2018ApJ...869..148S

We present the results of a single dish interstellar maser survey toward ∼200 molecular clouds (MCs) in the extreme outer Galaxy (EOG) possibly lying in the Outer Scutum-Centaurus (OSC) arm. The observations resulted in the detections of two new 22.2 GHz water masers, two new 6.7 GHz class II methanol masers, and one new 4765 MHz excited-state OH maser. These new detections include one high Galactic latitude (b > 2°) methanol maser, one farthest Galactic methanol maser, and one farthest Galactic OH maser from the Sun. The velocities of the masers and the CO emission are in agreement with each other, which strongly indicates an association between the maser and the EOG cloud. The presence of the masers provides reliable evidence for ongoing massive star-forming activity in the OSC arm. The EOG clouds with detected interstellar masers are all associated with young stellar objects. Masers are primarily detected toward the brighter and more massive MCs. Because of the large distances involved, the "faint" masers with luminosity below 10^–6 L_☉ are not accessible by our current observations. The low number of masers in the EOG regions supports the scenario of decreasing metallicity and decreasing star formation rate/efficiency at large Galactocentric radii. Our new maser detections are valuable targets for future VLBI parallax and proper motion measurements.