The Detection of H$_2$O Maser Emission from mid-IR Red Galaxies

TL;DR

This study reports the detection of water maser emission in a subset of mid-infrared red galaxies, highlighting the effectiveness of specific IR color criteria for identifying potential maser-hosting AGNs.

Contribution

It introduces refined mid-IR color selection criteria that significantly improve the detection rate of water masers in Type-2 AGNs.

Findings

Detected H$_2$O masers in 4 of 77 mid-IR red galaxies.

Achieved a maser detection rate of 13-18% among optically classified Type-2 AGNs.

IR color criteria are the most effective to date for maser searches.

Abstract

We report the detection of H$_2$O maser emission in 4 out of 77 (5.2%) mid-IR red galaxies that meet the color criteria of $W1-W2 > 0.5$ and $W1-W4 > 7$ and are classified as Type-2 AGNs based on optical, near-IR, and mid-IR spectral energy distribution (SED) fitting. Here, $W1$, $W2$, and $W4$ represent the IR magnitudes at 3.4, 4.6, and 22 micron, respectively, as measured by the Wide-field Infrared Survey Explorer. Three of the four newly identified maser galaxies are classified as either Seyfert 2 or LINER systems, but none are disk maser systems. Our analysis indicates that AGN identifications based solely on SED fitting are unreliable, resulting in an unexpectedly low detection rate. By restricting our sample to optically classified Type 2 AGNs that satisfy the mid-IR color criteria, we achieve a maser detection rate of ~13-18%, aligning with previous predictions for mid-IR red sources. These selection criteria are the most effective to date for facilitating new maser detections, particularly in light of the recent identification of additional Type 2 AGNs identified from ongoing galaxy and AGN surveys.