The UVES Large Program for testing fundamental physics – III. Constraints on the fine-structure constant from three telescopes

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, ∆α/α, in 3 absorption systems at redshifts z abs = 1.143, 1.342, and 1.802. The nine measurements of ∆α/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: ∆α/α = −5.4 ± 3.3 stat ± 1.5 sys ppm, consistent with both zero and previous, large samples. We also average all Large Programme results measuring ∆α/α = −0.6±1.9 stat ±0.9 sys 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.

Subaru Telescope limits on cosmological variations in the fine-structure constant

“…For each quasar, the extracted flux from all exposures was combined with variance weighting using _ (Murphy 2016, version 0.73) to form a final, 1-dimensional "master spectrum", including a 1σ uncertainty spectrum. Evans et al (2014) and Murphy et al (2016) describe the details of this process. Briefly, for our HDS spectra, the most important aspects are: (i) each echelle order, of each exposure, was redispersed onto a common, log-linear vacuumheliocentric wavelength grid with a 1.75 km s −1 pix −1 dispersion; (ii) after initial, automatic 'cleaning' of the spectra, artefacts in the flux arrays, such as 'cosmic rays', 'ghosts' (internal reflections within the spectrograph), spurious or outlying pixels, and residual blaze removal effects, were masked and/or normalised out manually; these steps were recorded and are fully reproducible; and (iii) an initial, automatically set, polynomial continuum is manually refitted, if required, in regions where metal absorption is present, to ensure profile fits can be established as accurately as possible.…”

“…Criterion (i) allows a somewhat larger χ 2 ν than allowed by Murphy et al (2016) who analysed only Zn and Cr transitions, which have similar strengths and are more easily modelled. However, χ 2 ν values of ≈1.5 were achieved in Molaro et al (2013) and Evans et al (2014) where all available transitions, with widely varying strengths, were considered. Similar to Evans et al (2014), we also find that some parts of the extracted Subaru/HDS exposures have slightly (factors of 1.1-1.3) larger root-mean-square (RMS) flux variations than expected in unabsorbed regions.…”

“…We follow many recent works (e.g. Malec et al 2010;Molaro et al 2013;Bagdonaite et al 2014;Evans et al 2014;Murphy et al 2016;Kotuš et al 2017) and adopt a basic saw-tooth model with a velocity shift amplitude of +v saw at the echelle order centre with a linear fall-off to −v saw at the order edges. This distortion is applied to each echelle order, of each quasar exposure, and these are then combined with _ in the usual way.…”

“…However, important systematic errors have recently been discovered in the high-resolution, slit-based spectrographs on the Keck, VLT and also Subaru telescopes, manifesting as long-range distortions in the wavelength scales of quasar spectra (e.g. Rahmani et al 2013;Bagdonaite et al 2014;Evans et al 2014;Songaila & Cowie 2014;Whitmore & Murphy 2015;Kotuš et al 2017). This is despite earlier studies finding no evidence for distortions of a similar size (Molaro et al 2008b;Griest et al 2010;Whitmore et al 2010), suggesting that the distortions vary over time (or other observational parameters).…”

Current and future constraints on extended Bekenstein-type models for a varying fine-structure constant

“….14 −7.5 ± 5.5 UVES/HIRES/HDS[19] HE0515−4414 1.15 −1.4 ± 0.Available dedicated measurements of α. Listed are, respectively, the object along each line of sight, the redshift of the measurement, the measurement itself (in parts per million), the spectrograph, and the original reference.…”