ROSATX−Ray Observations of the Radio Galaxy NGC 1316 (Fornax A)

We have observed NGC 1316 (Fornax A) with the ROSAT HRI. In this paper, wepresent the results of these observations and we complement them with thespectral analysis of the archival PSPC data. The spectral properties suggestthe presence of a significant component of thermal X-ray emission (> 60%),amounting to ~10^{9}Mo of hot ISM. Within 3' from the nucleus of NGC 1316, theHRI X-ray surface brightness falls as r^{-2}. In the inner ~40", the X-raysurface brightness is significantly elongated (e ~ 0.3). This flattened X-rayfeature is confirmed by a straightforward statistics test as well as momentanalysis. By comparing the morphology of the X-ray emission with thedistribution of optical dust patches, we find that the X-ray emission issignificantly reduced at the locations where the dust patches are morepronounced, indicating that at least some of the X-ray photons are absorbed bythe cold ISM. We also compare the distribution of the hot and cold ISM withthat of the ionized gas, using recently obtained Ha CCD data. We find that theionized gas is distributed roughly along the dust patches and follows the largescale X-ray distribution at r > 1' from the nucleus. However, there is noone-to-one correspondence between ionized gas and hot gas. Both morphologicalrelations and kinematics suggest different origins for hot and cold ISM. Theradio jets in projection appear to pass perpendicularly through the centralX-ray ellipsoid. Comparison of thermal and radio pressures suggests that theradio jets are confined by the surrounding hot gaseous medium.Comment: 28 pages, 12 figures, to be published in the Astrophysical Journa