The kinetic temperature of a molecular cloud at redshift 0.9: ammonia in the gravitational lens PKS 1830–211

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, 520 Edgemont Rd., Charlottesville, VA 22903, USA
³ CSIRO, Australia Telescope National Facility, Cnr. Vimiera & Pembroke Roads, Marsfield, NSW 2122, Australia

Received: 24 November 2007
Accepted: 18 March 2008

Abstract

Context. Little is known about the structure of the interstellar medium and the nature of individual clouds in galaxies at intermediate redshifts. The gravitational lens toward PKS 1830–211 offers the unique possibility to study this interstellar gas with high sensitivity and angular resolution in a molecular cloud that existed half a Hubble time ago.

Aims. This multi-line study aims at a better definition of the physical properties of a significantly redshifted cloud.

Methods. Using the Green Bank Telescope (GBT), we searched for ammonia (NH₃) toward PKS 1830–211.

Results. We have detected all ten observed metastable 2 cm ammonia lines. The (J, K) = (1, 1) to (10, 10) transitions, up to ~1030 K above the ground state, were measured in absorption against the radio continuum of the lensed background source. The ammonia absorption appears to be optically thin, with absolute peak flux densities up to 2.5% of the total continuum flux density. Measured intensities are consistent with a kinetic temperature of ~80 K for 80–90% of the ammonia column. The remaining 10–20% are warmer, with at least some of this gas reaching kinetic temperatures of 600 K. Toward the south-western continuum source, the column density is N(NH₃) ~ (5-10)10¹⁴ cm^-2, which implies a fractional abundance of X(NH₃) ~ (1.5-3.0)10^-8. Similarities with a hot NH₃ absorption component toward the Sgr B2 region close to our Galactic center, observed up to the (18, 18) line, suggest that the Sgr B2 component also consists of warm diffuse low-density gas. The warm absorption features from PKS 1830–211 are unique in the sense that they originate in a spiral arm.