DARK-MATTER AROUND GALAXIES - THE EDGE-ON SPIRAL NGC-5907

Using H-band (1.68 mum) surface photometry, we successfully model the rotation curve of the late-type, edge-on spiral NGC 5907. Previous attempts have not been able to match the inner 10 kpc of the rotation curve, which suggests the presence of a massive component inside 10 kpc undetected in visible light, presumably because it is obscured by the dust lane. At near-infrared wavelengths, we measure the starlight from an oblate spheroid bulge and detect more light from the inner portions of the disk than found in visible images. The mass of these components significantly contributes to the calculated gravitational potential, allowing us to match the inner portion of the rotation curve. However, to obtain a good match to the entire rotation curve, which extends to 32 kpc from the galaxy's center, we must add a dark halo for which we adopt a pseudo-isothermal sphere. With the mass distributions for the bulge, disk, atomic gas, and halo, we match our calculated rotation curve to the measured curve by adjusting four parameters: the mass-to-light ratios of the bulge and disk, the halo core radius, and the halo central mass density. We obtain a good fit with a model which does not include the bulge, although we obtain better fits with models containing a small bulge. Using the derived parameters, we construct mass distributions for the dark and luminous matter which show that the dark matter is the dominant mass component inside the optical disk radius by a ratio of about 2:1. We also investigate the mass-to-light ratio of the halo from limiting K-band (2.2 mum) surface brightness, deriving a lower limit M/L = 24M./L.,K. This suggests that at east 30% of the dark halo is composed of objects fainter than late-type main-sequence stars.