H I-to-H2 Transition Layers in the Star-forming Region W43

Shmuel Bialy, Simon Bihr, Henrik Beuther, Thomas Henning, Amiel Sternberg

Research output: Contribution to journalArticlepeer-review


The process of atomic-to-molecular (H i-to-H2) gas conversion is fundamental for molecular-cloud formation and star formation. 21 cm observations of the star-forming region W43 revealed extremely high H i column densities, of 120-180 , a factor of 10-20 larger than predicted by H i-to-H2 transition theories. We analyze the observed H i with a theoretical model of the H i-to-H2 transition, and show that the discrepancy between theory and observation cannot be explained by the intense radiation in W43, nor be explained by variations of the assumed volume density or H2 formation rate coefficient. We show that the large observed H i columns are naturally explained by several (9-22) H i-to-H2 transition layers, superimposed along the sightlines of W43. We discuss other possible interpretations such as a non-steady-state scenario and inefficient dust absorption. The case of W43 suggests that H i thresholds reported in extragalactic observations are probably not associated with a single H i-to-H2 transition, but are rather a result of several transition layers (clouds) along the sightlines, beam-diluted with diffuse intercloud gas.

Original languageEnglish
Article number126
JournalAstrophysical Journal
Issue number2
StatePublished - 1 Feb 2017


  • ISM: clouds
  • ISM: individual objects (W43)
  • ISM: structure
  • galaxies: star formation
  • photondominated region (PDR)


Dive into the research topics of 'H I-to-H2 Transition Layers in the Star-forming Region W43'. Together they form a unique fingerprint.

Cite this