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

We have previously presented evidence for a varying fine-structure constant, , in two independent samples of Keck/HIRES quasi-stellar object (QSO) absorption spectra. Here we present a detailed many-multiplet 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 < z abs < 3.7. The results, with raw statistical errors, indicate a smaller weighted mean in the absorption clouds: / = (-0.574 0.102) 10 -5 . All three samples separately yield consistent and significant values of /. The analyses of low-z (i.e. z abs < 1.8) and high-z systems rely on different ions and transitions with very different dependences on , yet they also give consistent results. We identify an additional source of random error in 22 high-z systems characterized by transitions with a large dynamic range in apparent optical depth. Increasing the statistical errors on / for these systems gives our fiducial result, a weighted mean / = (-0.543 0.116) 10 -5 , representing 4.7 evidence for a varying . Assuming that / = 0 at z abs = 0, the data marginally prefer a linear increase in with time rather than a constant offset from the laboratory value: / = (6.40 1.35) 10 -16 yr -1 . The two-point correlation function for is consistent with zero over 0.2-13 Gpc comoving scales and the angular distribution of / shows no significant dipolar anisotropy. We therefore have no evidence for spatial variations in /.