OH Megamasers in H i Surveys: Forecasts and a Machine-learning Approach to Separating Disks from Mergers.

OH megamasers (OHMs) are rare, luminous masers found in gas-rich major galaxy mergers. In untargeted neutral hydrogen (H i) emission-line surveys, spectroscopic redshifts are necessary to differentiate the λrest = 18 cm masing lines produced by OHMs from H i 21 cm lines. Next-generation H i surveys will detect an unprecedented number of galaxies, most of which will not have spectroscopic redshifts. We present predictions for the numbers of OHMs that will be detected and the potential "contamination" they will impose on H i surveys. We examine the Looking at the Distant Universe with the MeerKAT Array (LADUMA), a single-pointing deep-field survey reaching redshift zH I = 1.45, as well as potential future surveys with the Square Kilometre Array (SKA) that would observe large portions of the sky out to redshift zH I = 1.37. We predict that LADUMA will potentially double the number of known OHMs, creating an expected contamination of 1.0% of the survey's H i sample. Future SKA H i surveys are expected to see up to 7.2% OH contamination. To mitigate this contamination, we present methods to distinguish H i and OHM host populations without spectroscopic redshifts using near- to mid-IR photometry and a k-Nearest Neighbors algorithm. Using our methods, nearly 99% of OHMs out to redshift zOH ∼ 1.0 can be correctly identified. At redshifts out to zOH ∼ 2.0, 97% of OHMs can be identified. The discovery of these high-redshift OHMs will be valuable for understanding the connection between extreme star formation and galaxy evolution. [ABSTRACT FROM AUTHOR]