The formation of the W43 complex: constraining its atomic-to-molecular transition and searching for colliding clouds
Laboratoire AIM Paris-Saclay, CEA/IRFU – CNRS/INSU – Université Paris
Diderot, Service d’Astrophysique, Bât. 709, CEA-Saclay,
2 Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
3 OASU/LAB-UMR 5804, CNRS/INSU – Université Bordeaux 1, 2 rue de l’Observatoire, BP 89, 33270 Floirac, France
4 Department of Physics and Astronomy, University of North Carolina Chapel Hill, Phillips Hall, Chapel Hill, NC 27599-3255, USA
5 Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
6 Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
7 Joint ALMA Observatory, 3107 Alonso de Cordova, Vitacura, Santiago, Chile
8 Max-Planck-Institut für Astronomie, Königsstuhl 17, 69117 Heidelberg, Germany
Accepted: 17 April 2014
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, ΣH 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, RH2 ~ 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.
Key words: ISM: clouds / ISM: structure / stars: formation / stars: massive / ISM: atoms / evolution
© ESO, 2014