SURFACE BRIGHTNESS FLUCTUATIONS IN GLOBULAR-CLUSTERS AND STELLAR POPULATIONS IN GALAXIES

This project describes the spatial surface brightness fluctuations (SB Fs) in 19 Galactic globular clusters. The first important result of th is investigation is that the globular cluster fluctuation measurements are not discordant with the empirically determined extragalactic SBF distance scale of Tonry (1991), which gives the absolute fluctuation m agnitude M(I)BAR, as a function of a stellar population's integrated ( Y - I)0 color. For globular clusters, M(I)BAR, = -2.02 +/- 0.04 and is different from that of galaxies, but the globular cluster results are unable to place constraints on the zero point or slope of the relatio n for galaxies. In addition, a measurement of m(I)BAR, for a globular cluster can yield a distance modulus to the cluster accurate to about 0.25 mag. We also discuss at length why the Revised Yale Isochrones (R YI) fail to predict the correct slope in the M(I)BAR, relation; we fin d the primary cause to be that the giant branches of the RYI in the V and I bands fail to turn over at high metallicity. In addition, based on the behavior of the giant branch in very metal-rich globular cluste rs, we argue that the SBF distance-scale slope is reasonable. Another result is that ''fluctuation colors,'' which are the differences betwe en fluctuation magnitudes in two bandpasses, are related to the metal abundance of a stellar population, and this analysis has attempted to understand populations in galaxies from their fluctuation colors and t he corresponding results in globular clusters. The (VBAR-IBAR) fluctua tion color grows redder with increasing metallicity among the globular clusters studied. When (VBAR-IBAR) is plotted against integrated (V-I )0, we find a trend from the blue, metal-poor globular clusters to the red, metal-rich galaxies. Also, there may be a spread in (VBAR-IBAR) among the galaxies, especially among the reddest ones. When (VBAR-BAR) is plotted against the Mg2 index, again we find a trend with (VBAR-IB AR) growing redder as the Mg2 index increases. Finally, we discuss sep arating age and metallicity among globular clusters by using a fluctua tion color like (VBAR-IBAR) and an integrated color like (U-I), and pr esent an anaLysis of eight clusters. Although the data presented here have proved to be inadequate for this purpose, a study of fluctuation colors in Magellanic Cloud clusters, which cover a wide range in age, may prove valuable in trying to untangle the age and metallicity of a population with fluctuation colors.