  We present H-band images obtained with NICMOS of a field 75'' (5kpc) above
the plane of the disk of the edge-on spiral galaxy NGC 5907. Ground-based
observations have shown that NGC 5907 has a luminous halo with a shallow radial
profile between 4 and 8 kpc that roughly traces the dark matter distribution of
the galaxy deduced from its rotation curve. Our NICMOS observations were
designed to resolve bright giants in the halo of NGC 5907 to constrain its
stellar composition with the goal of understanding its nature and origin. More
than 100 stars are expected in the NICMOS images if the dwarf-to-giant ratio in
the halo of NGC 5907 is consistent with that expected from standard stellar
initial mass functions, and if ground-based estimates of the distance to NGC
5907 and the integrated colors of its halo are correct. Instead we observe only
one candidate giant star. This apparent discrepancy can be resolved by assuming
either a significantly larger distance than suggested by several studies, or a
halo metallicity much lower than suggested by ground-based colors and as low as
metal-poor Galactic globular clusters. If previous distance and halo color
estimates for NGC 5907 are correct, our NICMOS results suggest that its
extended light is composed of stars that formed with an initial mass function
different than that observed locally, leading to a much higher ratio of dwarfs
to giants. We describe how these three possible explanations for the absence of
bright giants in our NICMOS images of the halo of NGC 5907 might be constrained
by future observations.
