2014A&A...571A..32M

Numerical simulations have explored the possibility of forming molecular clouds through either a quasi-static, self-gravitating mechanism or the collision of gas streams or lower density clouds. They also quantitatively predict the distribution of matter at the transition from atomic to molecular gases. We aim to observationally test these models by studying the environment of W43, a molecular cloud complex recently identified near the tip of the Galactic long bar. Using Galaxy-wide HI and ¹²CO 1-0 surveys, we searched for gas flowing toward the W43 molecular cloud complex. We also estimated the HI and H₂ mass surface densities to constrain the transition from atomic to molecular gas around and within W43. We found three cloud ensembles within the position-velocity diagrams of ¹²CO and HI gases. They are separated by ∼20km/s along the line of sight and extend into the ¹³CO velocity structure of W43. Since their velocity gradients are consistent with free fall, they could be nearby clouds attracted by and streaming toward the W43∼10⁷M_☉ potential well. We show that the HI surface density, Σ_HI=45-85M_☉/pc², does not reach any threshold level but increases when entering the 130 pc-wide molecular complex previously defined. This suggests that an equilibrium between H₂ formation and photodissociation has not yet been reached. The H₂-to-HI ratio measured over the W43 region and its surroundings, R_H2∼3.5±₂³, is high, indicating that most of the gas is already in molecular form in W43 and in structures several hundred parsecs downstream along the Scutum-Centaurus arm. The W43 molecular cloud complex may have formed and, in fact may still be accreting mass from the agglomeration of clouds. Already in the molecular-dominated regime, most of these clouds are streaming from the Scutum-Centaurus arm. This clearly disagrees with quasi-static and steady-state models of molecular cloud formation.