Constraint on a cosmological variation in the proton-to-electron mass ratio from electronic CO absorption

Carbon monoxide (CO) absorption in the sub-damped Lyman-$α$ absorber at redshift $z_{abs} \simeq 2.69$, 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 time-scale. 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 kms$^{-1}$ per pixel at $\sim 5000$ Å. Thirteen CO vibrational bands in this absorber are detected: the A$^{1}Π$ - X$^{1}Σ^{+}$ ($ν'$,0) for $ν' = 0 - 8$, B$^{1}Σ^{+}$ - X$^{1}Σ^{+}$ (0,0), C$^{1}Σ^{+}$ - X$^{1}Σ^{+}$ (0,0), and E$^{1}Π$ - X$^{1}Σ^{+}$ (0,0) singlet-singlet bands and the d$^{3}Δ$ - X$^{1}Σ^{+}$ (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 $Δμ/μ= (0.7 \pm 1.6_{stat} \pm 0.5_{syst}) \times 10^{-5}$ 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 $Δμ/μ= (-5.6 \pm 5.6_{stat} \pm 3.1_{syst}) \times 10^{-6}$, which is consistent with no variation over a look-back time of $\sim 11.4$ Gyrs.

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