VLT/UVES constraints on the cosmological variability of the fine-structure constant

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Abstract
We propose a new methodology for probing the cosmological variability of α from pairs of lines (SIDAM, single ion differential α measurement) observed in individual exposures from a high resolution spectrograph. By this we avoid the influence of the spectral shifts due to ionization inhomogeneities in the absorbers; and non-zero offsets between different exposures. Applied to the lines of the metal absorption line system at z abs = 1.839 in the spectrum of Q 1101–264 obtained by means of the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT), SIDAM provides = . The z abs = 1.15 system toward HE 0515–4414 has been re-analyzed by this method thus obtaining for the combined sample = . These values are shifted with respect to the Keck/HIRES mean = (Murphy et al. 2004) at very high confidence level (95%). The fundamental photon noise limitation in the measurement with the VLT/UVES is discussed to figure the prospects for future observations. It is suggested that with a spectrograph of ~10 times the UVES dispersion coupled to a 100 m class telescope the present Oklo level ( ≥ 4.5 10-8) can be achieved along cosmological distances with differential measurements of .

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