Ionization of large-scale absorbing haloes and feedback events from high-redshift radio galaxies

Abstract

Aims. We present photoionization calculations for the spatially-extended absorbers observed in front of the extended emission-line spectrum of two high-redshift radio galaxies, 0943–242 (ze = 2.922) and 0200+015 (ze = 2.230), with the aim of reproducing the absorber column ratio, NCIV/NHI. Methods. We explore the effects of using different UV continua in the photoionization calculations. A comparison is made between the absorber in 0200+015 and the two absorbers observed near the lensed Lynx arc nebula at redshift 3.36, which present very similar NCIV/NHI ratios. Results. We find that hot stars from a powerful starburst, or a metagalactic background radiation (mbr) in which stars dominate quasars, are equally successful in reproducing the observed NCIV/NHI, assuming subsolar gas metallicities for each absorber. These softer seds eliminate the difference of a factor 1000 in metallicity between the two absorbers encountered in earlier work where a power-law sed was assumed. The detection of continuum flux in 0943–242 suggests that the level of ionizing photons is consistent with a stellar ionizing source. Conclusions. If the mbr is responsible for the ionization of the radio galaxy absorbing shells, their radii (if spherical) would be large (>100 kpc) and their mass huge >10 12 M� , implying that the feedback mechanism initiated by the central galaxy has caused the expulsion of more baryonic mass than that left in the radio galaxy. If, as we believe is more likely, stellar ionizing sources within the radio galaxy are responsible for the absorber’s ionization, smaller radii of ∼25 kpc and much smaller masses (∼10 8 −10 10 M� ) are inferred. This radius is consistent with the observed transition in radio source size between the smaller sources in which strong H i absorption is almost ubiquitous and the larger sources where it is mostly lacking. Finally, we outline further absorption-line diagnostics that could be used to further constrain the properties of the haloes and their source of ionization.