Redshifted H i and OH absorption in radio galaxies and quasars

S. J. Curran 3 M. T. Whiting 1 3 M. T. Murphy 0 7 J. K. Webb 3 C. Bignell 6 A. G. Polatidis 4 5 T. Wiklind 2 8 9 P. Francis 10 G. Langston 6 0 Institute of Astronomy, University of Cambridge , Madingley Road, Cambridge CB3 0HA 1 CSIRO Australia Telescope National Facility , PO Box 76, Epping, NSW 1710 , Australia 2 Joint ALMA Observatory , Santiago , Chile 3 School of Physics, University of New South Wales , Sydney, NSW 2052 , Australia 4 Netherlands Institute for Radio Astronomy (ASTRON) , Postbus 2, 7990 AA Dwingeloo , the Netherlands 5 Max-Planck-Institut fu r Radioastronomie , Postfach 2024, D-53010 Bonn , Germany 6 National Radio Astronomy Observatory , PO Box 2, Rt. 28/92 Green Bank, WV 24944-0002 , USA 7 Centre for Astrophysics and Supercomputing, Swinburne University of Technology , PO Box 218, Hawthorn, VIC 3122 , Australia 8 Space Telescope Science Institute , 3700 San Martin Drive, Baltimore, MD 21218-2463 , USA 9 Onsala Space Observatory , S-439 92 Onsala , Sweden 10 Department of Physics, Australian National University , ACT 0200 , Australia A B S T R A C T From a survey for the redshifted H I 21-cm and OH 18-cm absorption in the hosts of a sample of radio galaxies and quasars, we detect H I in three of the 10 and OH in none of the 14 sources for which useful data were obtained. As expected from our recent result, all of the 21-cm detections occur in sources with ultraviolet (UV) continuum luminosities of LUV 1023 W Hz1. At these 'moderate' luminosities, we also obtain four non-detections, although, as confirmed by the equipartition of detections between the type 1 and type 2 objects, this near-50 per cent detection rate cannot be attributed to unified schemes of active galactic nuclei (AGNs). All of our detections are at redshifts of z 0.67, which, in conjunction with our faint source selection, biases against UV luminous objects. The importance of the UV luminosity (over AGN type) in the detection of the 21-cm absorption is further supported by the nondetections in the two high-redshift (z 3.6-3.8) radio galaxies, which are both type 2 objects, while having LUV > 1023 W Hz1. Our 21-cm detections in combination with those previously published give a total of eight (associated and intervening) H I-absorbing sources searched and undetected in OH. Using the detected 21-cm line strengths to normalize the limits, we find that only two of these eight sources may have been searched sufficiently deeply in OH, even though these are marginal. 1 I N T R O D U C T I O N Although opaque to optical light, the dusty Universe is transparent to radiation at radio wavelengths, thus making the spectroscopic study of the 21-cm spin-flip transition of neutral hydrogen (H I) a very useful tool in probing the far reaches of the cosmos. The low probability of the transition compounded by the inverse square law renders H I 21-cm absorption currently undetectable in emission at redshifts of z 0.1. However, in the absorption of radio waves emitted from background quasars, the line strength depends only upon the column density of the absorber and the flux of the an order of magnitude of the sensitivity provided by the best optical data (see Tzanavaris et al. 2007). This offers one of the few experimental tests of current Grand Unified Theories, thus having profound implications for modern physics. The latter point requires the comparison of the redshift of the 21-cm line with other transitions, which may be optical/ultraviolet (UV) [from singly ionized metals, giving ( 2gp)/2gp], where is the fine structure constant, the electron-to-proton mass ratio and gp the proton g-factor, millimetre-wave [rotational transition of molecules, giving ( 2gp)/2gp] or other decimetre transitions (see Curran, Kanekar & Darling 2004a and references therein), specifically, transitions arising from the hydroxyl radical (OH), which can also be intracompared (Darling 2003), thus avoiding possible line-of-sight effects which could mimic a change in the constants. Thus, highly redshifted H I 21-cm and OH 18-cm absorbers are of great interest, although these are currently very rare, with only 73 H I 21-cm absorption systems at z 0.1 known 41 of which occur in galaxies intervening the sight-lines to more distant quasars (see table 1 of Curran 2010), with the remainder arising in the host galaxies of the quasars themselves (see table 1 of Curran & Whiting 2010). In the case of OH, the situation is more dire with only five absorbers currently known (Chengalur, de Bruyn & Narasimha 1999; Kanekar & Chengalur 2002; Kanekar et al. 2003, 2005). Four of these were originally found through millimetre-wave molecular absorption, although further surveys have proven futile (see Curran et al. 2004b), which we suggest is due to the traditional optical selection of the sources. The target of choice in many previous surveys has been damped Lyman absorption systems (DLAs), since these are known to contain large columns of neutral hydrogen (NHI 2 1020 cm2, by definition) at precisely determined redshifts. Although 19 DLAs have been detected in the H2 Lyman and Werner UV bands (see Noterdaeme et al. 2008; Jorgenson et al. 2009; Srianand et al. 2010), these are at molecular fractions well below the detection thresholds of current microwave instruments (Curran et al. 2004b). Furthermore, the molecular abundances appear to be correlated with the colour of the background quasar in that the DLAs 2NH2 have molecular fractions of F 2NH2 +NH I 1070.3 and V K 4, whereas the millimetre (and OH) absorbers have molecular fractions F 0.61 and opticalnear-infrared (opticalnear-IR) colours of V K 5 (see fig. 3 of Curran, Whiting & Webb 2010), that is, not only are the radio-band absorbers redder than those of the optical band, but there may also be a correlation between the normalized OH line strength and opticalnear-IR colour (Curran et al. 2006), although this requires a larger number of detections for confirmation. These points strongly suggest that the quasar light is reddened by the dust in the foreground absorber, which prevents the dissociation of the molecules by the ambient UV field. From this it is apparent that in order to detect redshifted molecular absorption with current radio instruments, targets must be selected on the basis of their optical and near-IR photometry, where we select the reddest objects. However, the obscuration responsible for the quasar reddening could be located anywhere between us and the quasar redshift (the three intervening systems are the strongest absorbers, see Section 4.2) and, although wide-band decimetre scans are more efficient than at millimetre wavelengths (Murphy, Curran & Webb 2003; Curran et al. 2005), these are very susceptible to radio frequency interference (RFI). Therefore, in addition to our programme of using the wide-band spectrometer on the Green Bank Telescope (GBT) to perform 200-MHz wide frequency scans of the entire redshift space towards very red, radio-loud ob (...truncated)