Molecular fraction limits in damped Lyman α absorption systems

S. J. Curran 2 M. T. Murphy 1 Y. M. Pihlstr om 0 J. K. Webb 2 A. D. Bolatto 3 G. C. Bower 3 0 National Radio Astronomy Observatory , Socorro, NM 87801 , USA 1 Institute of Astronomy , Madingley Road, Cambridge CB3 0HA 2 School of Physics, University of New South Wales , Sydney NSW 2052 , Australia 3 Radio Astronomy Laboratory , 601 Campbell Hall , University of California , Berkeley, CA 94720 , USA A B S T R A C T We have used the Green Bank Telescope (GBT) and Berkeley-Illinois-Maryland Association (BIMA) array to search for redshifted millimetre absorption in a sample of damped Lyman absorption systems (DLAs). This increases the number of published systems searched from 18 to 30. In 17 cases we reach 3 limits of 0.1, which is a significant improvement over the previous searches and more than sufficient to detect the four known redshifted millimetre absorbers ( 1). Although the CO rotational (millimetre) column density limits obtained are weaker than the electronic (optical) limits, they may provide useful limits below the atmospheric cut-off for the Lyman and Werner H2 bands in the UV (z abs 1.8). Using a model for the DLA metallicity evolution combined with assumed HCO+/H2 and CO/H2 conversion ratios, we use the molecular column density limits to calculate plausible H2 molecular fraction limits. Finally, we use these results to discuss the feasibility of detecting rotational CO transitions in DLAs with the next generation of large radio telescopes. 1 I N T R O D U C T I O N Molecular absorption lines trace, and provide detailed physical and chemical information about, the cold dense component of the interstellar medium (ISM). Although many detailed studies exist for molecular clouds within our own Galaxy, only relatively recently has detailed information emerged for molecular abundances at highredshift, through absorption studies of redshifted UV molecular hydrogen lines (e.g. Ledoux, Petitjean & Srianand 2003; Reimers et al. 2003) and millimetre-band rotational lines from molecular tracers (e.g. Wiklind & Combes 1996c). Observations of a range of different molecular transitions in gas clouds at high redshift would provide a wealth of information on star formation activity in external galaxies, potentially viewed at epochs when chemical abundances and environments were markedly different to today. Such information is invaluable for a detailed understanding of galactic formation and evolution. Furthermore, the narrowness of molecular lines reveals information about small-scale structure in the ISM. Comparison of relative line strengths yields information about the excitation mechanism, and, in particular, probes the temperature of the cosmic microwave background (CMB), and hence the expected (1 + z)-dependence (e.g. Wiklind & Combes 1996c). Finally, comparisons of the relative observed frequencies of millimetre molecular lines (with each other, and/or with atomic transitions arising in the same cloud) can be used to check on any possible variation in certain combinations of the fundamental constants (Drinkwater et al. 1998; Murphy et al. 2001). This last point was the prime motivation for our search for new redshifted millimetre absorbers reported in this paper, although we use the upper limits obtained to yield molecular fraction limits in low-redshift damped Lyman absorption systems (DLAs), which is not possible using the UV H2 lines, which fall below the atmospheric cut-off of z abs 1.8. Currently, only four redshifted millimetre absorption systems are known (see Wiklind & Combes 1999, and references therein), of which the highest redshift is 0.886 (PKS 1830211). As a means of approaching a search for new high-redshift radio absorbers systematically, we produced a catalogue of DLAs (Curran et al. 2002b),1 where large column densities (N H I 2 1020 cm2) are known to exist, and shortlisted those which are illuminated by radio-loud quasars (i.e. those with a measured radio flux density >0.1 Jy). This yielded 60 DLAs and sub-DLAs occulting radio-loud quasars. Of these, 37 have been searched for 21-cm absorption (see Kanekar & Chengalur 2003; Curran et al., in preparation). Selecting those of 1 A version of this catalogue is kept updated on-line and is available from http://www.phys.unsw.edu.au/sjc/dla. 12-mm and 3-mm flux densities 0.1 Jy gives 18 systems which have previously been searched for millimetre absorption (Curran et al. 2003), this number now being increased in total to 30 with the observations we present in this paper. 2 O B S E RVAT I O N S A N D D ATA R E D U C T I O N 2.1 12-mm observations In Table 12 we list all of the known DLAs and sub-DLAs which are illuminated by background sources of 0.1 Jy at centimetre wavelengths and have molecular transitions which could be detected in the Green Bank Telescope (GBT) K band. In all cases, this is both the HCN and HCO+ J = 0 1 rotational transitions.3 We chose to observe the latter transition because this tends to be slightly stronger than HCN in the four known millimetre absorption systems. Apart from the sources listed in Table 1, there are only three other such DLAs known which have millimetre transitions redshifted to 12 mm: Q 0438436, RX J1028.60844 and LBQS 1213+0922. Although strong (S 3 Jy at 12 mm), the first source is too far south to observe with the GBT, and the other two were estimated to have flux densities too low to yield good optical depth limits within a reasonable integration time. Therefore, using the GBT, we hoped for a near complete survey of 12-mm absorption in known DLAs, although due to time constraints we did not observe some of the sub-DLAs (Turnshek et al. 1989; Lanzetta et al. 1991) towards 1017+109, 1021006 and 1402+044, nor the N H I = 4 1020 cm2 absorber at z abs = 3.533 towards 1251407 (Ellison et al. 2001). The observations were performed in 2003 April, during several consecutive days. Typically, under acceptable observing conditions, the system temperature was 40100 K. At the time of observing, the wide-bandwidth spectrometer was unfortunately unavailable, and so we used the digital spectral processor with an observing bandwidth of 40 MHz, over 1024 channels. This gave a redshift coverage of z abs 0.004, sufficiently wide to cover the absorption redshift uncertainty in most of the DLAs. However, the optical redshifts of 1614+051 and 2131045 are poorly constrained (Table 1), and so we observed these with five and three overlapping bands, respectively. Dual polarization was used to optimize the sensitivity and beam switching facilitated the background emission removal. The data were reduced using the AIPS++ single-dish package and baselines were removed from the spectra by fitting a polynomial function. The continuum levels were measured either during GBT pointing scans of the brightest sources or with the Very Large Array (VLA) in its most compact (D) configuration. These latter data were taken in 2003 May, and thus the continuum levels might have altered d (...truncated)