New techniques for deriving bulge and disk models of two-component galaxies

Observatoire de la Cote d'Azur, Dept. Augustin Fresnel, BP. 229, F-06304 Nice Cedex 4, France

Send offprint request to: R. Michard

Received: 24 December 1996
Accepted: 11 April 1997

Abstract

A new method is introduced to "separate" the bulge and disk components of early-type galaxies, i.e. S0's and disky E's, viewed at such inclinations that their isophotes depart significantly from ellipses. In our approach, the two components are assumed to have concentric and coaxial ellipitical isophotes of arbitrary surface brightness (SuBr) and ellipticity profiles. These four functions are to be determined by comparing calculated and easily measured quantities, i.e. the harmonics of the SuBr along suitable reference ellipses. 
The principle of the method is straightforward and its implementation very easy in favourable cases. There are difficulties however with truly edge-on galaxies, where the assumption of elliptical isophotes is perhaps no more valid for the disk, and with objects of such inclinations that the geometrical signature of the two-components structure tends to vanish. 
Our techniques have been tested with a number of calculated model objects, and with 9 real galaxies, observed at sub-arcsec resolution at the CFHT or at the Pic du Midi. The results are compared with those of other authors, sometimes involving more uncertain assumptions: the one of negligible disk contributions to the minor axis SuBr is unsatisfactory for S0 galaxies.