W43: the closest molecular complex of the Galactic Bar?

Context. In the framework of the multi-wavelength Galactic surveys of star-formation presently underway, complexes of molecular clouds that stretch over up to hundreds of parsecs are of particular interest. This is because a large population of stars is forming within them, thus all at the same distance from the Sun and under similar physical conditions. Aims. We study the Galactic plane between approximate to 29.5 and 31.5 degrees of longitude, which is especially rich in terms of molecular clouds and star-formation activity. It is located within what is sometimes called the molecular ring and contains the Galactic mini-starburst region W43, as well as the prominent hot core G29.96-0.02 with its associated compact HII region. Methods. We used a large database extracted from Galaxy-wide surveys of HI, (CO)-C-13 1-0, 8 mu m, and 870 mu m continuum to trace diffuse atomic gas, low-to medium-density molecular gas, high-density molecular gas, and star-formation activity, which we complemented by dedicated (CO)-C-12 2-1, 3-2 observations of the region. Results. From the detailed 3D (space-space-velocity) analysis of the molecular and atomic cloud tracers through the region and despite its wide velocity range (FWHM similar to 22.3 km s(-1) around V-LSR similar to 95.9 km s(-1)), we identified W43 as a large (equivalent diameter similar to 140 pc) and coherent complex of molecular clouds that is surrounded by an atomic gas envelope (equivalent diameter similar to 290 pc). We measured both the total mass of this newly identified molecular complex (M-total similar to 7.1 x 10(6) M-circle dot) and the mass contained in dense 870 mu m clumps (<5 pc dense cloud structures, M-clumps similar to 8.4 x 10(5) M-circle dot), and conclude that W43 is particularly massive and concentrated. The distance that we assume for the W43 complex is 6 kpc from the Sun, which may place it at the meeting point of the Scutum-Centaurus (or Scutum-Crux) Galactic arm and the bar, a dynamically complex region where high-velocity streams could easily collide. We propose that the star-formation rate of W43 is not steady but increases from similar to 0.01 M-circle dot yr(-1) (measured from its 8 mu m luminosity) to similar to 0.1 M-circle dot yr(-1) (measured from its molecular content). From the global properties of W43, we claim that it is an extreme molecular complex in the Milky Way and it might even form starburst clusters in the near future. Conclusions. W43 is the perfect testbed to investigate (1) the star-formation process occuring through bursts as well as (2) the formation of such an extreme complex in the framework of converging flows scenarios.