Identifying OH Imposters in the ALFALFA Neutral Hydrogen Survey

TL;DR

This paper reports the first detection of OH megamasers in a blind spectral line survey, validating infrared-based methods to distinguish them from neutral hydrogen signals without optical spectroscopy, and discusses implications for future high-redshift surveys.

Contribution

It introduces a novel optical spectroscopy approach to identify OH megamasers in blind HI surveys and validates infrared color criteria for future detection without optical data.

Findings

Detected five new OH megamasers in the ALFALFA survey.

Confirmed the consistency of OHM occurrence with luminosity function predictions.

Validated infrared color criteria for OHM identification in blind surveys.

Abstract

OH megamasers (OHMs) are rare, luminous molecular masers that are typically observed in (ultra) luminous infrared galaxies and serve as markers of major galaxy mergers. In blind emission line surveys such as the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFALFA) survey for neutral hydrogen (HI), OHMs at z~0.2 can mimic z~0.05 HI lines. We present the results of optical spectroscopy of ambiguous HI detections in the ALFALFA 40% data release detected by the Wide Field Infrared Survey Explorer (WISE) but with uncertain optical counterparts. The optical redshifts, obtained from observations at the Apache Point Observatory, revealed five new OHMs and identified 129 HI optical counterparts. Sixty candidates remain ambiguous. The new OHMs are the first detected in a blind spectral line survey. The number of OHMs in ALFALFA is consistent with predictions from the OH luminosity function. Additionally, the mid-infrared magnitudes and colors of the OHM host galaxies found in a blind survey do not seem to differ from those found in previous targeted surveys. This validates the methods used in previous IR-selected OHM surveys and indicates there is no previously unknown OHM-producing population at z~0.2. We also provide a method for future surveys to separate OH megamasers from 99% of HI line emitters without optical spectroscopy by using WISE infrared colors and magnitudes. Since the fraction of OHMs found in flux-limited HI surveys is expected to increase with the survey's redshift, this selection method can be applied to future flux-limited high-redshift hydrogen surveys.