The 6-GHz multibeam maser survey – II. Statistical analysis and Galactic distribution of 6668-MHz methanol masers

Abstract

The Methanol Multibeam survey has produced the largest and most complete census of
\nmethanol and excited-state hydroxyl masers in the Galaxy to date. Observing the entire
\nGalactic plane visible from the Southern hemisphere for 6668-MHz methanol and 6030/6035-
\nMHz hydroxyl masers, to an rms sensitivity of 0.015 Jy km s−1, the survey has detected a
\ntotal of 972 methanol maser sources, implying a total Galactic population of ∼1290 sources
\nwith flux densities above the survey 3σ peak flux density limit of 0.51 Jy. We present here
\nthe statistical properties of the methanol detections of the survey, including distributions in
\nflux density, variability and range of source velocities. The data suggest that the weaker
\nmasers exhibit greater variability. We also present an analysis of the Galactic distribution
\nof 6668-MHz methanol masers. For the Galactic distribution, we present kinematic distance
\nresolutions to an additional 202 sources to those published previously, and collate these with
\nprevious allocations, as well as exploring a recent Bayesian distance approach based on maser
\nparallaxes to separately determine distances. We examine Galactic structure and determine
\nthe luminosity function of the Galactic population of methanol masers. We find that more
\nluminous masers have an evenly distributed wide range of velocity widths compared with less
\nluminous masers being dominated by narrow velocity ranges, with the implication that this
\nmay be tied to the evolution of the host protostar(s). We also see an indication that brighter
\nsources are seen towards the arm origins.