2.1. Does the PN Number Density Follow Light?
Although PNe are representative for the bulk of all stars, there are some population effects that can bias their number density profile with respect to the galaxy's luminosity profile. The PN luminosity function (PNLF) is observed to have a near-universal form and bright cut-off magnitude . Then the quantity X,n is defined as the ratio of the number of PN in a stellar population, n magnitudes down the PNLF from the cut-off magnitude, to its total stellar luminosity, in a specified wavelength band X. While the bolometric bol, is expected to be nearly independent of stellar population , Hui et al. show that the blue B,2.5 depends on colour B-V, decreasing by a factor of 4 from the M31 bulge to the reddest elliptical galaxies . Age, or metallicity, or both, must influence B,2.5. Theoretical models predict that the brightest PNe in B have central stars with progenitor masses ~ 2.5 M , with luminosity also dependent on uncertain post-AGB evolution parameters and metallicity. The presence of a younger population in elliptical galaxies, whether made in situ or accreted, could thus change the number of bright PNe significantly. Real ellipticals can have colour and metallicity gradients. Therefore it is prudent to check carefully whether the observed (usually magnitude-limited) PNe samples do indeed trace light.
Observational tests of this question have been carried out in a number of galaxies, including M31, Cen A, the Leo group galaxies NGC 3377, 3379 and 3384, several Virgo ellipticals, the Fornax galaxies NGC 1399 and NGC 1404, and NGC 4697 [2, 7, 9, 11]. Generally, the number of PNe as a function of isophote follows the luminosity distribution for radii outside ~ 1 - 2', in some cases very well. At smaller radii some PNe are lost against the bright background of the galaxy, and the PNe no longer track the luminosity profile. A colour gradient such that the inner parts of the galaxy are redder with fewer PN per blue light may further decrease the number of PNe per unit light there; NGC 4697, for example, has a slight gradient of this kind .