7. Effective surface brightness versus magnitude
This relationship was cited by Binggeli & Jerjen as being of comparable value to the profile-shape parameter, n, as a distance indicator. We do not deny that it is a reasonably useful relationship. In fact, it is related to the L -n and R -n relationships, and probably a direct consequence of them. However, it can be expected to be significantly harder than the L -n and R -n relationships to measure accurately, because it invokes the effective surface-brightness parameter, which is a tertiary parameter (unlike n and which are primary parameters and total magnitude which is a secondary one).
In order to measure effective surface brightness accurately, a model profile must first be fitted, then the profile must be extrapolated to obtain a total-light estimate and then the profile model must be integrated to the half-light radius. Clearly, an extra stage is involved. We therefore cannot accept Binggeli & Jerjen's assertion that no profile modeling is required in the measurement of either total magnitude or effective surface brightness. Young (1997) and Young et al. (1998) have already demonstrated that for dwarf galaxies in particular, total magnitude values (and therefore effective parameters too) are critically dependent on the profile model adopted.
© European Southern Observatory (ESO) 1998
Online publication: April 28, 1998