CONSTRAINT on A COSMOLOGICAL VARIATION in the PROTON-TO-ELECTRON MASS RATIO from ELECTRONIC CO ABSORPTION

Carbon monoxide (CO) absorption in the sub-damped Lyα absorber at redshift zabs ≃ 2.69 toward the background quasar SDSS J123714.60+064759.5 (J1237+0647) was investigated for the first time in order to search for a possible variation of the proton-to-electron mass ratio, μ, over a cosmological timescale. The observations were performed with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of 40 per 2.5 km s-1 per pixel at ∼5000 Å. Thirteen CO vibrational bands in this absorber are detected: the A1Π -X1 + (ν', 0) for ν' = 0-8, B1 + - X1 + (0, 0), C1 + - X1 + (0, 0), and E1Π- X1 + (0, 0) singlet-singlet bands and the d3Δ-X1 + (5, 0) singlet-triplet band. An updated database including the most precise molecular inputs needed for a μ-variation analysis is presented for rotational levels J = 0-5, consisting of transition wavelengths, oscillator strengths, natural lifetime damping parameters, and sensitivity coefficients to a variation of the proton-to-electron mass ratio. A comprehensive fitting method was used to fit all the CO bands at once and an independent constraint of Δμ/μ = (0.7 ± 1.6stat ± 0.5syst) ∼ 10-5 was derived from CO only. A combined analysis using both molecular hydrogen and CO in the same J1237+0647 absorber returned a final constraint on the relative variation of Δμ/μ = (-5.6 ± 5.6stat ± 3.1syst) ∼ 10-6, which is consistent with no variation over a look-back time of ∼11.4 Gyr.