STATISTICAL PROPERTIES OF STELLAR POPULATIONS AND SURFACE-BRIGHTNESS FLUCTUATIONS IN GALAXIES

Some relevant concepts dealing with the statistical properties of stel lar populations are approached and explored in detail defining the eff ective stellar contributors N(eff), and the effective luminosity l(eff ) of a stellar aggregate. These two quantities strictly relate to the distinctive properties of a stellar population, and basically modulate the intrinsic statistical scatter in a pixel-to-pixel surface photome try of a galaxy (providing that photon scatter and other perturbing ef fects are excluded). The effective luminosity and number contributors in simple stellar populations have been calculated through a set of 20 6 models based on Buzzoni's (1989) code for population synthesis, incl uding also the case of composite stellar populations. The results have been applied to the Tonry-Schneider (1988) method on galaxy surface-b rightness fluctuations deriving new distances for M 32 and galaxies in the Virgo cluster as well as in the Leo, Fornax, and Eridanus groups observed by Tonry and colleagues. The distance of Virgo is found to be 15.3 Mpc supporting Tonry's (1991) previous estimate. Also the distan ce of the Galactic globular cluster M 3 has been derived resting on th e complete sample of 10000 stars by Buonanno et al. (1986,1988). We ob tain a distance modulus (m - M)0 = 15.01 +/- 0.13 in excellent agreeme nt with other current estimates confirming the reliability of the Tonr y-Schneider method once accurate theoretical models for population sys thesis are considered.