The formation of the W43 complex: constraining its atomic-to-molecular transition and searching for colliding clouds

Context. 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.
Aims: 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.
Methods: Using Galaxy-wide H i and ¹²CO 1-0 surveys, we searched for gas flowing toward the W43 molecular cloud complex. We also estimated the H i and H₂ mass surface densities to constrain the transition from atomic to molecular gas around and within W43.
Results: We found three cloud ensembles within the position-velocity diagrams of ¹²CO and H i gases. They are separated by ~20 km s^-1 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 ~10⁷ M_⊙ potential well. We show that the H i surface density, Σ_{H i} = 45-85 M_⊙pc^-2, 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-H i 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.
Conclusions: 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.