THE PROXIMITY EFFECT AND THE MEAN INTENSITY OF IONIZING-RADIATION AT LOW REDSHIFTS

We report the first tentative detection of the proximity effect in the distribution of Lyalpha forest lines at low redshifts (z less than or similar to 1). This is based on observations of 13 quasars by Bahcall et al. with the Faint Object Spectrograph on the Hubble Space Telesco pe as part of the Quasar Absorption Line Key Project. We estimate the mean intensity of ionizing radiation on the usual assumption that the proximity effect is caused entirely by the increased photoionization o f absorbers near the quasars. The result is J(L) approximately 6 x 10( 24) ergs s-1 cm-2 sr-1 Hz-1 at z almost-equal-to 0.5. However, the unc ertainties are large, and even at the 1 sigma level, J(L) could be low er by a factor of 3 or higher by a factor of 6. Our estimate could be improved with a larger sample of absorbers and more accurate redshifts of a few of the quasars.