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 ≈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, ¹³CO 1-0, 8 μm, and 870 μm 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 ¹²CO 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 ~ 22.3 km s^-1 around V_LSR ~ 95.9 km s^-1), we identified W43 as a large (equivalent diameter ~140 pc) and coherent complex of molecular clouds that is surrounded by an atomic gas envelope (equivalent diameter ~290 pc). We measured both the total mass of this newly identified molecular complex (M_total ~ 7.1 × 10⁶ M_⊙) and the mass contained in dense 870 μm clumps (<5 pc dense cloud structures, M_clumps ~ 8.4 × 10⁵ M_⊙), 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 ~0.01 M_⊙ yr^-1 (measured from its 8 μm luminosity) to ~0.1 M_⊙ 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.

Appendix A is only available in electronic form at http://www.aanda.org