Positive feedback from AGN
We have seen that negative feedback from AGN helps account for the black hole mass-а correlation and for the luminosity function of massive galaxies. At the same
Fig. 9.24 (a) Midplane density slices of the evolution of a 1044 ultrafast outflow for a two-phase ISM with spherically distributed clouds. (b) Same as (a), but for an ISM with disk-distributed clouds. From Wagner et al. (2013). For the figure in color, please see the online version of the lectures.
time, AGN activity could result in positive feedback on the star formation rate (Silk and Norman, 2009; Silk, 2013). A phase of positive feedback is motivated by evidence for AGN triggering of star formation (Feain et al., 2007; Zinn et al., 2013), discussed further below.
AGN outflows can trigger star formation by compressing dense clouds. Propagation of jets into a clumpy interstellar medium will lead to the formation of an expanding, over-pressurized cocoon at vco, which is much larger than the velocity field associated with the gravitational potential well. Therefore, protogalactic clouds that are above the Jeans, or the more appropriate Bonnor-Ebert, mass may be induced to collapse.
The region where AGN feedback can be positive is determined by the condition that the AGN-induced pressure exceeds the dynamical pressure that controls the ambient ISM. A key ingredient in star formation is molecular hydrogen. The molecular hydrogen fraction correlates with interstellar pressure in nearby star-forming galaxies (Blitz and Rosolowsky, 2006). Enhanced pressure from AGN is likely to accelerate molecular cloud formation and thereby star formation.
If one replaces the gas pressure pg by the AGN-driven pressure pagn , then the AGN-driven-star-formation-enhancement factor is (pAGN/pg)1/2 ~ (vco/a)r1/2, where т is the optical depth. Since eSN ~ &, the fraction of stars formed per dynamical time is boosted for spheroids relative to disks. Numerical simulations (Gaibler et al., 2012) of the interaction of a powerful AGN jet with the massive gaseous disk of a high-redshift galaxy demonstrate that such enhanced AGN-driven pressure from jets is effectively able to compress the disk gas and to enhance star formation, as shown in Fig. 9.25.