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Register a new userReality and Myths of AGN Feedback
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Authors
Bernd Husemann
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Feedback from active galactic nuclei (AGN) remains controversial despite its wide acceptance as necessary to regulate massive galaxy growth. A dedicated workshop was held on 16-20 October 2017 at the Lorentz Center in Leiden to distinguish between the reality and myths of AGN feedback from the observational side. Here, we summarize briefly all the sessions and outcome of the stimulating workshop. More details on the outcome of the discussions are provided in a series of articles.
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We perform adaptive mesh refinement (AMR) and smoothed particle hydrodynamics (SPH) cosmological zoom simulations of a region around a forming galaxy cluster, comparing the ability of the methods to handle successively more complex baryonic physics. In the simplest, non-radiative case, the two methods are in good agreement with each other, but the SPH simulations generate central cores with slightly lower entropies and virial shocks at slightly larger radii, consistent with what has been seen in previous studies. The inclusion of radiative cooling, star formation, and stellar feedback leads to much larger differences between the two methods. Most dramatically, at z=5, rapid cooling in the AMR case moves the accretion shock well within the virial radius, while this shock remains near the virial radius in the SPH case, due to excess heating, coupled with poorer capturing of the shock width. On the other hand, the addition of feedback from active galactic nuclei (AGN) to the simulations results in much better agreement between the methods. In this case both simulations display halo gas entropies of 100 keV cm^2, similar decrements in the star-formation rate, and a drop in the halo baryon content of roughly 30%. This is consistent with AGN growth being self-regulated, regardless of the numerical method. However, the simulations with AGN feedback continue to differ in aspects that are not self-regulated, such that in SPH a larger volume of gas is impacted by feedback, and the cluster still has a lower entropy central core.
AGN feedback from supermassive black holes (SMBHs) at the center of early type galaxies is commonly invoked as the explanation for the quenching of star formation in these systems. The situation is complicated by the significant amount of mass injected in the galaxy by the evolving stellar population over cosmological times. In absence of feedback, this mass would lead to unobserved galactic cooling flows, and to SMBHs two orders of magnitude more massive than observed. By using high-resolution 2D hydrodynamical simulations with radiative transport and star formation in state-of-the-art galaxy models, we show how the intermittent AGN feedback is highly structured on spatial and temporal scales, and how its effects are not only negative (shutting down the recurrent cooling episodes of the ISM), but also positive, inducing star formation in the inner regions of the host galaxy.
Powerful relativistic jets in radio galaxies are capable of driving strong outflows but also inducing star-formation by pressure-triggering collapse of dense clouds. We review theoretical work on negative and positive active galactic nuclei feedback, discussing insights gained from recent hydrodynamical simulations of jet-driven feedback on galaxy scales that are applicable to compact radio sources. The simulations show that the efficiency of feedback and the relative importance of negative and positive feedback depends strongly on interstellar medium properties, especially the column depth and spatial distribution of clouds. Negative feedback is most effective if clouds are distributed spherically and individual clouds have small column depths, while positive feedback is most effective if clouds are predominantly in a disc-like configuration.
[abridged] Aims: We test the effects of re-orienting jets from an active galactic nucleus (AGN) on the intracluster medium in a galaxy cluster environment with short central cooling time. We investigate appearance and properties of the resulting cavities, and the efficiency of jets in providing near-isotropic heating to the cooling cluster core. Methods: We use numerical simulations to explore four models of jets over several active/inactive cycles. We keep the jet power and duration fixed, varying only the jet angle prescription. We track the total energy of the intracluster medium (ICM) in the cluster core over time, and the fraction of the jet energy transferred to the ICM, paying attention to where the energy is deposited. We also compare synthetic X-ray images of the simulated cluster to actual observations. Results: Jets whose re-orientation is minimal ($lesssim 20^{circ}$) typically produce conical structures of interconnected cavities, with the opening angle of the cones being $sim 15-20^{circ}$, extending to $sim 300$ kpc from the cluster centre. Such jets transfer about $60%$ of their energy to the ICM, yet they are not very efficient at heating the cluster core, as the jet energy is deposited further out. Jets that re-orient by $gtrsim 20^{circ}$ generally produce multiple pairs of detached cavities. Although smaller, these cavities are inflated within the central 50~kpc and are more isotropically distributed, resulting in more effective heating of the core. Such jets, over few hundreds Myr, can deposit up to $80%$ of their energy where it is required. Consequently, these models come the closest to an heating/cooling balance and to mitigating runaway cooling of the core, even though all models have identical power/duration profiles. Additionally, the corresponding synthetic X-ray images exhibit structures closely resembling those seen in real cool-core clusters.
We present a spatially-resolved analysis of ionized and molecular gas in a nearby Seyfert 2 galaxy NGC 5728, using the VLT/MUSE and ALMA data. We find ionized gas outflows out to ~kpc scales, which encounter the star formation ring at 1 kpc radius. The star formation rate of the encountering region is significantly high (~1.8 M$_{rm sol}/yr/kpc^2$) compared to other regions in the ring. In contrast, the CO (2-1) emission is significantly weaker by a factor of ~3.5, indicating very high star formation efficiency. These results support the positive feedback scenario that the AGN-driven outflows compress the ISM in the ring, enhancing the star formation activity. In addition, we detect outflow regions outside of spiral arms, in which gas is likely to be removed from the spiral arms and no clear sign of star formation is detected. The overall impact of AGN outflows on the global star formation in NGC 5728 is limited, suggesting the feedback of the low-luminosity AGN is insignificant.
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