  To empirically calibrate the IR surface brightness fluctuation (SBF) distance
scale and probe the properties of unresolved stellar populations, we measured
fluctuations in 65 galaxies using NICMOS on the Hubble Space Telescope. The
early-type galaxies in this sample include elliptical and S0 galaxies and
spiral bulges in a variety of environments. Absolute fluctuation magnitudes in
the F160W (1.6 micron) filter were derived for each galaxy using
previously-measured I-band SBF and Cepheid variable star distances. F160W SBFs
can be used to measure distances to early-type galaxies with a relative
accuracy of ~10% provided that the galaxy color is known to ~0.035 mag or
better. Near-IR fluctuations can also reveal the properties of the most
luminous stellar populations in galaxies. Comparison of F160W fluctuation
magnitudes and optical colors to stellar population model predictions suggests
that bluer elliptical and S0 galaxies have significantly younger populations
than redder ones, and may also be more metal-rich. There are no galaxies in
this sample with fluctuation magnitudes consistent with old, metal-poor (t>5
Gyr, [Fe/H]<-0.7) stellar population models. Composite stellar population
models imply that bright fluctuations in the bluer galaxies may be the result
of an episode of recent star formation in a fraction of the total mass of a
galaxy. Age estimates from the F160W fluctuation magnitudes are consistent with
those measured using the H-beta Balmer line index. The two types of
measurements make use of completely different techniques and are sensitive to
stars in different evolutionary phases. Both techniques reveal the presence of
intermediate-age stars in the early-type galaxies of this sample.
