Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...429..557a&link_type=abstract
The Astrophysical Journal, vol. 429, no. 2, pt. 1, p. 557-571
Other
39
Color, Distance, Galaxies, Globular Clusters, Luminous Intensity, Metallicity, Populations, Stellar Structure, Variations, Calibrating, Charge Coupled Devices, Computerized Simulation, Errors, Photometry, Spatial Resolution, Telescopes
Scientific paper
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 mean value MI as a function of a stellar population's integrated (V - I)0 color. For globular clusters, MI = -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 mI 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 mean value MI 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 abundances 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 (mean value V - mean value I) fluctuation color grows redder with increasing metallicity among the globular clusters studied. When (mean value V - mean value I) 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 (mean value V - mean value I among the galaxies, especially among the reddest ones. When (mean value V - mean value I) is plotted against the Mg2 index, again we find a trend with (mean value V - mean value I) growing redder as the Mg2 index increases. Finally, we discuss separating age and metallicity among globular clusters by using a fluctuation color like (mean value V - mean value I) and an integrated color (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.
Ajhar Edward A.
Tonry John L.
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