Astronomy & Astrophysics

Free access article

A&A 440, 893-899 (2005)
DOI: 10.1051/0004-6361:20052816

The kinetic temperature of a molecular cloud at redshift 0.7: ammonia in the gravitational lens B0218+357

C. Henkel¹, N. Jethava¹, A. Kraus¹, K. M. Menten¹, C. L. Carilli², M. Grasshoff³, D. Lubowich⁴ and M. J. Reid⁵

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: chenkel@mpifr-bonn.mpg.de
² National Radio Astronomy Observatory, Socorro, New Mexico 87801, USA
³ Visiting the Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Department of Physics and Astronomy, Hofstra University, Hempstead, NY 11549, USA
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MS 42, Cambridge, MA 02138, USA

(Received 3 February 2005 / Accepted 1 June 2005 )

Abstract
Using the Effelsberg 100-m telescope, absorption in the ( J,K) = (1,1), (2,2) and (3,3) inversion lines of ammonia (NH₃) was detected at a redshift of z = 0.6847 toward the gravitational lens system B0218+357. The $\lambda \sim2$ cm absorption peaks at 0.5-1.0% of the continuum level and appears to cover a smaller fraction of the radio continuum background than lines at millimeter wavelengths. Measured intensities are consistent with a rotation temperature of ~35 K, corresponding to a kinetic temperature of ~55 K. The column density toward the core of image A then becomes N(NH $_3) \sim 1\times10^$ cm^-2 and fractional abundance and gas density are of order X(NH $_3) \sim$ 10^-8 and n(H $_2) \sim 5\times10^{3}$ cm^-3, respectively. Upper limits are reported for the (2,1) and (4,4) lines of NH₃ and for transitions of the SO, DCN, OCS, SiO, C₃N, H₂CO, SiC₂, HC₃N, HC₅N, and CH₃OH molecules. These limits and the kinetic temperature indicate that the absorption lines are not arising from a cold dark cloud but from a warm, diffuse, predominantly molecular medium. The physical parameters of the absorbing molecular complex, seen at a projected distance of ~2 kpc to the center of the lensing galaxy, are quite peculiar when compared with the properties of clouds in the Galaxy or in nearby extragalactic systems.

Key words: galaxies: abundances -- galaxies: ISM -- quasars: individual: B0218+357 -- quasars: absorption lines -- radio lines: galaxies

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