  As part of the Advanced Camera for Survey (ACS) Virgo Cluster Survey, we have
measured Surface Brightness Fluctuations (SBF) in a sample of 100 early-type
Virgo galaxies. Distances derived from these measurements are needed to explore
the three-dimensional structure of the Virgo Cluster, study the intrinsic
parameters of globular clusters associated with the program galaxies, and
compare with the galaxy distances derived from globular cluster luminosity
functions. Our SBF measurements have been performed in the F850LP bandpass of
the Wide Field Channel of the ACS on the Hubble Space Telescope. These are the
first measurements of this kind, and we present the first SBF calibration for
this bandpass. The measured fluctuations depend on galaxy stellar population
variations, which we quantify by galaxy color (g_475-z_850)_0, where g_475 and
z_850 are the galaxy magnitudes, respectively, in the F475W and F850LP ACS
filters. We derive the following calibration for the absolute SBF magnitude
M_850: M_850 = -2.06 +/- 0.04 + (2.0 +/ 0.2) [ (g_475-z_850)_0-1.3 ] in the
range 1.3 <(g_475-z_850)_0 <= 1.6, and M_850 = -2.06 +/- 0.04 + (0.9 +/- 0.2) [
(g_475-z_850)_0-1.3 ] in the range 1.0 <=(g_475-z_850)_0 <= 1.3. Physically,
the two different color regimes correspond to different galaxy types: giant
ellipticals and S0s at the red end, and early-type dwarfs at the blue end. For
the first time in SBF studies, we are able to provide a firm empirical
calibration of SBF in early-type dwarf galaxies. Our results agree with stellar
population model predictions from Bruzual & Charlot (2003).
