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Recent numerical models of the multiphase ISM underline the importance of cosmic rays and magnetic fields for the physics of the ISM in disc galaxies. Observations of properties of the ISM in galactic halos constrain models of the expected exchange of matter between the star-forming disc and the environment (circumgalactic medium, CGM). We present new observational evidence from radio-continuum polarization studies of edge-on galaxies on magnetic field strength and structure as well as cosmic ray electron transport in galactic halos. The findings are discussed in the context of the disk-halo interaction of the interstellar medium. In addition, it is also briefly demonstrated how recent LOFAR observations of edge-on galaxies further constrain the extent of magnetic fields in galactic halos.
The interstellar medium in galactic halos is described as a consequence of feedback mechanisms from processes related to star-formation in the disk. The presence of gas in galactic halos is also expected due to accretion of gas from the circumgalactic environment. The observational evidence for gas in galactic halos - from the hot X-ray emitting coronal phase to cool molecular gas and dust - is reviewed and discussed in the context of current models of the ISM and the “infall vs. outflow“ debate.
Over the last couple of years Diffuse Ionized Gas (DIG) has been identified as an important constituent of the interstellar medium (ISM) in the halos of spiral galaxies. Imaging in and spectroscopy of optical emission lines allow us to study the distribution and excitation of this gas with a spatial resolution not achievable for other phases of the ISM in external galaxies. Its origin and ionization is under debate and give important constraints for models of the ISM in general and on the large scale exchange of matter between disk and halo in particular. This review summarizes more recent observational results and compares them with model predictions. The data available now demonstrate that the presence of DIG in the disk-halo interface of spiral galaxies is related to star formation processes in the underlying disk. While photoionization by OB stars in the disk seems a viable source for the power required to ionize the DIG, additional processes are needed to explain some of the spectral features. The observed correlation with properties of the non-thermal radio continuum indicate that magnetic fields and cosmic rays could play a role for the physics of this medium.
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