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

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 (, (2,2) and (3,3) inversion lines of ammonia (NH₃) was detected at a redshift of toward the gravitational lens system B0218+357. The  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 cm^-2 and fractional abundance and gas density are of order X(NH 10^-8 and n(H 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.