Observation and Modelling of Starburst Driven Galactic Winds: A Review in honour of John Dyson

Abstract

Starburst galaxies are generally associated with extended X-ray and radio halos, giving a clear hint of an outflow of gas and relativistic particles from the disk into the halo. The driving agents are, not surprisingly, active star forming regions, injecting hot gas and cosmic rays generated by supernova remnant and superbubble shock waves. The dynamics of the outflow and the thermal evolution of the plasma are strongly coupled, and therefore a self-consistent model is necessary for a satisfactory description. Since the plasma temperature is between one and a few million Kelvin, the halo is most conspicuous in soft X-rays. It will be shown that X-ray data obtained recently with XMM-Newton EPIC pn bear clear spectral signatures that are in strong disagreement with an isothermal halo in collisional ionization equilibrium (CIE). Instead a temperature structure in X-ray halos is observed. It will be demonstrated that a galactic wind outflow model in which the non-equilibrium ionization structure is calculated self-consistently can give a satisfactory and physical explanation for the X-ray spectral characteristics, e.g. in case of the local starburst galaxy NGC 3079. In particular, high subsolar abundances, that have been reported from X-ray observations in some cases, are shown to be artefacts of CIE spectral fit models. It is found that spectral models are strongly constrained by the presence of diagnostic lines such as OVII and OVIII as well as Fe-L line complexes. Thus a detailed spectral X-ray model will help to provide important galactic wind parameters, such as mass loss rate and velocity profile, which may also serve as valuable input for galactic outflows in the early universe.