A survey for redshifted molecular and atomic absorption lines – I. The Parkes half-Jansky flat-spectrum red quasar sample

S. J. Curran 2 M. T. Whiting 1 2 M. T. Murphy 0 J. K. Webb 2 S. N. Longmore 1 2 Y. M. Pihlstr om 5 R. Athreya 4 C. Blake 3 0 Institute of Astronomy , Madingley Road, Cambridge CB3 0HA 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 Department of Physics & Astronomy, University of British Columbia , 6224 Agricultural Road, Vancouver, B.C. , Canada V6T 1Z1 4 National Centre for Radio Astrophysics , Pune 411 007, Maharashtra, India 5 Department of Physics and Astronomy, University of New Mexico, 800 Yale Boulevard NE , Albuquerque, NM 87131, USA A B S T R A C T We are currently undertaking a large survey for redshifted atomic and molecular absorption lines at radio frequencies. In this paper, we present the results from the first phase of this survey: the search for H I 21-cm and OH 18-cm absorption lines in the hosts of reddened quasars and radio galaxies. Although we observed each source for up to several hours with two of the world's most sensitive radio telescopes, the Giant Metrewave Radio Telescope (GMRT) and Westerbork Synthesis Radio Telescope (WSRT), only one clear and one tentative detection were obtained: H I absorption at z = 0.097 in PKS 1555140 and OH absorption at z = 0.126 in PKS 2300189, respectively, with the Australia Telescope Compact Array (ATCA). For the latter, no H I absorption was detected at the same redshift as the borderline OH detection. In order to determine why no clear molecular absorption was detected in any of the 13 sources searched, we investigate the properties of the five redshifted systems currently known to exhibit OH absorption. In four of these, molecules were first detected via millimetre-wave transitions, and the flat radio spectra indicate compact background continuum sources, which may suggest a high degree of coverage of the background source by the molecular clouds in the absorber. Furthermore, for these systems we find a relationship between the molecular line strength and red optical-near-infrared (V K) colours, thus supporting the notion that the reddening of these sources is due to dust, which provides an environment conducive to the formation of molecules. Upon comparison with the V K colours of our sample, this relationship suggests that, presuming the reddening occurs at the host galaxy redshift at least in some of the targets, many of our observations still fall short of the sensitivity required to detect OH absorption, although a confirmation of the 'detection' of OH in 2300189 could contravene this. 1 I N T R O D U C T I O N Unlike optical spectral lines, the H I 21-cm spin-flip transition of neutral hydrogen and molecular rotational transitions are transparent to interstellar dust, thus providing very useful probes of the early Universe. For example, these can be used in the following ways. (i) Investigate neutral gas at high redshift allowing a measure of the baryonic content of the early Universe, when neutral gas outweighed the stars: at low redshift, most of the gas has been consumed by star formation and H I represents only a small fraction of the total mass in baryons. Conditions were very different at high redshift, and quasar absorption line studies have demonstrated that the comoving density of H I is many times higher at z 3 (e.g. Peroux et al. 2001). (ii) Probe the evolution of large-scale structure. In the most successful models, massive galaxies build up through merging and accretion. This process is most vigorous at high redshift where interactions occur more frequently. H I observations are a powerful probe of the dynamics of these mergers, and thus constitute a basic test of theories of galaxy formation and evolution. (iii) High redshift observations H I can give a lower limit to the epoch of reionization, when neutral hydrogen collapsed to form the first structures (stars, galaxies/quasars), the upper limit of which is constrained by Cosmic Microwave Background (CMB) measurements (e.g. Griffiths, Barbosa & Liddle 1999). (iv) The relative populations of molecular rotational transitions can measure density and temperature at high redshift, thus providing a probe of the CMB and the abundance of cold star forming gas in the early Universe. (v) Monitoring absorbers which act as gravitational lenses can yield time delay studies giving measurements of the Hubble parameter (e.g. Wiklind & Combes 2001). (vi) Various combinations of H I, OH and millimetre rotational lines can give accurate measurements of several fundamental constants; the electronproton mass ratio, the proton g-factor and the fine structure constant (see Curran, Kanekar & Darling 2004, and references therein). These can provide at least an order of magnitude increase in accuracy over the current optical results, which may suggest that fine structure constant has undergone cosmological evolution (Murphy, Webb & Flambaum 2003, although see also Srianand et al. 2004). Unfortunately, redshifted radio absorption systems are currently very rare, with only 50 H I absorption systems known for redshifts of z 0.1 (summarized in Table 3). Five of these systems also exhibit OH absorption, four of which constitute the only known redshifted millimetre absorption systems (see Table 4). In addition to these, molecular absorption has also been detected in 10 known optical absorbers through H 2 vibrational transitions redshifted into the optical band at z 1.8 (see Reimers et al. 2003; Cui et al. 2005). These occur in damped Lyman absorption systems (DLAs), which have high neutral hydrogen column densities (NH I 2 1020 cm2) known to exist at precisely determined redshifts. However, extensive millimetre-wave observations of DLAs have yet to detect absorption from any rotational molecular transition (Curran et al. 2004b), leading us to suspect that using optically selected objects selects against dusty environments, which are more likely to harbour molecules in abundance. This is apparent when one compares the DLAs in which H2 has been detected, which have 2NH2 molecular fractions F 2NH2 +NH I 107 102 (see Curran et al. 2004a, and references therein) and colours of V K = 2.23.4 (typical for optically bright quasars) with the five known OH absorbers; F 0.71.0 (using NH2 107NOH; see Combes & Wiklind 1998; Kanekar & Chengalur 2002; Kanekar et al. 2005) and V K 5.07 (Fig. 1). This indicates substantial reddening of the quasar light in these cases, possibly by the absorber, which is either intrinsic to the source (in the cases of 1504+377 and 1413+135) or due to the lensing galaxy (for 0132097, 0218+357 and 1830211). We have therefore commenced a survey for redshifted atomic and molecular absorption lines towards reddened objects. In this paper, we present the results of the first phase of this survey: searching for H I and OH lines due to cold dense dust residing in the hosts of quasars and radio galaxies. 2 O B S E RVA (...truncated)