v 1 1 0 N ov 1 99 7 ROSAT X - RAY OBSERVATIONS OF THE RADIO GALAXY NGC

We have observed NGC 1316 (Fornax A) with the ROSAT HRI. In this paper, we present the results of these observations and we complement them with the spectral analysis of the archival PSPC data. The spectral properties suggest the presence of a significant component of thermal X-ray emission (> 60%), amounting to ∼ 10 9 M ⊙ of hot ISM. Within 3 ′ from the nucleus of NGC 1316, the HRI X-ray surface brightness falls as r −2. In the inner ∼40 ′′ , the X-ray surface brightness is significantly elongated (ǫ ∼ 0.3). This flattened X-ray feature is confirmed by a straightforward statistics test as well as moment analysis. By comparing the morphology of the X-ray emission with the distribution of optical dust patches, we find that the X-ray emission is significantly reduced at the locations where the dust patches are more pronounced, indicating that at least some of the X-ray photons are absorbed by the cold ISM. We also compare the distribution of the hot and cold ISM with that of the ionized gas, using recently obtained H α CCD data. We find that the ionized gas is distributed roughly along the dust patches and follows the large scale X-ray distribution at r > 1 ′ from the nucleus. However, there is no one-to-one correspondence between ionized gas and hot gas. Both morphological relations and kinematics suggest different origins for hot and cold ISM. The radio jets in projection appear to pass perpendicularly through the central X-ray ellipsoid. Comparison of thermal and radio pressures suggests that the radio jets are confined by the surrounding hot gaseous medium.