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

We propose a new methodology for probing the cosmological variability of α from pairs of Fe  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 (i) ionization inhomogeneities in the absorbers; and (ii) non-zero offsets between different exposures. Applied to the Fe  lines of the metal absorption line system at zabs = 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 ∆α/α = (2.4 ± 3.8stat) × 10 −6 .T hezabs = 1.15 Fe  system toward HE 0515-4414 has been re-analyzed by this method thus obtaining for the combined sample ∆α/α = (0.4 ± 1.5stat) × 10 −6 . These values are shifted with respect to the Keck/HIRES mean ∆α/α = (−5.7 ± 1.1stat) × 10 −6 (Murphy et al. 2004) at very high confidence level (95%). The fundamental photon noise limitation in the ∆α/α 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 (∆α/α ≥ 4.5 × 10 −8 )