Prof. Dr. Philipp Richter
Project Draft UV-Spectroscopy
The kinematic structure of diffuse molecular clouds and the HI-to-H2 transition
The transition from the neutral to the molecular gas phase in the ISM of galaxies is a fundamentally important process that regulates the star-formation activity of galaxies and thus their overall evolution. FUV absorption spectroscopy allows us to directly study the HI-to- H2 transition in the ISM of the Milky Way and other nearby galaxies using the Lyman- and Werner band of H2 between 90 and 112 nm. While previous FUV satellites such as FUSE have substantially increased the amount of H2 absorption-line data for the local Universe, the interpretation of these data are difficult because of the relatively low spectral resolution of FUSE spectral data. In particular, one key aspect in our understanding of H2 absorbing structures, namely the internal spatial structure of H2 absorbing clouds and its role for the self-shielding process of diffuse H2 that strongly influences the HI-to-H2 transition in galaxies for a given ambient dissociating UV radiation field, is not well understood.
What is needed are H2 absorption-line measurements with very high spectral resolution to analyze systematic velocity shifts and line-profile variations between H2 lines from the rotational ground states (J=0, J=1; tracing the inner core of the absorbing structures where the gas is efficiently shielded from the UV radiation) and higher rotational levels (J>3, possibly tracing the more diffuse outer layers that absorb most of the UV radiation from outside and thus represent the most important shielding layer).
From the UV observations of a large number of sightlines towards background stars in the Milky Way and in the Magellanic Clouds we will obtain a statistically relevant sample to investigate the H2 molecular gas fraction and H2 rotational excitation in various interstellar environments. Together with the velocity information from the UV spectra this will enable us to reconstruct the 3D structure of HI-to-H2 transition regions in the ISM.
Systematic measurements of the kinematical and spatial structure of such H2 absorbers will provide crucial information that is required to realistically model the self-shielding of H2 in diffuse interstellar structures, to statistically compare the observed velocity structure in such absorbers with high-resolution MHD models of a dynamic ISM, and to characterize the HI-to-H2 transition in environments that have different metallicities and gas-to-dust ratios.
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