Abstract
The co-evolution between supermassive black holes and their environment is most directly traced by the hot atmospheres of dark matter halos. The cooling of the hot atmosphere supplies the central regions with fresh gas, igniting active galactic nuclei (AGN) with long duty cycles. Outflows from the central engine tightly couple with the surrounding gaseous medium and provide the dominant heating source preventing runaway cooling by carving cavities and driving shocks across the medium. The AGN feedback loop is a key feature of all modern galaxy evolution models. Here, we review our knowledge of the AGN feedback process in the specific context of galaxy groups. Galaxy groups are uniquely suited to constrain the mechanisms governing the cooling–heating balance. Unlike in more massive halos, the energy that is supplied by the central AGN to the hot intragroup medium can exceed the gravitational binding energy of halo gas particles. We report on the state-of-the-art in observations of the feedback phenomenon and in theoretical models of the heating-cooling balance in galaxy groups. We also describe how our knowledge of the AGN feedback process impacts galaxy evolution models and large-scale baryon distributions. Finally, we discuss how new instrumentation will answer key open questions on the topic.
Highlights
Structure formation in the Universe operates as a bottom-up process in which small halos formed at high redshift progressively merge and accrete the surrounding material to form the massive halos we see today [1]
These results suggest that group-central active galactic nuclei (AGN) have the potential to drive gas out of the group core, and perhaps out of the group altogether, unless some mechanism reduces their effectiveness in heating the gas
While we review the kinematical features in a companion review (Gastaldello et al.), here we focus on its thermodynamical impact, namely the formation of chaotic cold accretion (CCA) and related multiphase rain, a key process driving the bulk of AGN feeding and, the recurrent AGN feedback triggering
Summary
Structure formation in the Universe operates as a bottom-up process in which small halos formed at high redshift progressively merge and accrete the surrounding material to form the massive halos we see today [1]. The radiative cooling time can become much shorter than the Hubble time, even at relatively low gas densities, and the supply of gas to the SMBH can be sustained more For all of these reasons, studying the feedback loop across a wide range of halo masses is necessary for informing our theoretical models. For the purpose of this review, we define galaxy groups as galaxy concentrations with halo masses in the range 1013–1014 M and Universe 2021, 7, 142 with an X-ray bright intragroup medium (IGrM) Such masses correspond to virial temperatures of ∼0.5–2 keV. Most of the processes that are discussed in this review are relevant in the case of X-ray bright isolated elliptical galaxies and massive spirals with kT ∼ 0.3–0.5 keV Whenever it is appropriate, we will discuss halos of lower masses as well.
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