Molecular gas at intermediate redshifts

Free access article

Issue		A&A Volume 381, Number 3, January III 2002


Page(s)		L73 - L76
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20011697

A&A 381, L73-L76 (2002)
DOI: 10.1051/0004-6361:20011697

Letter

Molecular gas at intermediate redshifts

N. Kanekar and J. N. Chengalur

National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, Pune 411 007, India

(Received 8 November 2001 / Accepted 28 November 2001)

Abstract
We present Giant Metrewave Radio Telescope (GMRT) observations of OH absorption in B3 1504+377 ( $z \sim 0.673$ ) and PKS 1413+135 ( $z \sim 0.247$ ). OH has now been detected in absorption towards four intermediate redshift systems, viz. the lensing galaxies towards B 0218+357 ( $z \sim 0.685$ ; Kanekar et al. 2001) and 1830-211 ( $z \sim 0.886$ ; Chengalur et al. 1999), in addition to the two systems listed above. All four systems also give rise to well studied millimetre wavelength molecular line absorption from a host of molecules, including HCO ⁺. Comparing our OH data with these millimetre line transitions, we find that the linear correlation between $N_{\rm OH}$ and $N_{\rm HCO^+}$ found in molecular clouds in the Milky Way (Liszt & Lucas 1996) persists out to $z \sim 1$ . It has been suggested (Liszt & Lucas 1999) that OH is a good tracer of ${\rm H_2}$ , with $N_{\rm H_2}/N_{\rm OH} \approx 10^7$ under a variety of physical conditions. We use this relationship to estimate $N_{\rm H_2}$ in these absorbers. The estimated $N_{\rm H_2}$ is $\ga$ 10²² in all four cases and substantially different from estimates based on CO observations.

Key words: galaxies: evolution: -- galaxies: formation: -- galaxies: ISM -- cosmology: observations -- radio lines: galaxies

Offprint request: N. Kanekar, nissim@ncra.tifr.res.in

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