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For refcode 1994ApJ...429..557A:
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1994ApJ...429..557A SURFACE BRIGHTNESS FLUCTUATIONS IN GLOBULAR CLUSTERS AND STELLAR POPULATIONS IN GALAXIES EDWARD A. AJHAR Kitt Peak National Observatory, National Optical Astronomy Observatories, PO Box 26732, Tucson, AZ 85726; and Massachusetts Institute of Technology AND JOHN L. TONRY Massachusetts Institute of Technology, Room 6-204, 77 Massachusetts Avenue, Cambridge, MA 02139 Received 1993 December 3; accepted 1994 January 18 ABSTRACT This project describes the spatial surface brightness fluctuations (SBFs) in 19 Galactic globular clusters. The first important result of this 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 magnitude M, as a function of a stellar population's integrated (V-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 relation 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 (RYI) fail to predict the correct slope in the M_I_^bar^, relation; we find 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 clusters, we argue that the SBF distance-scale slope is reasonable. Another result is that "fluctuation colors," which are the differences between 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 the corresponding results in globular clusters. The (V^bar^ - I^bar^) fluctuation color grows redder with increasing metallicity among the globular clusters studied. When (v^bar^ - I^bar^) 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 (V^bar^-I^bar^) among the galaxies, especially among the reddest ones. When (V^bar^ - I^bar^) is plotted against the Mg_2_ index, again we find a trend with (v^bar^ - I^bar^) growing redder as the Mg_2_ index increases. Finally, we discuss separating age and metallicity among globular clusters by using a fluctuation color like (V^bar^-I^bar^) and an integrated color like (U - I), and present 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. Subject headings: distance scale - globular clusters: general - stars: abundances - techniques: photometric
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