T. M. Evans, M. T. Murphy, J. B. Whitmore, T. Misawa, M. Centurión, S. D'Odorico, S. Lopez, C. J. A. P. Martins, P. Molaro, P. Petitjean, H. Rahmani, R. Srianand, M. Wendt

Abstract
Large statistical samples of quasar spectra have previously indicated possible cosmological variations in the fine-structure constant, α . A smaller sample of higher signal-to-noise ratio spectra, with dedicated calibration, would allow a detailed test of this evidence. Towards that end, we observed equatorial quasar HS 1549+ 1919 with three telescopes: the Very Large Telescope, Keck and, for the first time in such analyses, Subaru. By directly comparing these spectra to each other, and by `supercalibrating' them using asteroid and iodine-cell tests, we detected and removed long-range distortions of the quasar spectra's wavelength scales which would have caused significant systematic errors in our α measurements. For each telescope we measure the relative deviation in α from the current laboratory value, Δα/α , in 3 absorption systems at redshifts zabs=1.143 , 1.342, and 1.802. The nine measurements of Δα/α are all consistent with zero at the 2-σ level, with 1-σ statistical (systematic) uncertainties 5.6--24 (1.8--7.0) parts per million (ppm). They are also consistent with each other at the 1-σ level, allowing us to form a combined value for each telescope and, finally, a single value for this line of sight: Δα/α=−5.4±3.3stat±1.5sys ppm, consistent with both zero and previous, large samples. We also average all Large Programme results measuring Δα/α=−0.6±1.9stat±0.9sys ppm. Our results demonstrate the robustness and reliability at the 3 ppm level afforded by supercalibration techniques and direct comparison of spectra from different telescopes.

Keywords
quasars: absorption lines – quasars: individual: HS 1549+1919 – intergalactic medium – cosmology: miscellaneous – cosmology: observations

Monthly Notices of the Royal Astronomical Society
Volume 445, Issue 1, Page 128
2014 November

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