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A search for molecules in damped Lyman-alpha absorbers occulting millimetre-loud quasars.pdf

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A search for molecules in damped Lyman-alpha absorbers occulting millimetre-loud quasars

a r X i v : a s t r o - p h / 0 2 0 9 1 7 5 v 1 1 0 S e p 2 0 0 2 Astronomy Astrophysics manuscript no. paper February 2, 2008 (DOI: will be inserted by hand later) A search for molecules in damped Lyman-alpha absorbers occulting millimetre-loud quasars? S. J. Curran1, M. T. Murphy1, J. K. Webb1, F. Rantakyro?2,3, L. E. B. Johansson4 and S. Nikolic?4,5 1 School of Physics, University of New South Wales, Sydney NSW 2052, Australia 2 European Southern Observatory, Casilla 19001, Santiago 19, Chile 3 Observatorio Cerro Calan, Universidad de Chile, Santiago, Chile 4 Onsala Space Observatory, Chalmers University of Technology, S 439 92 Onsala, Sweden 5 Astronomical Observatory, Volgina 7, 11160 Belgrade, Serbia email: sjc@.au Received ; accepted Abstract. We have used the SEST 15-metre and Onsala 20-metre telescopes to perform deep (r.m.s. ～ 30 mJy) integrations of various molecular rotational transitions towards damped Lyman-alpha absorption systems (DLAs) known to occult millimetre-loud quasars. We have observed 6 new systems and improved the existing limits for 11 transitions. These limits may be approaching the sensitivities required to detect new systems and we present a small number of candidate systems which we believe warrant further observation. Key words. quasars: absorption lines–Cosmology: observations–Cosmology: early Universe 1. Introduction Millimetre-band molecular absorption systems along the line-of-sight toward quasars can provide a powerful probe of cold gas in the early Universe. Wiklind Combes (1994; 1995; 1996b) have used such absorption lines to study a variety of properties of the absorbers themselves (e.g. rela- tive column densities, kinetic and excitation temperatures, filling factors). Constraints on the cosmic microwave back- ground temperature can also be obtained by comparing the optical depths of different rotational transitions (e.g. Wiklind Combes 1996a). If the background quasar is gravitationally lensed, time delay studies can