A SEARCH FOR DIFFUSE H-ALPHA EMISSION FROM LY-ALPHA ABSORPTION CLOUDSTOWARD 3C-273

The Hubble Space Telescope has identified a series of low-velocity Ly alpha absorption lines in the direction of 3C 273. We have investigate d a recent claim (Williams and Schommer 1993) that there is diffuse Ha emission with surface brightness 1.2 x 10(-18) ergs cm(-2) s(-1) arcs ec(-2) at a radial velocity within 130 km s(-1) of one of these Ly alp ha features, The observations were made with the TAURUS-2 imaging Fabr y-Perot interferometer and a Tek 1024(2) CCD at the 3.9 m Angle-Austra lian Telescope. We do not detect Ha emission at the claimed wavelength lambda 6600.3 down to a 3 sigma upper limit of 4 x 10(-19) ergs cm(-2 ) s(-1) arcsec(-2) (E(m) approximate to 0.2 cm(-6) pc), nor do we dete ct emission at the absorption-line velocity. We show that the Jacquino t advantage of spectroscopic interferometers makes 3 sigma detection t hresholds as low as E(m)(H alpha) = 0.02 cm(-6) pc quite feasible. Eve n at this very deep limit, it is unlikely that Ly alpha absorbers irra diated by the UVX background will be detectable in optical line emissi on at any redshift. However, our sensitivity limit now brings other im portant detections into the realm of possibility for the first time, i n particular, the detection of ionized disks beyond the H I truncation radius in spiral galaxies.