English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

Unlike at lower redshift, where there is a 40% detection rate, surveys for21-cm absorption arising within the hosts of z > 1 radio galaxies and quasarshave been remarkably unsuccessful. Curran et al.(2008) suggest that this is dueto the high redshift selection biasing towards the most optically brightobjects (those most luminous in the ultra-violetin the rest-frame), where thegas is ionised by the active galactic nucleus. They therefore argue that theremust be a population of fainter objects in which the hydrogen is not ionisedand which exhibit a similar detection rate as at lower redshifts. In order tofind this "missing" gas at high redshift, we have therefore undertaken a surveyof z > 2 radio sources, selected by optical faintness. Despite having opticalmagnitudes which indicate that the targets have ultra-violet luminosities belowthe threshold where all of the gas is ionised, there were no detections in anyof the eight sources for which useable data were obtained. Upon an analysis ofthe spectral energy distributions, ionising photon rates can only be determinedfor three of these, all of which suggest that the objects are above the highestluminosity of a current 21-cm detection. The possibility that the other fivecould be located at lower photon rates cannot be ruled out, although zerodetections out of five is not statistically significant. Another possible causeof the non-detections is that our selection biases the sample towards sourceswhich are very steep in the radio band, with a mean spectral index of = -1.0,cf. -0.3 for both the 21-cm detections and UV luminous non-detections. Thisadds the further possibility that the sources have very extended emission,which would have the effect of reducing the coverage by the putative absorbinggas, thus decreasing the sensitivity of the observation.

A survey for the missing hydrogen in high redshift radio sources