Calibration of the surface brightness fluctuation method for use with the Hubble Space Telescope

This paper presents the calibration of the surface brightness fluctuation (SBF) technique for use with the Wide Field and Planetary Camera-2 (WFPC-2) on the Hubble Space Telescope (HST), The calibration has a color term and zero point significantly different from the Kron-Cousins I-band SBF technique developed for ground-based observations. It is a required first step for using SBF at the larger distances now made possible with HST. WFPC-2 observations of 16 nearby galaxies through the F555W and F814W filters were tied to the V and I photometry of the ground-based SBF Survey (Tonry ct al. 1997, ApJ, 475. 399), The calibration of the photometry agrees with that of Holtzman et al. (1995, PASP, 107, 1065) to within a few percent. We used a composite WFPC-2 point spread function (PSF) to measure (m) over bar, and we found that SBF measurements are relatively insensitive to small variations in the PSF, The identification of globular clusters and dust is much easier with HST*s high spatial resolution, We computed (M) over bar(1F814W) for the sample from the measured (I) over bar(F814W) and the ground-based SBF Survey distance modulus, The best fitting calibration has (M) over bar(1F814W) = (-1.73 +/- 0.07) + (6.5 +/- 0.7)[(V - I)(0) - 1.15], This color dependence is significantly steeper than the one for I-KC used on the ground, The total rms about this fit is 0.16 mag, reflecting not only the error in WFPC-2 measurements presented here but also the errors in the ground-based distance moduli, After accounting for additional sources of error of similar to 0.05-0.10 mag, the HST observations and ground-based observations are seen to contribute roughly equally to the residual error. We measured (I) over bar(F814W) for NGC 3379 from the ground to check this calibration, The distance obtained using the F814W filter and the HST calibration agreed well with that obtained in the SBF Survey. For NGC 3379 we also compared the difference in fluctuation magnitudes Delta (m) over bar between the I-KC and I-F814W filters to theoretical predictions for a typical old, metal rich population of a giant elliptical galaxy and found excellent agree mont. (C) 1997 American Astronomical Society.