Dust and gas in the MilkyWay: a full-sky view on the multiphase interstellar medium

Abstract

The Milky Way (MW) Galaxy is a unique laboratory to study the evolution of spiral galaxies, offering great sensitivities and angular resolutions with respect to observations of other galaxies. There, the large distance and consequently low linear resolution makes it impossible to study the accretion of matter and gas phase transitions through observations of individual clouds. The role of these high- and intermediate-velocity clouds (IVCs and HVCs), located in the Galactic halo, is still not fully understood. By using data of atomic neutral hydrogen (HI) and far-infrared (FIR) emission from dust grains, it is possible to probe a large range of densities, temperatures and gas phases. Here, the newly released data of the Effelsberg-Bonn HI Survey (EBHIS) and the FIR data from the Planck satellite allow to take this analysis to a new level in terms of sensitivity, spectral coverage, and angular resolution.<br /> Classically, it is believed that HVCs originate from beyond the Galactic disk, either from the Galactic halo that surrounds the disk or from structures located beyond the MW. IVCs on the other hand are thought to be related to Galactic fountain processes in which supernovae eject gas into the halo where it eventually cools and returns to the disk.<br /> We present evidence for the impact of a HVC onto a well studied IVC located close to the disk. The correlation of dust and gas allows us to infer the distribution and column density of molecular hydrogen. At the impact region, increased pressure reduces the formation time of molecular hydrogen. The morphology and velocity structure of the IVC/HVC system, combined with the dusty and molecular content of the IVC, points towards a physical connection between the two clouds.<br /> To further investigate the properties of HVCs and their role in MW evolution, we conduct a detailed analysis of a very cold, bright, and clumpy HVC that is located at high Galactic latitude. These properties make this particular cloud one of the best candidates to detect FIR emission from interstellar dust. Owing to their large distance from the interstellar radiation field and their low metallicity, there have been only tentative detections of dust in very few HVCs. Here, the biggest challenge is to remove the foreground dust emission associated with the MW in order to detect the very faint signal from the HVC. We present a new approach to model the FIR foreground, demonstrate its advantages over previous methods, and verify its accuracy and robustness via simulations. Despite these advancements in cleaning the map from foreground emission, we find that no dust emission is present in that HVC. For this type of analysis, the fluctuations from the cosmic infrared background (CIB) are the limiting factor.<br /> The work on dust in HVCs is the foundation for the final chapter of this thesis in which we present the first results on large-scale studies of the CIB. This background radiation is dominated by galaxies at redshifts between z=1..3. Up to now, CIB maps were not corrected for foreground emission at all or only by a simple model that required manual inspection. Using the full spectral information of the HI data and modern methods of machine learning, we present a fully-automated approach to remove galactic foreground emission from CIB data for a large (20%) fraction of the sky. We compare our results on the CIB with previous studies to validate our approach. Finally, we compute the deconvolved, binned, and unbiased angular power spectrum that is used to compare the data to cosmological simulations. For the first time, fully cleaned images of the CIB on large scales and their power spectra are available to study phenomena such as the star-formation history and the connection between luminous and dark matter at large scales. We thereby improve previous constraints on the CIB at the largest scales and further expand the angular range that is probed. This allows future cosmological models to be probed to a new level and with greater accuracy.