2013A&A...560A..24C

We aim to fully describe the distribution and location of dense molecular clouds in the giant molecular cloud complex W43. It was previously identified as one of the most massive star-forming regions in our Galaxy. To trace the moderately dense molecular clouds in the W43 region, we initiated W43-HERO, a large program using the IRAM30 m telescope, which covers a wide dynamic range of scales from 0.3 to 140 pc. We obtained on-the-fly-maps in ¹³CO (2-1) and C¹⁸O (2-1) with a high spectral resolution of 0.1 km/s and a spatial resolution of 12". These maps cover an area of ∼1.5 square degrees and include the two main clouds of W43 and the lower density gas surrounding them. A comparison to Galactic models and previous distance calculations confirms the location of W43 near the tangential point of the Scutum arm at approximately 6 kpc from the Sun. The resulting intensity cubes of the observed region are separated into subcubes, which are centered on single clouds and then analyzed in detail. The optical depth, excitation temperature, and H₂ column density maps are derived out of the ¹³CO and C¹⁸O data. These results are then compared to those derived from Herschel dust maps. The mass of a typical cloud is several 10⁴M_☉ while the total mass in the dense molecular gas (>10²cm^–3) in W43 is found to be ∼ 1.9x10⁶M_☉. Probability distribution functions obtained from column density maps derived from molecular line data and Herschel imaging show a log-normal distribution for low column densities and a power-law tail for high densities. A flatter slope for the molecular line data probability distribution function may imply that those selectively show the gravitationally collapsing gas.