OH Megamasers in HI Surveys: Forecasts and a Machine Learning Approach to Separating Disks from Mergers

Abstract

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 $z_{\mathrm{OH}}\sim 2.0$, 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.

14 pages, 11 figures, accepted for publication in ApJ

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• The FAST all sky HI survey (FASHI): The first release of catalog
• The First Large Absorption Survey in HI (FLASH): I. Science Goals and Survey Design
• GOALS-JWST: Unveiling Dusty Compact Sources in the Merging Galaxy IIZw096
• LADUMA: Discovery of a luminous OH megamaser at $z>0.5$
• Radio properties of the OH megamaser galaxy IIZw 096
• A serendipitous discovery of HI-rich galaxy groups with MeerKAT
• FAST Ultra-Deep Survey (FUDS): Data Release for FUDS0
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• Testing OH Megamaser Identification Methods in HI Surveys: Updated Source-Flagging Algorithms and New Detections in ALFALFA