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.Comment: 24 pages, 11 figures, 9 table