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

Aims. Abundances of the Mg isotopes 24 Mg, 25 Mg, and 26 Mg can be used to test models of chemical enrichment of interstellar/intergalactic gas clouds. Additionally, because the position of the Mg ii 2796, 2803 lines is often taken as a reference in computations of possible changes of the fine-structure constant , 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 z abs = 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, Fe ii, Ca i, Ca ii): V Mg II = -0.44 0.05 km s -1 , and V Mg I = -0.17 0.17 km s -1 . This translates into the isotopic ratio 24 Mg: 25 Mg: 26 Mg = (19 11):(22 13):(59 6) with a strong relative overabundance of heavy Mg isotopes, ( 25 Mg+ 26 Mg)/ 24 Mg = 4, as compared to the solar ratio 24 Mg: 25 Mg: 26 Mg = 79:10:11, and ( 25 Mg+ 26 Mg)/ 24 Mg = 0.3. In the systems at z abs = 1.58 and z abs = 1.65 enriched by AGB-stars we find only upper limits on the content of heavy Mg isotopes ( 25 Mg+ 26 Mg)/ 24 Mg < 0.7 and ( 25 Mg+ 26 Mg)/ 24 Mg < 2.6, respectively. At z abs = 1.58, we also put a strong constraint on a putative variation of : / = (-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 is subject to systematic wavelength-dependent drifts.