ATLASGAL — Environments of 6.7 GHz methanol masers

Using the 870-μm APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), we have identified 577 submillimetre continuum sources with masers from the methanol multibeam (MMB) survey in the region 280 < l < 20; | b | < 1.5. 94 per cent of methanol masers in the region are associated with sub-millimetre dust emission. We estimate masses for ∼450 maser-associated sources and find that methanol masers are preferentially associated with massive clumps. These clumps are centrally condensed, with envelope structures that appear to be scale-free, the mean maser position being offset from the peak column density by 0 ± 4. Assuming a Kroupa initial mass function and a star-formation efficiency of ∼30 per cent, we find that over two thirds of the clumps are likely to form clusters with masses >20 M⊙. Furthermore, almost all clumps satisfy the empirical mass-size criterion for massive star formation. Bolometric luminosities taken from the literature for ∼100 clumps range between ∼100 and 106 L⊙. This confirms the link between methanol masers and massive young stars for 90 per cent of our sample. The Galactic distribution of sources suggests that the starformation efficiency is significantly reduced in the Galactic-centre region, compared to the rest of the survey area, where it is broadly constant, and shows a significant drop in the massive star-formation rate density in the outer Galaxy. We find no enhancement in source counts towards the southern Scutum-Centaurus arm tangent at l ∼ 315, which suggests that this arm is not actively forming stars.