The Intracluster Plasma: probing its amount and thermal state in X‐rays and in μwaves

Rich clusters of galaxies shine in X‐rays since the large masses m of hot intracluster plasma (ICP) they contain radiate strong luminosities L_X ∝ m²T^1/2/R³ by thermal bremsstrahlung. The same ICP may be also probed in μwaves through the Sunyaev‐Zel’dovich (SZ) effect, whose intensity parameter y ∝ mT/R² is a measure of the thermal energy content. But the small groups are found to be underluminous in X‐rays when compared to clusters; they emit far less than expected from the law L_X ∝ T² that would hold if m passively scaled with the dark mass. In other words, the ICP in groups is considerably underdense relative to the cluster value m/R³ ∼ 10⁻³ cm⁻³. This behavior may be traced back to substantial energy fed back into the ICP, when the baryons in the member galaxies condense into stars then exploding as supernovae, or accrete onto central supermassive black holes energizing active galactic nuclei (AGNs). Such energy inputs deplete the ICP density in the shallower potential wells by causing thermal outflow and dynamical blowout. Here we focus on the energy feedback from AGNs and show how the latter dominantly affect the ICP amount, temperature, and distribution. We compute the gas heating and ejection caused by the AGNs and the resulting, steep correlation L_X ∝ T^4.5. By comparison with the X‐ray data we evaluate an average ∼ 5% of the full AGN output to be coupled to the ICP. In parallel, we compute the SZ depressions caused by the same density depletions. On the other hand, we show the AGN heating to yield transient SZ enhancements in galaxies and small groups. We discuss the observability of such events with present and future instruments. We link the behavior of L_X and y to the parallel effects of the AGN feedback on the gas in the host galaxy, finding that these yield the correlation $M• ∝ σ⋆4$ of the central black hole masses with the galactic velocity dispersions.