Cosmic ionizing background

The emission measure from the surface of a cloud embedded in a bath of ionizing radiation gives a direct gauge, independent of distance, of the ambient radiation field beyond the Lyman continuum edge. If we overlook problems of cloud geometry and covering fraction, a truly extragalactic HI cloud provides a crucial probe of the metagalactic ionizing flux.

The current best 2 upper limit (Vogel et al. 1995) for the cosmic ionizing background is < 0.08 ( phot cm s). is the ionizing flux density of the cosmic background at the Lyman limit in units of 10 erg cm s Hz sr; (/h) is the equivalent photon flux at face of a uniform, optically thick slab. For an electron density and an ion density , the expected emission measure from the slab is where L is the thickness of the ionized region. The resulting emission measure for an ionizing flux is then . The upper limit on is sufficiently deep that it begins to challenge various models for UV background (Haardt & Madau 1996). While one would certainly want to believe that `staring' provides a fundamental constraint, there is some uncertainty as to whether the Giovanelli-Haynes cloud is a suitable screen for probing the background radiation. The HI complex appears to be highly inclined to our line of sight (Chengalur, Giovanelli & Haynes 1995). What matters is the HI covering fraction seen by the ionizing photons, in other words, along an axis in the plane of the sky.

More recently, we obtained (McCain et al. 1995) a similar 2 upper limit ( = 0.09) from observations of a putative extragalactic HI cloud in the NGC 3256 group. This was originally `discovered' by Jayanne English (1995) but the cloud has yet to be confirmed by follow-up observations. We are wary of Sancisi's dictum that `most extragalactic HI clouds are not confirmed in subsequent observations'.

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