A survey for redshifted molecular and atomic absorption lines – II. Associated H i, OH and millimetre lines in the z≳ 3 Parkes quarter-Jansky flat-spectrum sample

S. J. Curran 2 M. T. Whiting 1 2 T. Wiklind 0 6 J. K. Webb 2 M. T. Murphy 4 5 C. R. Purcell 2 3 0 Onsala Space Observatory , S-439 92 Onsala , Sweden 1 CSIRO Australia Telescope National Facility , PO Box 76, Epping, NSW 1710 , Australia 2 School of Physics, University of New South Wales , Sydney, NSW 2052 , Australia 3 Jodrell Bank Centre for Astrophysics, University of Manchester , Alan Turing Building, Oxford Road, Manchester M13 9PL 4 Centre for Astrophysics and Supercomputing, Swinburne University of Technology , PO Box 218, Hawthorn, VIC 3122 , Australia 5 Institute of Astronomy , Madingley Road, Cambridge CB3 0HA 6 Space Telescope Science Institute , Baltimore, MD 21218 , USA A B S T R A C T We present the results of a z 2.9 survey for H I 21-cm and molecular absorption in the hosts of radio quasars using the Giant Metrewave Radio Telescope and the Tidbinbilla 70-m telescope. Although the atomic gas has been searched to limits capable of detecting most known absorption systems, no H I was detected in any of the 10 sources. Previously published searches, which are overwhelmingly at redshifts of z 1, exhibit a 42 per cent detection rate (31 out of 73 sources), whereas the inclusion of our survey yields a 17 per cent detection rate (two out of 12 sources) at z > 2.5. We therefore believe that our high-redshift selection is responsible for our exclusive non-detections, and find that at ultraviolet (UV) luminosities of LUV 1023 W Hz1, 21-cm absorption has never been detected. We also find this to not only apply to our targets, but also those at low redshift exhibiting similar luminosities, giving zero detections out of a total of 16 sources over z = 0.24 to 3.8. This is in contrast to the LUV 1023 W Hz1 sources where there is a near 50 per cent detection rate of 21-cm absorption. The mix of 21-cm detections and non-detections is currently attributed to orientation effects, where according to unified schemes of active galactic nuclei, 21-cm absorption is more likely to occur in sources designated as radio galaxies (type 2 objects, where the nucleus is viewed through dense obscuring circumnuclear gas) than in quasars (type 1 objects, where we have a direct view to the nucleus). However, due to the exclusively high UV luminosities of our targets it is not clear whether orientation effects alone can wholly account for the distribution, although there exists the possibility that the large luminosities are indicative of a changing demographic of galaxy types. We also find that below luminosities of LUV 1023 W Hz1, both type 1 and type 2 objects have a 50 per cent likelihood of exhibiting 21-cm absorption. Finally, we do not detect molecular gas in any of the sources. The lack of H I absorption, combined with the results from Paper I, suggests these sources are not conducive to high molecular abundances. 1 I N T R O D U C T I O N Redshifted molecular and atomic absorption lines can provide excellent probes of the contents and nature of the early Universe. In particular, with redshifted radio and microwave lines we can Orienti, Morganti & Dallacasa (2006)a Carilli, Rupen & Yanny (1993) Lovell et al. (1996) Chengalur et al. (1999) Kanekar & Briggs (2003) Kanekar & Chengalur (2003)b Darling et al. (2004) Kanekar et al. (2006) Kanekar, Chengalur & Lane (2007) Gupta et al. (2007) Curran et al. (2007b) Curran et al. (2007a) York et al. (2007) Zwaan et al. (in preparation) Intervening absorbers 0.6 investigate the gaseous content and large-scale structure as well any possible variations in the values of the fundamental constants at large look-back times. However, these are currently rare, with only 67 H I 21-cm absorption systems at z 0.1 known, comprising of 37 associated systems and 30 intervening (Table 1). For molecular absorption in the radio band the situation is considerably worse with only five redshifted OH 18-cm systems currently known (Chengalur, de Bruyn & Narasimha 1999; Kanekar & Chengalur 2002; Kanekar et al. 2003), four of which also exhibit a plethora of molecular absorption lines in the millimetre regime (see Combes & Wiklind 1998). From our previous survey for radio absorption lines in the hosts of the sources in the Parkes half-Jansky flat-spectrum sample (PHFS), one H I absorption system was clearly detected (out of five searched) and one OH system tentatively detected (of the 13 searched), both at zabs 0.1 (Paper I). Upon examination of the previous detections, we concluded the following. (ii) However, for the OH 18-cm absorption there is a clear relationship, thus suggesting that the reddening of these quasars is due to dust, the amount of which is correlated with the molecular abundance. Furthermore, for the molecular absorbers: (a) all of the absorption lines were found at redshifts were absorption (usually H I) was already known to occur;1 (b) in all cases the absorption occurs towards flat spectrum radio sources, suggesting compact radio sources and thus a large effective coverage by the absorber. Unfortunately, prior to the analysis undertaken in Paper I, not all of the above criteria were fully formulated, and so we have also targeted sources for which only the last criterion is satisfied. In this paper we present the results of a survey for redshifted atomic and molecular absorption within the hosts of sources selected from the 1 Except in the case of PKS 1830211 where a gravitational lens of unde termined redshift was previously known (Subrahmanyan et al. 1990). The redshift was finally determined through a 14 GHz wide spectral scan of the 3-mm band (Wiklind & Combes 1996a). NH I(cm2) 3.4283.456 2.9352.953 3.0923.106 3.1093.115 4.2654.283 3.5163.532 3.5133.543 4.4424.478 3.6933.722 3.6953.719 3.4853.509 3.4863.514 3.7733.802 3.7753.795 3.5583.585 3.5613.583 Parkes quarter-Jansky flat-spectrum sample (PQFS; Jackson et al. 2002). Because of the search for coincident molecular absorption, we initially prioritized sources in which the HCO+ 0 1 transition would be redshifted into the 12-mm band of the Tidbinbilla 70-m telescope. This gave 70 targets at z > 2.3 out of the 878 PQFS sources, and limiting the sample further to sources with flux densities in excess of 0.5 Jy at 0.4 GHz, gave a total of 19 sources at z > 2.9. 2 O B S E RVAT I O N S A N D R E S U LT S 2.1 The redshifted decimetre wave observations The redshifted decimetre wave observations where performed with the Giant Metrewave Radio Telescope (GMRT)2 in 2004 March, during the first run as described in Paper I. Again, as per Paper I, we searched for absorption within the host at the emission redshift of the quasar. Our target sample therefore consisted (primarily) of objects in the PQFS where either the H I 21-cm or OH 18-cm (2 3/2 J = 3/2) transition was redshifted into the 90-cm (327 MHz) band. Because of time constraints during the observations, we prioritized the targets in which B 19, in order to preferentially select sources where the presence of dust, associated wit (...truncated)