Mid- and far-infrared fine-structure-line sensitivities to hypothetical variability of the fine-structure constant

Abstract

Sensitivity coefficients to temporal variation of the fine-structure constant $\ensuremath{\alpha}$ for transitions between the fine-structure (FS) sublevels of the ground states of C I, Si I, S I, Ti I, Fe I, N II, Fe II, O III, S III, Ar III, Fe III, Mg V, Ca V, Na VI, Fe VI, Mg VII, Si VII, Ca VII, Fe VII, and Si IX are calculated. These transitions lie in the mid- and far-infrared regions and can be observed in spectra of high-redshift quasars and infrared bright galaxies with active galactic nuclei. Using FS transitions to study $\ensuremath{\alpha}$ variation over cosmological time scales allows one to improve the limit on $\ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}$ by several times as compared to contemporaneous optical observations $(|\ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}|<{10}^{\ensuremath{-}5})$, and to suppress considerably systematic errors of the radial velocity measurements caused by the Doppler noise. Moreover, the far infrared lines can be observed at redshifts $z\ensuremath{\gtrsim}10$, far beyond the range accessible to optical observations $(z\ensuremath{\lesssim}4)$. We have derived a simple analytical expression which relates the FS intervals and the sensitivity of the FS transitions to the change of $\ensuremath{\alpha}$.