Volume 564, April 2014
|Number of page(s)||13|
|Section||Interstellar and circumstellar matter|
|Published online||08 April 2014|
A dynamical transition from atomic to molecular intermediate-velocity clouds⋆
1 Argelander-Institut für Astronomie (AIfA), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
2 Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
3 Institut d’Astrophysique Spatiale (IAS), Université Paris-Sud XI, 91405 Orsay, France
Received: 22 August 2013
Accepted: 5 February 2014
Context. Towards the high galactic latitude sky, the far-infrared (FIR) intensity is tightly correlated to the total hydrogen column density which is made up of atomic (H i) and molecular hydrogen (H2). Above a certain column density threshold, atomic hydrogen turns molecular.
Aims. We analyse gas and dust properties of intermediate-velocity clouds (IVCs) in the lower galactic halo to explore their transition from the atomic to the molecular phase. Driven by observations, we investigate the physical processes that transform a purely atomic IVC into a molecular one.
Methods. Data from the Effelsberg-Bonn H i-Survey (EBHIS) are correlated to FIR wavebands of the Planck satellite and IRIS. Modified black-body emission spectra are fitted to deduce dust optical depths and grain temperatures. We remove the contribution of atomic hydrogen to the FIR intensity to estimate molecular hydrogen column densities.
Results. Two IVCs show different FIR properties, despite their similarity in H i, such as narrow spectral lines and large column densities. One FIR bright IVC is associated with H2, confirmed by 12CO (1 → 0) emission; the other IVC is FIR dim and shows no FIR excess, which indicates the absence of molecular hydrogen.
Conclusions. We propose that the FIR dim and bright IVCs probe the transition between the atomic and molecular gas phase. Triggered by dynamical processes, this transition happens during the descent of IVCs onto the galactic disk. The most natural driver is ram pressure exerted onto the cloud by the increasing halo density. Because of the enhanced pressure, the formation timescale of H2 is reduced, allowing the formation of large amounts of H2 within a few Myr.
Key words: ISM: clouds / ISM: molecules / infrared: ISM / Galaxy: halo
Based on observations obtained with Planck (http://www.esa.int/Planck), an ESA science mission with instruments and contributions directly funded by ESA Member States, NASA, and Canada.
© ESO, 2014
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