The UVES Large Program for testing fundamental physics I. Bounds on a change in α towards quasar HE?2217?2818

摘要

Context. Absorption-line systems detected in quasar spectra can be used to compare the value of the fine-structure constant, α, measured today on Earth with its value in distant galaxies. In recent years, some evidence has emerged of small temporal and also spatial variations in α on cosmological scales. These variations may reach a fractional level of ?≈?10?ppm (parts per million). Aims. To test these claims we are conducting a Large Program of observations with the Very Large Telescope’s Ultraviolet and Visual Echelle Spectrograph (UVES), and are obtaining high-resolution (R?≈?60?000) and high signal-to-noise ratio (S/N?≈?100) UVES spectra calibrated specifically for this purpose. Here we analyse the first complete quasar spectrum from this programme, that of HE?2217?2818. Methods. We applied the many multiplet method to measure α in five absorption systems towards this quasar: zabs?=?0.7866, 0.9424, 1.5558, 1.6279 , and 1.6919. Results. The most precise result is obtained for the absorber at zabs?=?1.6919 where 3 Fe?ii transitions and Al?ii λ1670 have high S/N and provide a wide range of sensitivities to α. The absorption profile is complex with several very narrow features, and it requires?32 velocity components to be fitted to the data. We also conducted a range of tests to estimate the systematic error budget. Our final result for the relative variation in α in this system is Δα/α?=?+1.3?±?2.4stat?±?1.0sys?ppm. This is one of the tightest current bounds on α-variation from an individual absorber. A second, separate approach to the data reduction, calibration, and analysis of this system yielded a slightly different result of ?3.8?ppm, possibly suggesting a larger systematic error component than our tests indicated. This approach used an additional 3 Fe?ii transitions, parts of which were masked due to contamination by telluric features. Restricting this analysis to the Fe?ii transitions alone and using a modified absorption profile model gave a result that is consistent with the first approach, Δα/α?=?+1.1?±?2.6stat?ppm. The four other absorbers have simpler absorption profiles, with fewer and broader features, and offer transitions with a narrower range of sensitivities to α. They therefore provide looser bounds on Δα/α at the ??10?ppm precision level. Conclusions. The absorbers towards quasar HE?2217?2818 reveal no evidence of any variation in α at the 3-ppm precision level (1σ confidence). If the recently reported 10-ppm dipolar variation in α across the sky is correct, the expectation at this sky position is (3.2?5.4)?±?1.7?ppm depending on dipole model used. Our constraint of Δα/α?=?+1.3?±?2.4stat?±?1.0sys?ppm is not inconsistent with this expectation.