Researchers from the University of Durham, including Samuel Ruthven Ward, Tiago Costa, Chris M. Harrison, and Vincenzo Mainieri, have published a study in the Monthly Notices of the Royal Astronomical Society that sheds light on the interaction between active galactic nuclei (AGN) outflows and the interstellar medium (ISM) in galaxies. Their findings have implications for understanding galaxy evolution and the role of AGN in heating their host galaxies.
Active galactic nuclei are known to drive powerful outflows that can influence the evolution of their host galaxies. These outflows consist of multiple phases, including a hot, shocked phase and a cooler phase. The hot phase, with temperatures above 10^6 K, is thought to dominate the energy content but is difficult to observe due to its long cooling time and low emissivity. The cooler phase, below 10^4 K, is easier to detect but represents a less energetic component of the outflow.
The researchers conducted simulations to investigate the interaction between an energy-driven AGN outflow and a clumpy ISM. They found that mixing between the hot wind and cool ISM clouds produces a new, highly radiative phase at temperatures around 10^6-7 K. This phase fuels the formation of a long-lived cool outflow and generates significant X-ray emission through thermal Bremsstrahlung and high-ionization metal line emission.
The X-ray emission generated by this mixing is strongest in the part of the outflow propagating equatorially through the disc and is extended on scales of about 3-4 kiloparsecs. For quasar luminosities of 10^45-46 erg/s, the resulting X-ray luminosity could be equivalent to that expected from star formation rates of 10-200 solar masses per year. This suggests that the mixing-induced X-ray emission could be an important source of soft X-rays in AGN host galaxies.
The researchers suggest that this extended emission could be resolvable in local quasars (z < 0.11) using high spatial-resolution X-ray observatories such as Chandra, or proposed missions like AXIS and Lynx. This research provides new insights into the role of AGN outflows in galaxy evolution and highlights the potential for future X-ray observations to further our understanding of these processes. The study, titled "Mixing between AGN winds and ISM clouds produces luminous X-ray emission," was published in the Monthly Notices of the Royal Astronomical Society. This article is based on research available at arXiv.