On the absence of molecular absorption in high-redshift millimetre-band searches

Mon. Not. R. Astron. Soc. 416, 2143–2153 (2011) doi:10.1111/j.1365-2966.2011.19193.x On the absence of molecular absorption in high-redshift millimetre-band searches S. J. Curran,1 M. T. Whiting,1,2 F. Combes,3 N. Kuno,4 P. Francis,5 N. Nakai,6 J. K. Webb,1 M. T. Murphy1,7 and T. Wiklind8,9,10 1 School of Physics, University of New South Wales, Sydney, NSW 2052, Australia 2 CSIRO Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia 3 LERMA, Observatoire de Paris, 77 Avenue Denfert-Rochereau, 75014 Paris, France 4 Nobeyama Radio Observatory, Nagano 384-1305, Japan 6 Institute of Physics, University of Tsukuba, Ten-noudai, Tsukuba, Ibaraki 305-8571, Japan 7 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia 8 Space Telescope Science Institute, Baltimore, MD 21218, USA 9 Onsala Space Observatory, S-439 92 Onsala, Sweden 10 Joint ALMA Observatory, Santiago, Chile Accepted 2011 June 3. Received 2011 June 3; in original form 2011 March 31 ABSTRACT We have undertaken a search for millimetre-waveband absorption (through the CO and HCO+ rotational transitions) in the host galaxies of reddened radio sources (z = 0.405–1.802). Despite the colour selection (optical–near-infrared colours of V − K  5 in all but one source), no absorption was found in any of the eight quasars for which the background continuum flux was detected. On the basis of the previous (mostly intervening) H2 and OH detections, the limits reached here and in some previous surveys should be deep enough to detect molecular absorption according to their V − K colours. However, our survey makes the assumption that the reddening is associated with dust close to the emission redshift of the quasar and that the narrow millimetre component of this emission is intercepted by the compact molecular cores. By using the known millimetre absorbers to define the colour depth and comparing this with the ultraviolet luminosities of the sources, we find that, even if these assumptions are valid, only 12 of the 40 objects (mainly from this work) are potentially detectable. This is assuming an excitation temperature of T x = 10 K at z = 0, with the number decreasing with increasing temperatures (to zero detectable at T x  100 K). Key words: galaxies: abundances – galaxies: active – galaxies: high-redshift – quasars: absorption lines – cosmology: observations – radio lines: galaxies. 1 I N T RO D U C T I O N Millimetre-wave observations of molecular absorption systems along the sightlines to distant quasars provide a powerful probe of the cold, dense, star-forming gas in the distant Universe. Furthermore, a comparison of the redshifts of the rotational transitions of the molecules with those of the spin-flip transition of H I, as well as the electronic optical/UV transitions of metal ions, can be used to determine high-redshift values of the fundamental constants, to at least an order of magnitude the sensitivity of purely optical data (see Curran, Kanekar & Darling 2004a). However, despite much searching, only four such systems are currently known (Wiklind & Combes 1995; 1996ab; 1997), the highest redshift being at zabs = 0.89. Of these, two are intervening systems (gravitational lenses towards more distant quasars), with the other two systems arising through absorption within the host galaxy of the quasar. Subsequent searches at the redshifts of known high column density H I absorption systems, intervening the sightlines to more distant quasi-stellar objects (QSOs), have also failed to detect molecular absorption in the millimetreband (Curran et al. 2004b and references therein), despite the possibility that these so-called damped Lyman α systems (DLAs)1 20 cm−2 1 These have neutral hydrogen column densities of N H I ≥ 2 × 10  E-mail:  C 2011 The Authors C 2011 RAS Monthly Notices of the Royal Astronomical Society  and are usually detected at zabs  1.8, where the Lyman α transition is redshifted into the optical band. 5 Australian National University, Canberra, Australia 2144 S. J. Curran et al. may account for more than 80 per cent of the neutral gas content in the Universe (Prochaska, Herbert-Fort & Wolfe 2005). DLAs are, however, not devoid of molecular gas: to date, the Lyman and Werner ultraviolet bands of H2 have been detected in 19 DLAs (see Noterdaeme et al. 2008;2 Jorgenson et al. 2009; Srianand et al. 2010). These, however, have molecular abundances which are generally much lower than those detectable with current microwave and radio telescopes (Curran et al. 2004b and Fig. 1, top). Furthermore, in Curran et al. (2006) we showed that the 2NH2 ∼ H2 -bearing DLAs have molecular fractions of F ≡ 2NH +N H 2 I 2 One of which, J1439 + 113, has also been detected in the CO A − X UV band (Srianand et al. 2008). 2 O B S E RVAT I O N S 2.1 Target selection As per Curran et al. (2006), our sources were selected from the Parkes Half-Jansky Flat-spectrum Sample (PHFS, Drinkwater et al. 1997),4 on the basis of their optical–near-IR photometry (Francis, Whiting & Webster 2000). From these, we selected the 30 reddest sources (which correspond to an extinction of AV ≈ 4.1), in which the emission redshift of the quasar (zem ) would place a strong absorption line (CO or HCO+ ) into the 3-mm band. After culling these further, by selecting those of δ > −30◦ (thus being observable from northern latitudes)5 and with 3-mm flux densities expected to be 100 mJy, the 10 objects listed in Table 1 remained. 2.2 The IRAM 30-m observations From 2003 December to 2004 February we observed three of the targets with the IRAM (Institut de Radio Astronomie Millimetrique) 30-m telescope at Pico Veleta, Spain. We used two 3 V − K = 2.88 ± 1.04 in general and 3.05 ± 0.97 if radio-loud. 4 With the addition of 0500 + 019, included since it has been detected in 21-cm absorption (Carilli et al. 1998). We also included J0906 + 4952 and J1341 + 3301, which are two very red sources from Glikman et al. (2004) (Section 2.3). 5 We miss SEST.  C 2011 The Authors, MNRAS 416, 2143–2153 C 2011 RAS Monthly Notices of the Royal Astronomical Society  Figure 1. The H2 column density (top) and the molecular fraction (middle) versus the observed frame optical–near-infrared colour (where available) for high-redshift molecular absorption systems. The circles represent the H2 bearing DLAs (all optically selected intervening absorbers) and the squares and stars represent the OH absorbers (radio selected), with the inset in the middle panel showing the normalized OH line strength (Curran et al. 2006). These are comprised of the four systems originally identified in millimetre-wave transitions (with the least-squares fit to these shown) plus the gravitational lens at zabs = 0.764 towards 0132–097 (detected in OH decimetre but not HCO+ millimetre absorption, Kanekar et al. 2005). P(τ ) shows Kendall’s τ two-sided probability of the observed distribution (...truncated)