Molecules at z = 0.89

A 4-mm-rest-frame absorption-line survey toward PKS 1830−211^⋆,⋆⋆

¹ Onsala Space Observatory, 439-92 Onsala, Sweden
e-mail: mullers@chalmers.se
² Institut d’Astrophysique Spatiale, Bât. 121, Université Paris-Sud, 91405 Orsay Cedex, France
³ Institut de Radioastronomie Millimétrique, 300 rue de la piscine, 38406 St Martin d’Hères, France
⁴ Ecole Normale Supérieure/LERMA, 24 rue Lhomond, 75005 Paris, France
⁵ Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
⁶ School of Physics, University of New South Wales, Sydney NSW 2052, Australia
⁷ Physique des interactions ioniques et moléculaires, Université de Provence, Centre de Saint Jérôme, 13397 Marseille Cedex 20, France
⁸ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

Received: 17 April 2011
Accepted: 29 August 2011

Abstract

We present the results of a 7 mm spectral survey of molecular absorption lines originating in the disk of a z = 0.89 spiral galaxy located in front of the quasar PKS 1830−211. Our survey was performed with the Australia Telescope Compact Array and covers the frequency interval 30–50 GHz, corresponding to the rest-frame frequency interval 57–94 GHz. A total of 28 different species, plus 8 isotopic variants, were detected toward the south-west absorption region, located about 2 kpc from the center of the z = 0.89 galaxy, which therefore has the largest number of detected molecular species of any extragalactic object so far. The results of our rotation diagram analysis show that the rotation temperatures are close to the cosmic microwave background temperature of 5.14 K that we expect to measure at z = 0.89, whereas the kinetic temperature is one order of magnitude higher, indicating that the gas is subthermally excited. The molecular fractional abundances are found to be in-between those in typical Galactic diffuse and translucent clouds, and clearly deviate from those observed in the dark cloud TMC 1 or in the Galactic center giant molecular cloud Sgr B2. The isotopic ratios of carbon, nitrogen, oxygen, and silicon deviate significantly from the solar values, which can be linked to the young age of the z = 0.89 galaxy and a release of nucleosynthesis products dominated by massive stars. Toward the north-east absorption region, where the extinction and column density of gas is roughly one order of magnitude lower than toward the SW absorption region, only a handful of molecules are detected. Their relative abundances are comparable to those in Galactic diffuse clouds. We also report the discovery of several new absorption components, with velocities spanning between  − 300 and +170 km s^-1. Finally, the line centroids of several species (e.g., CH₃OH, NH₃) are found to be significantly offset from the average velocity. If caused by a variation in the proton-to-electron mass ratio μ with redshift, these offsets yield an upper limit |Δμ/μ| < 4×10^-6, which takes into account the kinematical noise produced by the velocity dispersion measured from a large number of molecular species.