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

TL;DR

This paper predicts the occurrence of OH megamasers in upcoming HI surveys and introduces a machine learning method to distinguish them from HI lines without spectroscopic redshifts.

Contribution

It provides forecasts for OHM detection rates in future surveys and develops a near- to mid-IR photometry based machine learning approach for separating OHMs from HI sources.

Findings

LADUMA may double known OHMs, with 1% contamination.

SKA surveys could have up to 7.2% OHM contamination.

Near- to mid-IR photometry with k-NN can identify 97-99% of OHMs.

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 $z_\mathrm{OH}\sim2.0$, 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.