Probing isotopic ratios at z = 0.89: molecular line absorption in front of the quasar PKS 1830-211

¹ Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), PO Box 23-141, 106 Taipei, Taiwan e-mail: muller@asiaa.sinica.edu.tw
² Institut de RadioAstronomie Millimétrique (IRAM), 300 rue de la piscine, 38406 St-Martin-d'Hères, France
³ European Southern Observatory (ESO), Alonso de Cordova 3107, Casilla 19001, Santiago 19, Chile
⁴ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France

Received: 13 March 2006
Accepted: 26 July 2006

Abstract

With the Plateau de Bure interferometer, we have measured the C, N, O and S isotopic abundance ratios in the arm of a spiral galaxy with a redshift of 0.89. The galaxy is seen face-on according to HST images. Its bulge intercepts the line of sight to the radio-loud quasar PKS 1830-211, giving rise at mm wavelengths to two Einstein images located each behind a spiral arm. The arms appear in absorption in the lines of several molecules, giving the opportunity to study the chemical composition of a galaxy only a few Gyr old. The isotopic ratios in this spiral galaxy differ markedly from those observed in the Milky Way. The ¹⁷O/¹⁸O and ¹⁴N/¹⁵N ratios are low, as one would expect from an object too young to let low mass stars play a major role in the regeneration of the gas.