EXTRAGALACTIC IONIZED HYDROGEN IN THE FORNAX CLUSTER

The radio galaxy Fornax A is well known for its giant radio lobes whic h extend almost a degree (similar to 300 kpc) across the sky. Fomalont et al, have shown that these lobes are linearly polarized on the larg est scales, although the western lobe is highly depolarized in discret e, resolved regions. The depolarized regions indicate the presence of ionized gas (Faraday screen) along the line of sight to the western lo be. We have now detected the warm gas at H alpha using a Fabry-Perot i nterferometer in a way that allows us to reach very low surface bright ness levels (<10(-19) erg cm(-2) s(-1) arcsec(-2)). The radial velocit y of the ionized gas (1610 km s(-1)) places it in the Fornax cluster a t a projected radius of 1.0 Mpc, The gas is spread over a region 10 kp c in diameter and has an ionized mass of 6 x 10(7) f(-0.5) M. (where f is the volume filling factor). For the inferred column density of ele ctrons (approximate to 2.6 x 10(20) f(-0.5) cm(-2)) and in light of th e H I upper limits (<4 x 10(19) cm(-2)), the cloud has to be mostly io nized. Possible sources of ionization are large-scale shocks or a puta tive hot component which confines the radio lobes. The high line-of-si ght magnetic field strength (B-parallel to approximate to 0.3-1.3 mu G ) deduced from the rotation measure (approximate to 20 rad m(-2)) and energy equipartition suggests that the cloud may have undergone signif icant turbulent heating from secondary shocks. The predicted bounds on the fraction of neutral material (10(-6)-10(-1)) indicates that this may be the first example of a spatially resolved, extragalactic ionize d cloud. Such objects are thought to be responsible for some of the Ly alpha absorption lines observed in the spectra of quasars.