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

¹ INAF-Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
² Ioffe Physical-Technical Institute, Polytekhnicheskaya Str. 26, 194021 Saint Petersburg, Russia
e-mail: lev@astro.ioffe.rssi.ru
³ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, CAS, 80 Nandan Road, Shanghai 200030, PR China

Received: 24 November 2010
Accepted: 14 February 2011

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 Mg ii λλ2796,2803 Å lines is often taken as a reference in computations of possible changes of the fine-structure constant α, 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 high-metallicity 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): ΔV_Mg   II = −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 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 (²⁵Mg+ ²⁶Mg)/²⁴Mg  ≲ 0.7 and (²⁵Mg+ ²⁶Mg)/²⁴Mg  ≲ 2.6, respectively. At z_abs = 1.58, we also put a strong constraint on a putative variation of α: Δα/α = (−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.