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

摘要

With the Plateau de Bure interferometer, we have measured the C, N, O and Sisotopic abundance ratios in the arm of a spiral galaxy with a redshift of0.89. The galaxy is seen face-on according to HST images. Its bulge interceptsthe line of sight to the radio-loud quasar PKS 1830-211, giving rise at mmwavelengths to two Einstein images located each behind a spiral arm. The armsappear in absorption in the lines of several molecules, giving the opportunityto study the chemical composition of a galaxy only a few Gyr old. The isotopicratios in this spiral galaxy differ markedly from those observed in the MilkyWay. The $^{17}$O/$^{18}$O and $^{14}$N/$^{15}$N ratios are low, as one wouldexpect from an object too young to let low mass stars play a major role in theregeneration of the gas.