5. Discussion and conclusion

The separation of galaxies into their bulge and disk components from image analysis, is of course an exercise of limited physical significance. It is essentially a quantitative form of morphology, and its usefulness will have to be appreciated for the hints it may give, or constraints it may suggest, in the elaboration of physically self-consistent models.

When a galaxy is projected at a large enough inclination to the line- of-sight, the presence of the two components, their relative importance, some details of their SuBr profiles and the inclination of the system, are apparent from the isophotal contours and from their analysis by Carter's method, or perhaps others. This has been shown by simulations (Rix & White 1990), or in the numerous papers quoted above in Sect. 1.1. (click here) It is then of interest to derive two-components galaxian models from some form of isophotal analysis of the image.

In our contribution to this approach, it was proven feasible to relax part of the approximations made in earlier work: it is here assumed only that the bulge and disk project are two systems of coaxial and concentric ellipses. The SuBr and profiles of the two system are not a priori constrained.

The problem of deriving these four unknown functions from image analysis could perhaps be tackled more efficiently than by the step by step procedure described in Sect.nbsp;2 (click here), It should be emphasized however than any approach will involve the derivation of a necessarily uncertain asymptotic model, and various alternatives to adapt the number of degree of freedoms to the available information: this heavily depends upon the inclination (too large or too small, it leads to difficulties!) and to the relative "strengths" of the bulge and disk (if one dominates too much uncertainties occur).

As it is, the techniques here presented have the following advantages:

1. They give, as explained in Sect. 2.3 (click here), a direct insight into the profiles of the axis ratios and , allowing a choice of plausible interpolated profiles for these quantities, including the "classical" one .
2. They provide a complete model for bulge and disk, allowing to calculate their respective contributions to the galaxian image at any position, and providing a stringent 2D test of the formal quality of the representation.
3. When the assumptions underlying our techniques do not lead to good solution, i.e. when the galaxian image cannot be well represented by two systems of elliptical isophotes, we are still left with an approximate bulge model, to be substracted from the image to provide an approximate disk model.

After experimenting with 9 galaxies of various characteristics, and considering also the results of previous work, it seems to us that the derivation of two-components models for galaxies at suitable inclinations, may lead to interesting indications about the followings topics:

1. the light profile of bulges, often very close to the r^1/4 law, but with possibly significant deviations in some cases (NGC 4350 and other S0's with spH envelopes?)
2. the light profiles of disks, with the structure of bright and dark rings, or lenses, that is found in all the studied S0's, although at various degrees of contrast,
3. the intrinsic thickness of disks, which for near edge-on galaxies, can be approached from their transverse scale-heights,
4. the colour contrast between a bulge and a disk, which could be more prcisely measured after their "separation" (see an example by Silva et al. 1989).

The present techniques might be perhaps applied to some spirals galaxies, provided their images can be cleaned from the troublesome effect of localized dust and star formation regions (possibly through observations in selected IR bands?). Only spirals where the projected disk does not largely deviate from an ellipse could be considered.

One should finally comment that poor seeing can lead to completely spurious results in the type of image analysis here considered. In the inner galaxian regions, the MajA light from the disk will be transfered to its MinA, and added to the bulge light. Eventually, a spurious hole in the disk light may result! Farther out, the contrast of an edge-on thin disk might be much reduced by seeing, as we experinced by comparing frames of NGC 3377, 3115, 3098, 4350. 5308... obtained at various resolutions.

Acknowledgements

Part of the frames used in this work where obtained by J.L. Nieto at the CFHT and J. Marchal at the OHP. I geatly benefited from their cooperation and their friendship until their untimely death. Dr. P. Poulain obtained the other CFHT and Pic du Midi frames as part of a cooperative study of colour distributions in early-type galaxies: I am glad for his permission to use this material in the present work.