The stellar content of the halo of NGC 5907 from deep Hubble Space Telescope NICMOS imaging

We present H-band images obtained with the Near Infrared Camera and Multi-O bject Spectrometer (NICMOS) of a field 75 " (5 kpc) 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 betwe en 4 and 8 kpc that roughly traces the dark matter distribution of the gala xy deduced from its rotation curve. Our NICMOS observations were designed t o resolve bright giants in the halo of NGC 5907 to constrain its stellar co mposition with the goal of understanding its nature and origin. More than 1 00 stars are expected in the NICMOS images if the dwarf-to-giant ratio in t he 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 b y assuming either a significantly larger distance than that suggested by se veral studies or a halo metallicity much lower than that suggested by groun d-based colors and as low as that in metal-poor Galactic globular clusters. If previous distance and halo color estimates for NGC 5907 are correct, ou r 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 t hree possible explanations for the absence of bright giants in our NICMOS i mages of the halo of NGC 5907 might be constrained by future observations.