A More Efficient Search for H2O Megamaser Galaxies: The Power of X-Ray and Mid-infrared Photometry

Abstract

Abstract We present an investigation of the dependence of H2O maser detection rates and properties on the mid-IR active galactic nucleus (AGN) luminosity, L AGN, and the obscuring column density, N H, based on mid-IR and hard X-ray photometry. Based on spectral energy distribution fitting that allows for decomposition of the black hole accretion and star formation components in the mid-infrared, we show that the megamaser (disk maser) detection rate increases sharply for galaxies with 12 μm AGN luminosity greater than 1042 erg s−1, from ≲3% (≲2%) to ∼12% (∼5%). By using the ratio of the observed X-ray to mid-IR AGN luminosity as an indicator of N H, we also find that megamaser (disk maser) detection rates are boosted to 15% (7%) and 20% (9%) for galaxies with N H ≥ 1023 cm−2 and N H ≥ 1024 cm−2, respectively. Combining these column density cuts with a constraint for high (≥1042 erg s−1) predicts further increases in the megamaser (disk maser) detection rates to 19% (8%) and 27% (14%), revealing unprecedented potential increases of the megamaser and disk maser detection rates by a factor of 7–15 relative to the current rates, depending on the chosen sample selection criteria. A noteworthy aspect of these new predictions is that the completeness rates are only compromised mildly, with the rates remaining at the level of ∼95%(∼50%) for sources with N H ≥ 1023 cm−2 (N H ≥ 1024 cm−2). Applying these selection methods to current X-ray AGN surveys predicts the detection of ≳15 new megamaser disks.