Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra

We have previously presented evidence for a varying fine-str ucture constant, α, in two independent samples of Keck/HIRES QSO absorption spectra. Here we present a detailed manymultiplet analysis of a third Keck/HIRES sample containing 78 absorption systems. We also re-analyse the previous samples, providing a total of 128 absorption systems over the redshift range 0.2 < zabs < 3.7. The results, with raw statistical errors, indicate a small er weighted mean α in the absorption clouds: �α/α = ( 0.574 ± 0.102) × 10 5 . All three samples separately yield consistent and significant values of �α/α. The analyses of low-z (i.e. zabs < 1.8) and high-z systems rely on different ions and transitions with very different dependencies on α, yet they also give consistent results. We identify an addit ional source of random error in 22 high-z systems characterized by transitions with a large dynamic range in apparent optical depth. Increasing the statistical erro rs on �α/α for these systems gives our fiducial result, a weighted mean �α/α = ( 0.543 ± 0.116) × 10 5 , representing 4.7 σ evidence for a varying α. Assuming that �α/α = 0 at zabs = 0, the data marginally prefer a linear increase in α with time rather than a constant offset from the laboratory value: u α/α = (6.40 ± 1.35) × 10 16 yr 1 . The two-point correlation function for α is consistent with zero over 0.2‐13 Gpc comoving scales and the angular distribution of �α/α shows no significant dipolar anisotropy. We therefore have no eviden ce for spatial variations in �α/α. We extend our previous searches for possible systematic errors, giving detailed analyses of potential kinematic effects, line blending, wavelength miscalibration, spectrograph temperature variations, atmospheric dispersion and isotopic /hyperfine structure effects. The latter two are potentially the most significant. However, overall, known systematic errors do not explain the results. Future many-multiplet analyses of indep endent QSO spectra from different telescopes and spectrographs will provide a now crucial check on our Keck/HIRES results.

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