Globular Cluster Systems As Distance Indicators: Metallicity Effects On The Luminosity Function

We investigate the universality of the globular cluster luminosity function (GCLF) and the use of this function as an extragalactic distance indicator. Previous studies have found an offset between GCLF distances and those obtained with other techniques. We show that this offset can be understood in terms of a metallicity effect. Specifically, the globular cluster systems used in distance scale studies have traditionally been those around elliptical galaxies. These systems have higher mean metallicities than the Milky Way globular cluster system. Consequently, the peak of the GCLF in the systems around ellipticals is significantly fainter in $B$ and $V$ than the GCLF peak in the Milky Way. We calculate the shift in the peak of the GCLF relative to the Milky Way globulars in $B$, $V$, $R$, $I$ and $J$ for a range of globular cluster metallicities. Applying these corrections, we find good agreement between GCLF distances and those obtained using the surface brightness fluctuations method. The similarity between metallicity-corrected GCLFs suggests that the underlying mass function of globular cluster systems is remarkably constant from one galaxy to another. Our results allow the GCLF to be employed as an improved distance indicator.