A dynamical transition from atomic to molecular intermediate-velocity clouds

Abstract

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 (HI) and molecular hydrogen (H${}_2)$. Above a certain column density threshold, atomic hydrogen turns molecular. 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. Data from the Effelsberg-Bonn HI-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. Two IVCs show different FIR properties, despite their similarity in HI, such as narrow spectral lines and large column densities. One FIR bright IVC is associated with H${}_2$, confirmed by ${}^{12}$CO $(1\to 0)$ emission; the other IVC is FIR dim and shows no FIR excess, which indicates the absence of molecular hydrogen. 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 H${}_2$ is reduced, allowing the formation of large amounts of H${}_2$ within a few Myr.

13 pages, 14 figures, accepted for publication by A&A

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• Understanding star formation in molecular clouds IV. Column density PDFs from quiescent to massive molecular clouds
• Galaxy Interactions in Compact Groups II: abundance and kinematic anomalies in HCG 91c
• All-sky census of Galactic high-latitude molecular intermediate-velocity clouds
• The Large-scale ISM of SS433/W50 Revisited
• Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium
• Discovery of two embedded clusters with WISE in the high Galactic latitude cloud HRK 81.4-77.8
• Dust in a compact, cold, high-velocity cloud: A new approach to removing foreground emission
• High-resolution HI and CO observations of high-latitude intermediate-velocity clouds
• Far-infrared excess emission as a tracer of disk-halo interaction
• Star formation in a diffuse high-altitude cloud?