Subaru Telescope limits on cosmological variations in the fine-structure constant

Previous, large samples of quasar absorption spectra have indicated some evidence for relative variations in the fine-structure constant ($\Delta\alpha/\alpha$) across the sky. However, they were likely affected by long-range distortions of the wavelength calibration, so it is important to establish a statistical sample of more reliable results, from multiple telescopes. Here we triple the sample of $\Delta\alpha/\alpha$ measurements from the Subaru Telescope which have been `supercalibrated' to correct for long-range distortions. A blinded analysis of the metallic ions in 6 intervening absorption systems in two Subaru quasar spectra provides no evidence for $\alpha$ variation, with a weighted mean of $\Delta\alpha/\alpha=3.0\pm2.8_{\rm stat}\pm2.0_{\rm sys}$ parts per million (1$\sigma$ statistical and systematic uncertainties). The main remaining systematic effects are uncertainties in the long-range distortion corrections, absorption profile models, and errors from redispersing multiple quasar exposures onto a common wavelength grid. The results also assume that terrestrial isotopic abundances prevail in the absorbers; assuming only the dominant terrestrial isotope is present significantly lowers $\Delta\alpha/\alpha$, though it is still consistent with zero. Given the location of the two quasars on the sky, our results do not support the evidence for spatial $\alpha$ variation, especially when combined with the 21 other recent measurements which were corrected for, or resistant to, long-range distortions. Our spectra and absorption profile fits are publicly available.