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Title:
Where is the cold neutral gas in the hosts of high redshift AGN? .
Authors:
Curran, S. J.; Whiting, M. T.; Webb, J. K.
Affiliation:
AA( School of Physics, University of New South Wales, Sydney NSW 2052, Australia ), AB(CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia), AC( School of Physics, University of New South Wales, Sydney NSW 2052, Australia )
Publication:
Memorie della Societa Astronomica Italiana, v.79, p.1113 (2008)
Publication Date:
00/2008
Origin:
MmSAI
Keywords:
radio lines: galaxies, galaxies: active, quasars: absorption lines, cosmology: observations, galaxies: abundances, galaxies: high redshift
Bibliographic Code:
2008MmSAI..79.1113C

Abstract

Previous surveys for HI 21-cm absorption in z > 0.1 radio galaxies and quasars yield a ≈40% detection rate, which is attributed to unified schemes of active galactic nuclei (AGN). In this paradigm absorption is only witnessed in (close to) type-2 objects, where the central obscuration is viewed (nearly) edge-on and thus absorbs the rest frame 1420 MHz emission along our sight-line. However, we find this mix of detections and non-detections to only apply at low redshift (z < 1): From a sensitive survey of eight z ≳3 radio sources we find no 21-cm absorption, indicating a low abundance of cold neutral gas in (the sight-lines searched in) these objects. Analysing the spectral energy distributions of these sources, we find that our high redshift selection introduces a bias where our sample consists exclusively of quasars with ultra-violet luminosities in excess of L_UV˜1023 W Hz-1. This may suggest that we have selected a class of particularly UV bright type-1 objects. Whatever the cause, it must also be invoked to explain the non-detections in an equal number of z < 0.7 sources, where we find, for the first time, the same exclusive non-detections at L_UV ≳ 1023 W Hz-1. These objects also turn out to be quasars and, from these exclusive high UV luminosity-21-cm non-detections, it is apparent that orientation effects alone cannot account for the mix of 21-cm detections and non-detections at any redshift.

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