2014ApJ...782...10B

Rovibronic molecular hydrogen (H₂) transitions at redshift z_abs≃ 2.659 toward the background quasar B0642-5038 are examined for a possible cosmological variation in the proton-to-electron mass ratio µ. We utilize an archival spectrum from the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph (UVES) with a signal-to-noise ratio of ∼35 per 2.5 km/s pixel at the observed H₂ wavelengths (335-410 nm). Some 111 H₂transitions in the Lyman and Werner bands have been identified in the damped Lyα system for which a kinetic gas temperature of ∼84 K and a molecular fraction log f = -2.18±0.08 are determined. The H₂absorption lines are included in a comprehensive fitting method, which allows us to extract a constraint on a variation of the proton-electron mass ratio Δµ/µ from all transitions at once. We obtain Δµ/µ = (17.1±4.5_stat± 3.7_sys)x10^–6. However, we find evidence that this measurement has been affected by wavelength miscalibration errors recently identified in UVES. A correction based on observations of objects with solar-like spectra gives a smaller Δµ/µ value and contributes to a larger systematic uncertainty: Δµ/µ = (12.7±4.5_stat± 4.2_sys)x10^–6.