The 6-GHz methanol multibeam maser catalogue I: Galactic Centre region, longitudes 345 to 6

Abstract

We have conducted a Galactic plane survey of methanol masers at 6668 MHz using a 7-beam receiver on the Parkes telescope. Here we present results from the first part, which provides sensitive unbiased coverage of a large region around the Galactic Centre. Details are given for 183 methanol maser sites in the longitude range 345$^{\circ}$ through the Galactic Centre to 6$^{\circ}$. Within 6$^{\circ}$ of the Centre, we found 88 maser sites, of which more than half (48) are new discoveries. The masers are confined to a narrow Galactic latitude range, indicative of many sources at the Galactic Centre distance and beyond, and confined to a thin disk population; there is no high latitude population that might be ascribed to the Galactic Bulge. Within 2$^{\circ}$ of the Galactic Centre the maser velocities all lie between -60 and +77 \kms, a range much smaller than the 540 \kms range observed in CO. Elsewhere, the maser with highest positive velocity (+107 \kms) occurs, surprisingly, near longitude 355$^{\circ}$ and is probably attributable to the Galactic bar. The maser with the most negative velocity (-127 \kms) is near longitude 346$^{\circ}$, within the longitude-velocity locus of the near side of the `3-kpc arm'. It has the most extreme velocity of a clear population of masers associated with the near and far sides of the 3-kpc arm. Closer to the Galactic Centre the maser space density is generally low, except within 0.25 kpc of the Centre itself, the `Galactic Centre Zone', where it is 50 times higher, which is hinted at by the longitude distribution, and confirmed by the unusual velocities.