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Distances to southern 6.7-GHz methanol masers through H i self-absorption




We present the kinematic distance ambiguity resolutions of over 400 6.7-GHz methanol masers. Using the data sets of the International Galactic Plane Survey we have extracted H i spectra towards the sites of methanol maser emission between longitudes 270° and 67° and examined them for evidence of H i self-absorption. Using well-established criteria we identify 442 resolutions (classified by reliability) with a 1.07:1.11 ratio between near and far kinematic distance assignments. For a sub-sample of 118, we compare our resolutions with those of the literature and find 85 ± 10 per cent agreement. Using the distances we examine the Galactocentric distribution and properties of the masers. We also compare 6.7-GHz maser velocities with those of CS, finding the maser mid-velocity is closely correlated with the CS velocity, typically to within 2.8 km s−1.