NEW UPPER LIMITS ON THE LOCAL METAGALACTIC IONIZING-RADIATION DENSITY

We have obtained Halpha observations with the Maryland-Caltech Fabry-P erot Spectrometer attached to the Cassegrain focus of the 1.5 m telesc ope at Palomar Observatory in order to set limits on the number of ion izing photons from the local metagalactic radiation field. We have obs erved the SW component of the Haynes-Giovanelli cloud H I 1225 + 01, a n intergalactic cloud which should be optimum for measuring the metaga lactic flux because it is nearly opaque to ionizing photons, it does n ot appear to be significantly shielded from the metagalactic radiation field, and the limits on embedded or nearby ionizing sources are unus ually low. For the area of the cloud with an H I column density greate r 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 th e 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 eff orts 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 t he 7' diameter region centered on 3C 273 in which Halpha emission at a velocity of approximately 1700 km s-1 was initially reported by Willi ams and Schommer. In agreement with T. B. Williams (private communicat ion) we find the initial detection was spurious. We obtain a 2 sigma u pper limit of 1.8 x 10(-19) ergs cm-2 s-1 arcsec-2 (32 mR) for the mea n surface brightness of diffuse Halpha, about a factor of 6 below the published value.