Ionized Hydrogen at Large Galactocentric Distances

J. Bland-Hawthorn, PASA, 14 (1), 64.

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Cosmic ionizing background

The emission measure tex2html_wrap_inline223 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 2tex2html_wrap_inline217 upper limit (Vogel et al. 1995) for the cosmic ionizing background is tex2html_wrap_inline277 < 0.08 (tex2html_wrap_inline281tex2html_wrap_inline283 phot cmtex2html_wrap_inline255 stex2html_wrap_inline257). tex2html_wrap_inline277 is the ionizing flux density of the cosmic background at the Lyman limit in units of 10tex2html_wrap_inline291 erg cmtex2html_wrap_inline255 stex2html_wrap_inline257 Hztex2html_wrap_inline257 srtex2html_wrap_inline257; tex2html_wrap_inline281 (tex2html_wrap_inline303tex2html_wrap_inline277/h) is the equivalent photon flux at face of a uniform, optically thick slab. For an electron density tex2html_wrap_inline309 and an ion density tex2html_wrap_inline311, the expected emission measure from the slab is tex2html_wrap_inline313 where L is the thickness of the ionized region. The resulting emission measure for an ionizing flux tex2html_wrap_inline281 is then tex2html_wrap_inline319tex2html_wrap_inline281tex2html_wrap_inline323. The upper limit on tex2html_wrap_inline277 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 2tex2html_wrap_inline217 upper limit (tex2html_wrap_inline277 = 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'.


Next Section: The ionized edges of
Title/Abstract Page: Ionized Hydrogen at Large
Previous Section: What is the Fabry-Perot
Contents Page: Volume 14, Number 1

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