NEW UPPER LIMITS ON THE LOCAL METAGALACTIC IONIZING-RADIATION DENSITY

We have obtained Halpha observations with the Maryland-Caltech Fabry-Perot Spectrometer attached to the Cassegrain focus of the 1.5 m telescope at Palomar Observatory in order to set limits on the number of ionizing photons from the local metagalactic radiation field. We have observed the SW component of the Haynes-Giovanelli cloud H I 1225 + 01, an intergalactic cloud which should be optimum for measuring the metagalactic flux because it is nearly opaque to ionizing photons, it does not appear to be significantly shielded from the metagalactic radiation field, and the limits on embedded or nearby ionizing sources are unusually low. For the area of the cloud with an H I column density greater than 10(19) cm-2 we set a 2 sigma limit of 1.1 x 10(-19) ergs cm-2 s-1 arcsec-2 (20 mR) for the surface brightness of diffuse Halpha. This implies a 2 sigma upper limit on the incident one-sided ionizing flux of PHI(ex) < 3 x 10(4) cm-2 s-1. For a radiation field of the form J(nu) approximately nu-1.4, this yields a firm 2 sigma upper limit on the local metagalactic photoionization rate of GAMMA < 2 x 10(-13) s-1, and an upper limit for the radiation field J(nu) at the Lyman limit of J(nu0) < 8 x 10(-23) ergs cm-2 s-1 Hz-1 sr-1. We discuss previous efforts to constrain the metagalactic ionizing flux using Halpha surface brightness observations and also other methods, and conclude that our result places the firmest upper limit on this flux. We also observed the 7' diameter region centered on 3C 273 in which Halpha emission at a velocity of approximately 1700 km s-1 was initially reported by Williams and Schommer. In agreement with T. B. Williams (private communication) we find the initial detection was spurious. We obtain a 2 sigma upper limit of 1.8 x 10(-19) ergs cm-2 s-1 arcsec-2 (32 mR) for the mean surface brightness of diffuse Halpha, about a factor of 6 below the published value.