Identifying OH Imposters in the ALFALFA Neutral Hydrogen Survey
  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.
