Context :

Aims : Abundances of Mg isotopes ^ { 24 } Mg , ^ { 25 } Mg , and ^ { 26 } Mg can be used to test models of chemical enrichment of interstellar/intergalactic gas clouds .
Additionally , since the position of the Mg ii \lambda \lambda 2796 , 2803 Å lines is often taken as a reference in computations of possible changes of the fine-structure constant \alpha , it should be clarified to what extent these lines are affected by isotopic shifts .

Methods : We use a high-resolution spectrum ( pixel size \approx 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 accounting for possible shifts between the individual exposures .
In the line fitting procedure , the lines of each ion are treated independently .
Due to 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 high-metallicity white dwarf ( s ) we measured velocity shifts of Mg ii and Mg i lines relative to other lines ( Fe i , Fe ii , Ca i , Ca ii ) : \Delta V _ { \scriptscriptstyle Mg { \sc II } } = -0.44 \pm 0.05 km s ^ { -1 } , and \Delta V _ { \scriptscriptstyle Mg { \sc I } } = -0.17 \pm 0.17 km s ^ { -1 } .
This translates into the isotopic ratio ^ { 24 } Mg : ^ { 25 } Mg : ^ { 26 } Mg = ( 19 \pm 11 ) : ( 22 \pm 13 ) : ( 59 \pm 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 \raise 1.29 pt \hbox { $ < $ \kern - 7.5 pt \raise - 4.73 pt \hbox { $ \sim$ } } 0.7 and ( ^ { 25 } Mg+ ^ { 26 } Mg ) / ^ { 24 } Mg \raise 1.29 pt \hbox { $ < $ \kern - 7.5 pt \raise - 4.73 pt \hbox { $ \sim$ } } 2.6 , respectively .
At z _ { abs } = 1.58 , we also put a strong constraint on a putative variation of \alpha : \Delta \alpha / \alpha = ( -1.5 \pm 2.6 ) \times 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 .

Conclusions :