Abstract
ABSTRACT Recent highly sensitive detections of line emission from extended gas in the localuniverse demonstrate the feasibility of detecting Hα emitting galactic halos out to z ∼1. We determine the form of the surface brightness vs. redshift dependence which takesinto account UV background evolution. Successful detections will have a major impacton a wide range of fields, in particular, the source of ionization in QSO absorptionsystems.Key words: galaxies: ISM—galaxies: haloes—techniques: spectroscopic 1 INTRODUCTIONThe optical line emission of extendedextragalactic photoion-ized sources has been recently considered in several boththeoretical and observational works (Hogan & Weymann1987; Maloney 1992; Binette et al. 1993; Bland-Hawthorn etal. 1994; Donahue,Aldering& Stocke 1995; Bland-Hawthornet al. 1995; Gould & Weinberg 1996; Bechtold et al. 1997;Bland-Hawthorn 1997; Bland-Hawthorn et al. 1997; Cirkovi´c´& Samurovi´c 1998). One of the most important possibili-ties of such observations are direct detections of extendedgaseous structures around normal luminous galaxies at var-ious epochs.Theoretical models of both galactic halo structure(Bregman 1981; Kovalenko, Shchekinov & Suchkov 1989;Norman & Ikeuchi 1989; Wolfire et al. 1995) and QSO ab-sorption line systems (Mo 1994; Mo & Miralda-Escud´e 1996;Chiba & Nath 1997) predict vast quantities of photoionizedgas at large galactocentric distances (from several kpc to∼ 10
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