Efficient selection of gravitationally lensed OH megamasers with MeerKAT and the Square Kilometre Array

Abstract: There has been a recent resurgence in hydroxyl (OH) megamaser research driven by Square Kilometre Array (SKA) precursor/pathfinder telescopes. This will continue in the lead-up to the SKA mid-frequency array, which will greatly expand our view of OH megamasers and their cosmic evolution over $\gtrsim80$ per cent of the age of the universe. This is expected to yield large scientific returns as OH megamasers trace galaxy mergers, extreme star formation, high molecular gas densities, and potentially binary/dual supermassive black hole systems. In this paper, we predict the distortion to the OH luminosity function that a magnification bias will inflict, and in turn, predict the distortion on the OH megamaser number counts as a function of redshift. We identify spectral flux density thresholds that will enable efficient lensed OH megamaser selection in large spectral line surveys with MeerKAT and SKA. The surface density of lensed galaxies that could be discovered in this way is a strong function of the redshift evolution of the OH megamaser luminosity function, with predictions as high as $\sim$1 lensed OH source per square degree at high redshifts ($z \gtrsim 1$) for anticipated SKA spectral line survey designs. This could enable efficient selection of some of the most highly-obscured galaxies in the universe. This high-redshift selection efficiency, in combination with the large survey speed of the SKA at $\lesssim$1 GHz frequencies and the high magnifications possible with compact OH emission regions ($\mu_{\rm OH} \gg 10$), will enable a transformational view of OH in the universe.