2. Limits on the generality of the indicators
As Binggeli & Jerjen have mis-interpreted certain aspects of the luminosity-n (L -n) and scalelength-n (R -n) distance indicators of Young & Currie (1994) (hereunder YC94) and YC95, it would probably be pertinent to re-emphasize the scope of the indicators and how they are related to one another.
The indicators can be interpreted as follows. Dwarf and intermediate elliptical galaxies of the same n are approximately the same physical size [i.e. they have similar ]. Only therefore, when such galaxies have very similar stellar populations can they be expected to have similar central and mean surface brightnesses, whence similar luminosities 1. It follows that galaxies of different colours can be expected to have different stellar populations and therefore cannot be compared directly using the L -n method. The converse is not always true however, as objects of the same overall colour may have different stellar populations. The L -n relationship appears to be most useful for those dwarf ellipticals with colours of , because other dwarfs are generally bluer (i.e. ) regardless of whether they are higher or lower surface-brightness objects (YC95).
Another important caveat is that if the stellar populations within a galaxy are not well mixed, the surface-brightness profile shape may deviate significantly from that which one might expect on the basis of its size. The n based distance indicators should therefore, ideally, not be applied to galaxies with internal colour gradients. Although the absence of any colour gradient within a galaxy does strictly not imply that its stars are well mixed, in such cases it is probably safe to assume that they are. This is because a conspiracy of many different factors would be required in order to balance the colour gradients that would otherwise inevitably arise from segregated stellar populations. With the above in mind, the Local Group early-type dwarf, NGC 205, should not be used as an absolute-distance calibrator for n -based scaling laws if those target galaxies possessing colour gradients can be screened out. However, it can be used as a calibrator if, as in YC94 and YC95, colour-gradient information is not available for target galaxies.
Since the distance indicators were first presented, Graham et al. (1996) and Gerbal et al. (1997) have found that the correlations on which they are based probably apply not only to dwarf and intermediate ellipticals, but also to classical ones, including the brightest cluster ellipticals. Also, Binggeli & Jerjen have also shown that the correlations probably apply to dwarf lenticulars as well, while de Jong (1996) has demonstrated that even the bulges of spiral galaxies appear to exhibit a continuous range of n values.
It should be remembered however, that colour gradients are more common and often much larger in classical ellipticals and lenticulars than in dwarfs. When dealing with samples of classical early-type galaxies, it is therefore even more important to screen them for objects with colour gradients as such objects cannot be expected to conform to the R -n relationship [or the L -n relationship].
© European Southern Observatory (ESO) 1998
Online publication: April 28, 1998