A survey of the galactic plane for 6.7-GHz methanol masers - I. l=325 deg-335 deg b=-0.˚53-0.˚53

Abstract

We report the results of the first complete survey of an area of the Galactic plane for maser emission from the 6.7-GHz 5_1--〉6_0 A^+ transition of CH_3OH. The survey covers a 10.6-deg^2 region of the Galactic plane in the longitude range 325 deg-335 deg and latitude range -0.˚53-0.˚53. The survey is sensitive to masers with a peak flux density greater than ~2.6 Jy. The weakest maser detected has a peak flux density of 2.3 Jy, and the strongest a peak flux density of 425 Jy. We detected a total of 50 distinct masers, 26 of which are new detections. We show that many 6.7-GHz CH_3OH masers are not associated with IRAS sources, and that some are associated with sources that have colours differing from those of a typical ultracompact HII (UCHII) region. We estimate that the number of UCHII regions in the Galaxy is significantly more than suggested by IRAS-based estimates, possibly by more than a factor of 2.