Probing Fundamental Constant Evolution With Redshifted Conjugate-satellite OH Lines

Abstract

We report Westerbork Synthesis Radio Telescope and Arecibo Telescope observations of the redshifted satellite OH-18cm lines at z∼0.247 towards PKS1413+135. The "conjugate" nature of these lines, with one line in emission and the other in absorption, but with the same shape, implies that the lines arise in the same gas. The satellite OH-18cm line frequencies also have different dependences on the fine structure constant α, the proton-electron mass ratio μ=mp/me, and the proton gyromagnetic ratio gp. Comparisons between the satellite line redshifts in conjugate systems can hence be used to probe changes in α, μ, and gp, with few systematic effects. The technique yields the expected null result when applied to Cen.A, a nearby conjugate satellite system. For the z∼0.247 system towards PKS1413+135, we find, on combining results from the two telescopes, that [ΔG/G]=(−1.18±0.46)×10−5 (weighted mean), where G=gp[μα2]1.85; this is tentative evidence (with 2.6σ significance, or at 99.1% confidence) for a smaller value of α, μ, and/or gp at z~0.247, i.e. at a lookback time of ~2.9 Gyrs. If we assume that the dominant change is in α, this implies [Δα/α]=(−3.1±1.2)×10−6. We find no evidence that the observed offset might be produced by systematic effects, either due to observational or analysis procedures, or local conditions in the molecular cloud.