AGN outflows & feedback

Different mechanisms can drive outflows in active galactic nuclei (AGNs), but it is often unclear which mechanism dominates, if any. To quantify the impact of AGN feedback on galaxy evolution, the driving mechanism of outflows must be better understood.

Observations and simulations show that outflows in active galactic nuclei contain gas in different phases. To understand their true impact on galaxy evolution, we advocate consistent and unbiased investigation of these multiphase winds in large active galactic nuclei samples.

Galaxy-scale outflows powered by actively accreting supermassive black holes are routinely detected, and they have been associated with both the suppression and triggering of star formation. Recent observational evidence and simulations are favouring a delayed mechanism that connects outflows and star formation.

Galaxies hosting actively accreting supermassive black holes make up roughly 10% of all galaxies in the Universe. Nevertheless, due to their immense energy output, active galactic nuclei are widely regarded as regulators of their host galaxy growth. But does observational evidence stack up?

Feedback from actively accreting supermassive black holes is thought to be important in the evolution of galaxies. Theoretical and observational results are reviewed with regard to the impact of this feedback on star formation in galaxies.

Quantifying the effect of active galactic nuclei (AGNs) on their hosts requires knowledge of their life cycle. This review on AGN archaeology summarizes the main recent findings regarding the AGN life cycle from optical and radio observations.

Virtually all massive galaxies host central black holes, the growth of which releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. However, a tiny fraction of this energy could halt star formation by heating and ejecting ambient gas; a central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies.

The star formation histories of galaxies, as encapsulated in their integrated optical spectra, depend on the mass of the black holes present at their centres.

Radiation pressure on dusty gas is the main physical mechanism that regulates the distribution of the circumnuclear material that obscures many supermassive black holes.

Star formation at a rate of more than 15 solar masses a year has been observed inside a massive outflow of gas from a nearby galaxy; this could also be happening inside other galactic outflows.

In order for quiescent galaxies to maintain their low-to-non-existent star formation, there must be a mechanism to remove or heat gas that would otherwise cool to form stars; now supermassive black hole winds that are sufficient to suppress star formation in such galaxies are reported.

Observations of an ultrafast accretion-disk wind in the X-ray spectrum of a nearby ultraluminous infrared galaxy support the theory that such winds affect the evolution of supermassive black holes and their host galaxies.

Observations confirm models of galaxy cooling in which cold clouds precipitate out of hot gas via thermal instability, and the precipitation threshold is incorporated into a theoretical framework that explains how precipitation and thermal conduction regulate star formation.

The observation of high-velocity warm molecular hydrogen in the galaxy IC 5063 supports the proposal that the powerful jets of particles launched by active galactic nuclei can both accelerate and heat the molecular outflows that influence the evolution of galaxies.

Observations at submillimetre and X-ray wavelengths show that rapid star formation was common in the host galaxies of active galactic nuclei when the Universe was 2–6 Gyr old, but that the most vigorous star formation is not observed around powerful black holes, thereby confirming a key prediction of models in which an active galactic nucleus expels the interstellar medium of its host galaxy.

Astronomical observations of a luminous galaxy that has a central, mass-accreting supermassive black hole reveal how such entities launch and propel gas through galaxies at high speeds. See Letter p.436

Galaxies contain fewer stars than predicted. The discovery of a massive galactic outflow of molecular gas in a compact galaxy, which forms stars 100 times faster than the Milky Way, may help to explain why. See Letter p.68

The discovery of stars being formed within a galactic outflow confirms that supermassive black holes can enhance the growth of their host galaxies.

Feedback from active galactic nuclei (AGNs) remains controversial despite its wide acceptance as necessary to regulate massive galaxy growth. Consequently, we held a workshop in October 2017, at Leiden’s Lorentz Center, to distinguish between the reality and myths of feedback.