Analysis of Molecular Hydrogen Absorption toward QSO B0642-5038 for a Varying Proton-to-Electron Mass Ratio

Abstract

Rovibronic molecular hydrogen (H${}_2$) transitions at redshift $z_{\mathrm{abs}}\simeq 2.659$ towards the background quasar B0642$-$5038 are examined for a possible cosmological variation in the proton-to-electron mass ratio, $\mu$. We utilise an archival spectrum from the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of $\sim$35 per 2.5-kms${}^{-1}$ pixel at the observed H${}_2$ wavelengths (335--410 nm). Some 111 H${}_2$ transitions in the Lyman and Werner bands have been identified in the damped Lyman $\alpha$ system for which a kinetic gas temperature of $\sim$84 K and a molecular fraction $\log f=-2.18\pm 0.08$ is determined. The H${}_2$ absorption lines are included in a comprehensive fitting method, which allows us to extract a constraint on a variation of the proton-electron mass ratio, $\Delta \mu /\mu$, from all transitions at once. We obtain $\Delta \mu /\mu =(17.1\pm 4.5_{\mathrm{stat}}\pm 3.7_{\mathrm{sys}})\times 10^{-6}$. 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 $\Delta \mu /\mu$ value and contributes to a larger systematic uncertainty: $\Delta \mu /\mu =(12.7\pm 4.5_{\mathrm{stat}}\pm 4.2_{\mathrm{sys}})\times 10^{-6}$.

34 pages, 9 figures. Accepted for publication in ApJ

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