First measurement of Mg isotope abundances at high redshifts and accurate estimate of Δ α/α

Abstract

Aims. Abundances of the Mg isotopes 24Mg, 25Mg, and 26Mg can be used to test models of chemical enrichment of interstellar/ intergalactic gas clouds. Additionally, because the position of the Mgii λ2796, 2803 Å lines is often taken as a reference in computations of possible changes of the fine-structure constant a, it should be clarified to which extent these lines are affected by isotopic shifts. Methods. We use a high-resolution spectrum (pixel size ∼1.3 km s-1) of the quasar HE0001-2340 observed with the UVES/VLT to measure Mg isotope abundances in the intervening absorption-line systems at high redshifts. Line profiles are prepared taking into account possible shifts between the individual exposures. In the line-fitting procedure, the lines of each ion are treated independently. Because of the unique composition of the selected systems - the presence of several transitions of the same ion - we can test the local accuracy of the wavelength scale calibration, which is the main source of errors in the sub-pixel line position measurements. Results. In the system at zabs = 0.45, which is probably a fragment of the outflow caused by SN Ia explosion of highmetallicity white dwarf(s), we measured velocity shifts of Mg ii and Mg i lines with respect to other lines (Fe I, Feii, CaI, CaII): δV MgII = -0.44 ± 0.05 km s-1, and δV Mg I = -0.17 ± 0.17 km s-1. This translates into the isotopic ratio 24Mg:25Mg:26Mg = (19 ± 11):(22 ± 13):(59 ± 6) with a strong relative overabundance of heavy Mg isotopes, (25Mg+26Mg)/ 24Mg = 4, as compared to the solar ratio 24Mg: 25Mg:26Mg = 79:10:11, and (25Mg+ 26Mg)/24Mg = 0.3. In the systems at zabs = 1.58 and z abs = 1.65 enriched by AGB-stars we find only upper limits on the content of heavy Mg isotopes (25Mg+ 26Mg)/24Mg ≤ 0.7 and (25Mg+ 26Mg)/24Mg ≤2.6, respectively. At zabs = 1.58, we also put a strong constraint on a putative variation of a: .a/a = (-1.5 ± 2.6) × 10-6, which is one of the most stringent limits obtained from optical spectra of QSOs.We reveal that the wavelength calibration in the range above 7500 Å is subject to systematic wavelength-dependent drifts. © 2011 ESO.