Identifying OH Imposters in the ALFALFA Neutral Hydrogen Survey

Suess, Katherine

Undergraduate Honors Thesis

Identifying OH Imposters in the ALFALFA Neutral Hydrogen Survey Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/undergraduate_honors_theses/5h73pw693

Abstract

OH megamasers (OHMs) are rare, luminous molecular masers that are typically observed in (ultra) luminous infrared galaxies ([U]LIRGs) 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) in the local universe, 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 (Haynes et. al 2011) detected by WISE but with uncertain optical counterparts. The optical redshifts, obtained from observations at the Apache Point Observatory 3.5m telescope, identified 127 HI optical counterparts and discovered five new OHMs. Fifty-six candidates remain ambiguous. The new OHMs are the first detected in a blind spectral line survey. The number of OHMs in ALFALFA matches predictions based on the OH luminosity function (Darling & Giovanelli 2002). Additionally, the OHMs found in a blind survey do not seem to differ from those found in previous targeted surveys. This provides validation of 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 and HI lines without expensive spectral observations. This method utilizes infrared colors and magnitudes, such as WISE (Wide Field Infrared Survey Explorer) mid-IR data. Since the fraction of OHMs found in flux-limited HI surveys is expected to increase with the redshift of the survey (Briggs 1998), this analysis can be applied to future flux-limited high-redshift hydrogen surveys.

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