2005A&A...440..893H

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 λ∼2cm 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 ∼35K, corresponding to a kinetic temperature of ∼55K. The column density toward the core of image A then becomes N(NH₃)∼1x10¹⁴cm^–2 and fractional abundance and gas density are of order X(NH₃)∼10^–8 and n(H₂)∼5x10³cm^–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 ∼2kpc 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.