The Mass Distribution of NGC 1399 from Optical and X-Ray Surface Photometry

We have obtained blue and red photographic plates and Einstein Observatory Imaging Proportional Counter (IPC) 0.5-4.5 keV X-ray images of the elliptical galaxy NGC 1399, which is the central and dominant member of the nearby Fornax cluster. With these data, we have modeled the X-ray emitting ISM by considering both constant and radially varying mass-to-luminosity ratios. Our conclusions depend crucially on the temperature profile of the hot gas for which there is unfortunately only limited information. From the IPC spectral data, the 90% confidence range for the temperature averaged over the inner 26 kpc is 1.7-6.4 x 10^7^ K. This temperature range and the assumption of either an isothermal or polytropic equation of state implies that dark matter is present in NGC 1399. However, should the IPC inferred temperature prove unreliable and the gas temperature be slightly lower at T ~ 10^7^ K, then isothermal and polytropic models in which dark matter is not required can be constructed. The isothermal model provided the best fit to the surface brightness data. At radii greater than 26 kpc, the only temperature constraint is that the gas be hotter than ~ 3 x 10^6^ K in order to allow detection by the IPC. This means that if T is roughly at this lower limit at r ~ 40 kpc (the maximum radius at which the X-ray data is of reasonable quality) and so significantly cooler than the central regions, it is possible that dark matter is not present. The optical and X-ray surface brightness profiles track each other remarkably well until r ~ 50 kpc. This may indicate that the X-ray emitting gas is sitting in a potential defined by the optical light distribution and that the gas temperature is of order twice the stellar virial temperature.