On the structure of the turbulent interstellar atomic hydrogen

II. First comparison between observation and theory. Are the characteristics of molecular clouds determined early in the turbulent 2-phase atomic gas?

¹ Laboratoire de radioastronomie millimétrique, UMR 8112 du CNRS, École normale supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France e-mail: patrick.hennebelle@ens.fr
² Service d'Astrophysique, CEA/DSM/DAPNIA/SAp, CE Saclay, 91191 Gif-sur-Yvette Cedex, France e-mail: edouard.audit@cea.fr
³ Institut d'astrophysique spatiale, Université Paris-Sud, Bat. 121, 91405 Orsay, France.

Received: 28 July 2006
Accepted: 13 December 2006

Abstract

Aims.It is necessary to understand the dynamics of atomic gas to use complex modeling and to carry out detailed comparisons between theoretical models and observations.

Methods.In a companion paper, we present high resolution bidimensional numerical simulations of the interstellar atomic hydrogen. Here, we further characterize these simulations and we compare our results with various observations.

Results.We give statistics of the column density and velocity along the line of sight and show that they compare favorably with observations of high-latitude lines of sight. We compute synthetic HI spectra and qualitatively discuss the information that could be inferred if these spectra were observed. Finally, we extract CNM clouds and study their physical properties finding strong similarities with real clouds. In particular, we find that the clouds follow Larson-type relations, i.e. , where (we propose a theory which predicts in 3D) and . We also find that the distribution, , of the column density, N, of the CNM structures formed in the simulation follows which is marginally compatible with the observational result obtained by Heiles & Troland ([CITE], ApJ, 624, 773). From the mass-size relation and the mass spectrum, we derive an exponent for the column density distribution close to the value obtained in the numerical simulation.

Conclusions.We conclude that the simulations reproduce various observational features reasonably well. An important implication suggested by our results is that the “turbulence” within the cold interstellar atomic gas is mainly the result of individual long living cloudlet (confined by an external warm medium) motions rather than supersonic turbulence within nearly isothermal clouds. Another important aspect is that the CNM structures produced in the simulation present various physical characteristics that are similar to the characteristics of the molecular clouds. This raises the question as to whether the physical properties of the molecular clouds are determined at a very early stage, before the gas becomes molecular.