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3. Models for 9 test galaxies

3.1. The data

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.


4.6 4.8
NGC Typ Env Tel Psf tex2html_wrap_inline1418 tex2html_wrap_inline1420 mean tex2html_wrap_inline1422
2549 S0 exD CFH 0.59 35 - - -
id id id OHP 2.06 132 82 +2.7
3098 S0 thD CFH 0.61 50 47 -7.3 10.0
3115 S0 spH CFH 0.55 37 - - -
id id id OHP 3.10 150 109 +3.1
3245 S0 thD CFH 0.60 35 - - -
id id id OHP 2.49 110 71 +3.5 4.2
3377 diE spH CFH 0.59 25 - - -
id id id CFH - 40 - - -
id id id OHP 2.35 140 30 +0.2 1.4
3585 diE spH CFH 1.07 43 37 -1.5 2.2
4350 S0 spH TBL 1.38 86 66 -3.2 4.2
5308 S0 thD TBL 0.95 90 71 -4.0 6.5
5422 S0 thD TBL 0.81 99 95 -4.0 7.2
Table 1: Studied galaxies: frames and models characteristics

  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).
  1. Column 1: NGC number of the object.
  2. Column 2: Morphological type from Michard & Marchal (1994b).
  3. Column 3: Classification of the envelope from the same source.
  4. 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.
  5. Column 5: FWHM of the frame PSF in arcsec.
  6. Column 6: Major axis of the outermost measured isophote.
  7. Column 7: Major axis of the valid range of the two-component model.
  8. Column 8: Mean value of the model residuals tex2html_wrap_inline1158 in %.
  9. 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.

3.2. The use of synthetic images

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 solution.

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.

3.3. Results

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.

  figure397  figure409  figure420
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 tex2html_wrap_inline1200 and tex2html_wrap_inline1204 parameters. The intermediate one gives the profiles of the total SuBr (full squares), the bulge SuBr (circles) and the disk SuBr (spades)

  1. NGC 2549 The data from one CFHT and one OHP frame were successfully merged to produce a synthetic "observed frame". The tex2html_wrap_inline1456 and e4 coeeficients have 3 maxima, at a circa 7, 33 and 80 arcsec as regards tex2html_wrap_inline1456. 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 tex2html_wrap_inline1200 values decreases while the tex2html_wrap_inline1204 values increases to form a thick disk envelope. The residuals of this solution are rather small.
  2. NGC 3098 It is the difficult case of an edge-on, disk dominated galaxy! The tex2html_wrap_inline1456 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.
  3. 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.
  4. NGC 3245 This galaxy of relatively low inclination and small Carter coefficients, was studied from a synthetic mixed frame. Approximate values of tex2html_wrap_inline1200 and tex2html_wrap_inline1204 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.
  5. 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.
  6. 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.
  7. 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 tex2html_wrap_inline1200 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.
  8. 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 tex2html_wrap_inline1200 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 relatively large.
  9. NGC 5422 is a very inclined S0 with a thD envelope (although there are hints of spiral structure in our unpublished tex2html_wrap_inline1524 maps). The tex2html_wrap_inline1456 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.

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