Low deuterium abundance in the zabs=3.514 absorber towards APM 08279+5255

Abstract

A high-resolution, high signal-to-noise HIRES-Keck spectrum of APM 08279+5255 reveals a feature in the HI Ly alpha profile of the zabs=3.514 absorber at the expected position of the corresponding DI Lyalpha line. The absorber shows a relatively simple velocity structure, with a major component detected in several metallic lines including CII, CIV, SiII, SiIII and Si IV. Modeling of the hydrogen column density with the minimum number of components yields D/H ~ 1.5 x 10E-5. However, a more complex structure for the hydrogen cloud with somewhat ad hoc components would allow a higher D/H. The system has a very low metallicity with [Si/H] ~ -3.5 and [C/H] ~ -4.0 and is therefore representative of essentially unprocessed material, with a deuterium abundance close to the primordial value. The present analysis favours the low value of D/H =3.39(+/-0.25) x 10E-5 measured towards QSO 1009+2956 and QSO 1937-1009 (Burles & Tytler 1998a, 1998b) as the fiducial value of deuterium at high redshift.