Surveying a Very Deep Patch

Taking up the last point, could we, with a Multibeam instrument, survey a limited piece of sky to sufficient depth either to find, or to rule out, a cosmologically interestingly population of dim or dark galaxies? According to (4) hours to reach 10 cm.

The maximum distance to which one could see such dim galaxies would correspond to that at which they would fill the beam (size in radians) in which case, with the beams of a multibeam survey one would survey a total volume, of

Such a volume would contain normal i.e. high SB Schecter (1976) galaxies. If therefore, we made 10 such pointings, we ought to pick up normal galaxies and a number of dim (i.e. ) galaxies such that

where the 's refer to their relative contribution to the cosmic density parameter, .

It follows that in a program of only 10 pointings, each lasting between and , the Parkes multibeam system will either pick up a healthy sample of an entirely new population of ``Iceberg galaxies'', or else rule them out, once and for all, as significant (i.e. > 10%) contributors to the cosmic density of all galaxies. Either way, this would be such an important result that we simply have to do it.

It has been postulated that all disk galaxies will have a sharp cut-off at a few , at which point the conversion of atomic into ionized hydrogen occurs. The explanation for this could lie with internal or external ionizing photons, and in particular the cosmic UV background. Two galaxies have been used to check for this abrupt cut-off (or ``sharp edge'') to the HI disk. Van Gorkom (1993) used the VLA to verify this characteristic in NGC 3198, by undertaking a 100 hour integration, reaching a sensitivity corresponding to . The sharp outer edge was observed to occur at a few . Corbelli and Salpeter (1989) corroborated this by performing a sensitive scan along the major axis of M33, the largest galaxy in angular extent that can be observed with Arecibo, and finding a cut-off at . To further substantiate these findings, Maloney (1993), Corbelli and Salpeter (1993), and Dove and Schull (1994) applied photoionization models to these two galaxies and predicted the observed edges. Note, however, that the intergalactic radiation field (IRF) is likely to vary considerably as between say the inside of a spiral-rich cluster and a void. More cogently, there appears to be no break at in the number density of QSOALs as a function of , which runs smoothly down, all the way from to (Tytler 1987) as . More recently, Corbelli and Salpeter (1996) have repeated the work on M33 to show that the effects of disk inclination affect earlier interpretations of the sharp fall-off in HI column density. Their observations, to the northwest and southeast of M33, render them incapable of excluding a faint level of HI, possibly of , although this may well be caused by distant sidelobe contamination. In short, there is currently little observational evidence to indicate that hydrogen columns below a few should cease to exist.

John Dickey's observations (1997) of the Hercules complex show how localised the neutral hydrogen may be. In the dense, X-ray emitting regions, all the HI appears to have vanished, whereas in the spiral-rich region he finds numerous patches associated either with very dim, or in some cases apparently invisible galaxies. This phenomenon, which he refers to as the ``Massacre of the Innocents'', poses a real challenge for Deep Patch enthusiasts. Where should we point the telescope?

© Copyright Astronomical Society of Australia 1997