals of this
solution are rather small.
- NGC 3098 It is the difficult case of an edge-on, disk dominated
galaxy! The and e4 coefficients show maxima near a = 1,
10 and 27 arcsec (the latter two almost merging). Accordingly we find a
small inner disk peaking above the main one. The range of the model is
limited by the field of the CFHT frame, but the envelope has been
classified spH so that the strength of the bulge component is expected
to increase in the envelope.
The residuals of the solution are large, possibly
because the assumptions made in our technique do not well apply.
- NGC 3115 It was studied from a synthetic frame, merging data from the
CFHT and OHP frames (the merging leaves a small defect in the run of
e6). This object has an spH envelope. The bulge follows the a1/4
law. As a result of the two maxima in the e4 coefficient, we find a
bright inner disk; the main disk has a bright ring near a = 16 arcsec,
and a cut-off near a = 100. The solution has low residuals.
- NGC 3245 This galaxy of relatively low inclination and small
Carter coefficients, was studied from a synthetic mixed frame.
Approximate values of and were derived from local models
and the complete solution was obtained with constant axis ratio for
both components. The solution has low residuals, but the bulge component
shows a surprising SuBr profile.
- NGC 3377 This well known diE and spH object was studied from a
synthetic mixed frame, involving the analysis of two CFHT and one OHP frame.
In agreement with Scorza (1992) the disk has an inner bright
and high gradient range, followed by a much
lower gradient range and a cut-off near a = 35 arcsec. In the high
gradient range, the disk has a transverse scale height below 1 arcsec.
The bulge follows the a1/4 law with an inner cusp. The residuals of
the solution are very small.
- NGC 3585 This diE was studied from a CFHT frame only.
The bulge follows the a1/4 law. The disk has a low contrast bright
ring. Our solution has very small residuals.
- NGC 4350 is an S0 with a spH envelope. The bulge is in two parts,
both following the a1/4 law, but with different effective scales and
values. It is clear that the spheroidal envelope is not simply a
continuation of the inner bulge. The disk thickens prior to its cut-off
near a = 70 arcsec. The residuals for our model are small.
- NGC 5308 is an edge-on S0 with a thD envelope: as explained in
Sect. 2.2 (click here) there are ambiguities in the choice of the asymptotic model. The solution was guided by assuming an outer bulge with constant and a
thickening disk. An inner disk well above the main disk is again found
(compare with NGC 3098, NGC 3115). The residuals of our solution are
- NGC 5422 is a very inclined S0 with a thD envelope (although
there are hints of spiral structure in our unpublished maps). The
and e4 profiles show 3 maxima as in NGC 2549. In our
solution, the inner bulge follows the a1/4 law but its SuBr shows
an uncertain tendency to level off in the outermost range.
There is a bright inner disk,
followed outwards by a bright ring, a lens...and a cut-off.
As for other nearly edge-on disk galaxies, the residuals from our model
are relatively large.
Up: New techniques for
Copyright by the European Southern Observatory (ESO)