Resolving the kinematic distance ambiguity of southern massive young stellar object candidates

We investigate the use of H i data to resolve the near/far ambiguity in kinematic distances of massive young stellar object (MYSO) candidates. Kinematic distances were obtained from 13CO 1-0 (and N2H+) spectral-line observations with the Mopra Telescope towards 94 candidates selected from the Red MSX Source (RMS) survey in the fourth Galactic quadrant (282° < l < 350°). H i data from the Southern Galactic Plane Survey (SGPS) were used in conjunction with the H i self-absorption (SA) technique to determine the near or far distance. We resolved the kinematic distance ambiguity to 70 per cent of the sources. We can also simultaneously solve for any multiple line-of-sight component sources. We discuss the advantages and disadvantages of this technique in comparison with other methods, and also perform confidence checks on the reliability of using the H i SA technique. We examined the projected location of these sources in both the Galactic plane and longitude–velocity (l–v) diagrams to ascertain any recognizable spiral arm pattern. Although no obvious spiral pattern was found when compared to that proposed by Cordes and Lazio, far distance sources tended to lie on or near spiral arm loci. Near distance sources, however, had peculiar velocity uncertainties consistent with the separation between the spiral arms themselves. The l–v plot shows a more consistent picture, with tangent points of the spiral arm loci easily seen. We conclude that using the H i SA technique to determine kinematic distance ambiguities is a quick and reliable method in most cases, with an 80 per cent success rate in determining the correct designation of whether an object is at the near or far distance.