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 $\ion{Fe}{ii}$ 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 $\ion{Fe}{ii}$ 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 $\Delta\alpha/\alpha$ = $(2.4\pm3.8_{\rm stat})\times10^{-6}$. The zabs = 1.15 $\ion{Fe}{ii}$ system toward HE 0515–4414 has been re-analyzed by this method thus obtaining for the combined sample $\Delta\alpha/\alpha$ = $(0.4\pm1.5_{\rm stat})\times10^{-6}$. These values are shifted with respect to the Keck/HIRES mean $\Delta\alpha/\alpha$ = $(-5.7\,\pm\,1.1_{\rm stat})\times10^{-6}$ (Murphy et al. 2004) at very high confidence level (95%). The fundamental photon noise limitation in the $\Delta\alpha/\alpha$ 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 ($\Delta\alpha/\alpha$ ≥ 4.5 $\times$ 10-8) can be achieved along cosmological distances with differential measurements of $\Delta\alpha/\alpha$.