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For refcode 1997ApJ...483..745N:
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1997ApJ...483..745N The Surface Brightness Fluctuations and Globular Cluster Population of NGC 4478 ERIC H. NEILSEN, JR., ZLATAN I. TSVETANOV, AND HOLLAND C. FORD Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218; neilsen@pha.jhu.edu, zlatan@pha.jhu.edu, ford@pha.jhu.edu Received 1996 September 3; accepted 1997 February 6 ABSTRACT We calculate the distance to NGC 4478, a neighbor of the giant Virgo elliptical M87, using the surface brightness fluctuations method on parallel data from the HST WFPC2 camera. The exposures considered were taken through the F814W filter, which approximates the Johnson I, and through the F606W filter, which is effectively a broad V filter. We describe the use of Fourier methods for separation of the fluctuations due to statistical variations in the projected number of stars per pixel (used to determine the distance) from fluctuations from other sources, which include photon noise, foreground stars, cosmic rays, and the variations due to the overall structure of the galaxy. The mean apparent magnitude of stars responsible for the measured fluctuations is m^bar^_I_ = 29.78 +/- 0.03, with an additional systematic uncertainty of 0.05 mag. Using the latest calibration of Tonry (1997), we obtain a distance of 15.6 +/- 1.0 Mpc, which is consistent with the distance determined through the use of this method in the infrared K band by Pahre & Mould (1994). We discuss the globular cluster (GC) luminosity function and color distribution, based on the ~130 candidates identified in the field. We find a roughly Gaussian luminosity function centered at m_V_ = 23.82 +/- 0.38 with {sigma} = 1.16 +/- 0.21. The GC color distribution is also roughly Gaussian, centered at V-I = 0.91 +/- 0.03, with {sigma} = 0.20 +/- 0.03, and it is coincident to within the error bars with the blue peak of the bimodal color distribution found in M87. Comparing our data with recent evaluations of the GC color distribution at two different locations in M87 we find a trend of the red peak disappearing with radial distance. We speculate that this could be understood if the blue portion of the M87 GC population were acquired from its smaller companions. The specific frequency of globular clusters is among the lowest for elliptical galaxies, comparable only to the value found in M32. In both cases this may be a result of tidal truncation by a giant neighbor. This further indicates that clusters have been stripped from NGC 4478 by M87 and likely now are part of its globular cluster population. Subject headings: galaxies: distances-galaxies: individual (NGC 4478)- galaxies: star clusters
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