ROSAT PSPC OBSERVATIONS OF NGC-4636 - INTERACTION WITH VIRGO GAS

ROSAT PSPC observations of NGC 4636 have provided a deep image of the galaxy and its surroundings, which reveal the presence of emission well outside the optical galaxy. The source emission is measured out to r approximately 18', where the instrument support structure prevents us at this time from following it farther out. The nature of this emission is not fully understood, but could be evidence that the Virgo cluster ps extends as far out as this galaxy (> 3 Mpc from M87). Spectral analysis of the X-ray emission suggests a relatively cool (kT approximately 0.5-0.9 keV) interstellar medium, with temperatures increasing with radius. However, the detailed properties of the interstellar gas cannot be unambiguously determined with the present data, since the results we obtain depend strongly on the choice of the spectral model. Comparison of the spectral distributions in concentric annuli clearly indicates significant differences with radius, which can be parameterized as a general increase of the temperature. For low cosmic abundance models, kT varies from approximately 0.55 keV in the inner 1' to approximately 0.8 keV at r approximately 6'-8'. Outside 8' the average temperature is higher than in the inner region (kT approximately 0.8-1.2 keV) and the low-energy absorption is significantly lower. For 100% cosmic abundance and galactic line-of-sight absorption, multi-temperature fits are required, suggesting the possibility that the interstellar medium is inhomogeneous.