Astron. Astrophys. 319, 435-449 (1997)

8. Discussion and conclusions

In the present paper we have shown that there is a type dependence in the zero-points of the diameter and also B - magnitude TF-relations. The new knowledge of the zero-point differences will have direct impact on our future attempt to use the TF-relation in the study of the kinematics of the local galaxy universe. The type dependences in both the diameter and magnitude relations were shown to be expected from simple disc + bulge + dark halo models, where the de Vaucouleurs disc determines the photometric diameter. This discussion of the origin of the type dependence should form a useful starting point for more detailed investigations of such models, including the fraction of dark mass needed, using large galaxy samples and the TF-relations. It is also clear that improved knowledge of the total gas fractions is important for the present kind of analysis of the dark mass fraction within the radius that usual TF-relations refer to. Our present calculations gave for values in the range 0.5-0.8 .

A concise summary of the main conclusions:

• the type dependence may be seen as a zero-point shift, while the slope remains the same in the range 1 8, and especially in the range 2 6, to be used in the KLUN analysis
• the slope of the inverse diameter TF-relation is quite close to 0.5 as expected from the simple model, where the dark mass fraction is a constant; the slope of the inverse magnitude TF-relation is close to 0.1, also as expected from the same model
• as the main ingredient of the model is the de Vaucouleurs exponential disc, we confirm that the bulge component does not generally influence the determination of the photometric diameter at the 25 mag (arcsec)^-2 level
• it is concluded that the kinematical distance scale used to calculate linear diameters (and absolute magnitudes) is sufficiently good so that at constant calculated diameter the intrinsic variations in (mass) dominate
• we do not see evidence for a curvature in the TF-relations
• the zero-point shifts may be understood in terms of the disc + bulge + dark halo model, with reasonable ratios for the disc and bulge components

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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