OH Megamasers in HI 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 ($\mathrm{HI}$) emission-line surveys,
spectroscopic redshifts are necessary to differentiate the
$\lambda_\text{rest}=18$ cm masing lines produced by OHMs from $\mathrm{HI}$ 21
cm lines. Next generation $\mathrm{HI}$ 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 $\mathrm{HI}$ surveys. We examine
Looking at the Distant Universe with the MeerKAT Array (LADUMA), a
single-pointing deep-field survey reaching redshift $z_\mathrm{HI}=1.45$, as
well as potential future surveys with the Square Kilometre Array (SKA) that
would observe large portions of the sky out to redshift $z_\mathrm{HI}=1.37$.
We predict that LADUMA will potentially double the number of known OHMs,
creating an expected contamination of 1.0% of the survey's $\mathrm{HI}$
sample. Future SKA $\mathrm{HI}$ surveys are expected to see up to 7.2% OH
contamination. To mitigate this contamination, we present methods to
distinguish $\mathrm{HI}$ 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 $z_\mathrm{OH} \sim 1.0$
can be correctly identified. At redshifts out to $z_\mathrm{OH}\sim2.0$, 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.
