AGN feedback: the relation between black holes and host galaxies


The interplay between AGN – powered by accreting SuperMassive Black Holes (SMBH) – and their host galaxies is a topic of great interest in modern astrophysics, with AGN feedback emerging as a pivotal mechanism governing galaxy formation and evolution. AGN-driven multi-phase and multi-scale winds are thought to play a fundamental role in this process, by shaping the SMBHs/galaxy co-evolution, removing and/or heating the cold gas from the host galaxy, quenching the growth of both the SMBH and the stellar component and possibly explaining the tight SMBH-host mass relation.

The average properties of the different outflow phases are summarized in Table. Credits: Cicone et al. 2018 Nat. As. 2, 176

Schematic viewing of the different outflow scales, from accretion-disc (a), to galaxy- (b) and halo-scales (c). Panels d, e, f show the UFO of PDS 456 observed in the X-rays (Nardini et al. 2015), the molecular outflow of Mrk 231 observed in the radio and mm bands (Cicone et al. 2012; Morganti et al. 2016), and the kpc-scales outflow in NGC 1365 observed in the optical (Venturi et al. 2018). Credits: Cicone et al. 2018 Nat. As. 2, 176.

Different tracers map different phases (density and temperature/ionization state) and physical scales of these winds (see Figure and Table, from Cicone et al. 2018). As an example, red and/or blue wings in IR, mm and sub-mm emission lines are often used to trace the cold, molecular gas phase, i.e., the raw material from which stars are born, while optical emission lines such as [OIII] and Hα trace the ionized phase depositing energy from the AGN into the Interstellar Medium (ISM) and possibly affecting the cold gas reservoir.

Finally, Ultra Fast Outflows (UFOs), launched in the inner regions (tens of gravitational radii, Rg) of the SMBH accretion disk, are thought to be the first engine powering the ionized and molecular outflow seen at larger scales. UFOs are fast (v_out > 10’000 km/s) and dense winds of gas at high ionization state, so that they can be observed only in the X-rays. They carry significant amounts of mass and kinetic energy away from the central engine and release them in the host ISM. Estimating the energetics of these nuclear winds and comparing it with that of larger-scale outflows provides crucial constraints on the amount of energy that is transferred to the kiloparsec scale and into the host galaxy.

So far, the characterization of UFOs has been limited – with few exceptions – to samples of low-power AGN (i.e., Seyfert galaxies) in the local Universe. Researchers at OAS are at the forefront of the community’s effort to expand our understanding of the UFO phenomenon beyond the local Universe and in luminous Quasars, that were much more common at the peak of the Star Formation History of the Universe and may have influenced more directly galaxy evolution. One example is the SUBWAYS program, observing for the first time for an adequate amount of time in X-rays, luminous QSOs at z=0.1-0.4 (in the past 1-4 Gyr), to investigate the outflow phenomenon for the first time with a statistical approach at these luminosity and redshift ranges, and to study the UFOs relation with the winds observed at larger scales and in different gas components.