Water Masers in the Andromeda Galaxy: I. A Survey for Water Masers, Ammonia, and Hydrogen Recombination Lines

TL;DR

This survey of 506 luminous regions in M31 found no new water masers, ammonia, or recombination lines, suggesting such masers are rare or weak in M31, with implications for future astrometric studies.

Contribution

First comprehensive survey of water masers, ammonia, and recombination lines in M31, providing constraints on their prevalence and variability.

Findings

No new water masers detected in M31.

Spectral stacks showed no significant ammonia or recombination lines.

Results align with Galactic extrapolations, indicating rarity of bright masers in M31.

Abstract

We report the results of a Green Bank Telescope survey for water masers, ammonia (1,1) and (2,2), and the H66-alpha recombination line toward 506 luminous compact 24 micron-emitting regions in the Andromeda Galaxy (M31). We include the 206 sources observed in the Darling (2011) water maser survey for completeness. The survey was sensitive enough to detect any maser useful for ~10 microarcsecond/yr astrometry. No new water masers, ammonia lines, or H66-alpha recombination lines were detected individually or in spectral stacks reaching rms noise levels of ~3 mJy and ~0.2 mJy, respectively, in 3.1-3.3 km/s channels. The lack of detections in individual spectra and in the spectral stacks is consistent with Galactic extrapolations. Contrary to previous assertions, there do not seem to be additional bright water masers to be found in M31. The strong variability of water masers may enable new maser detections in the future, but variability may also limit the astrometric utility of known (or future) masers since flaring masers must also fade.