Kung Su and colleagues have attempted a systematic exploration of different AGN jet models that inject energy into massive halos, quenching galaxies and suppressing cooling flows. They specifically considered models with pure kinetic jets, thermal energy dominated jets, magnetic jets and cosmic ray jets and found that cosmic ray jets quench more effectively than any other jets. They also explored the reason behind it. Their study published in ArXiv on dated 5 Feb, 2021.
Active galactic nucleus (AGN), is a small region at the centre of a galaxy that emits a prodigious amount of energy in the form of radio, optical, X-ray, or gamma radiation or high-speed particle jets. Many classes of “active galaxies” have been identified—for example, quasars, radio galaxies, and Seyfert galaxies.
A major outstanding problem in galaxy formation for decades has been how to “quench” massive galaxies (stellar masses & 10¹¹ M or above ∼ L∗ in the galaxy luminosity function) and keep them “red and dead” over a large fraction of cosmic time.
The difficulty lies in the classic “cooling flow” problem — X-ray observations have found significant radiative cooling in the hot gas of elliptical galaxies and clusters, indicating cooling times shorter than a Hubble time. However, compared to the inferred cooling flow (reaching up to ∼ 1000 Myr-¹ in clusters), neither sufficient cold gas from HI and CO observations nor sufficient star formation has been observed in galaxies. Simulations and semi-analytic models which do not suppress the cooling flows, and simply allow gas to cool into the galactic core, typically predict over an order of magnitude higher star formation rates (SFRs) than observed.
Some heat source or pressure support must be present to compensate for the observed cooling. Moreover, the heating must still preserve the cool core structure (e.g., density and entropy profiles) observed in the majority of galaxies. One way to achieve this is to suppress the cooling flow and maintain a very-low-SFR, stable cool-core (CC) cluster. Another possibility is that clusters undergo cool-core—non-cool-core (NCC) cycles: a stronger episode of feedback overturns the cooling flows, resulting in a non-cool-core cluster, which gradually recovers to a cool-core cluster and starts another episode of feedback.
The various non-AGN solutions to the cooling flow problem proposed in the literature generally belong to the former case, including: stellar feedback from shock-heated AGB winds, Type Ia supernovae (SNe), SNe-injected cosmic rays (CRs), magnetic fields and thermal conduction in the circum-galactic medium (CGM) or intra-cluster medium (ICM), or “morphological quenching” via altering the galaxy morphology and gravitational stability properties. Although these processes can slightly suppress star formation, or help suppress the cooling flows, most previous studies, including authors own exhaustive survey studying each of these in simulations similar to those presented in this recent paper (Su et al. 2019, hereafter Paper I), have shown that they do not fundamentally alter the classic cooling flow picture. In the end, the star formation is still regulated by cooling flows, and the star formation rate is orders of magnitude too high.
Consequently, AGN feedback seems to be the most promising candidate to solve the cooling flow problem, and there has been a tremendous amount of theoretical work on the topic. Observational studies also infer that the available energy budget from AGN can match the cooling rate. There are also observations of un-ambiguous cases of AGN expelling gas from galaxies, injecting thermal energy via shocks or sound waves, or via photo-ionization and Compton heating, or via “stirring” the CGM and ICM, and creating “bubbles” of hot plasma with non-negligible relativistic components which are ubiquitous around massive galaxies. However, despite its plausibility and the extensive work above, the detailed physics of AGN feedback remain uncertain, as do the relevant “input parameters.”
Now, Su and colleagues, in their recent paper, attempted a systematic exploration of different AGN jet models that inject energy into massive halos, quenching galaxies and suppressing cooling flows. They specifically considered models with pure kinetic jets, thermal energy dominated jets, magnetic jets and cosmic ray jets. They also systematically varied the mass loading, jet width, jet magnetic field strength and field geometry, precession angle and period, and jet duty cycle. These were studied in full-halo-scale but non-cosmological simulations including radiative heating and cooling, self-gravity, star formation, and stellar feedback from supernovae, stellar mass-loss, and radiation, enabling a truly “live” response of star formation and the multi-phase ISM to cooling flows. They used a hierarchical super-Lagrangian refinement scheme to reach ∼ 10⁴ M mass resolution, much higher than many previous global studies.
They found that a cosmic ray (CR) jet quenches more efficiently than a thermal jet with the same energy flux — i.e., CR jets require only about one-tenth of the energy flux as a thermal jet to quench the same galaxy. On the other hand, kinetic jets are the least efficient at stopping the cooling flow, and only marginally suppress the SFR unless the opening-angles or procession angles are quite wide.
They also discussed how different jet parameters affect jet propagation and galaxy quenching. They first provide a simple model for the jet propagation and cocoon expansion, which helps them to interpret the results of their numerical experiments.
They found that despite the different energy forms, the propagation of a jet builds up a pressurized region (cocoon) with the thermal, CR, or magnetic energy it carries at launch or gains through converting its kinetic energy through shocks. This both heats up gas within the cocoon, reversing the cooling and suppressing cooling instabilities, as well as building up a pressure gradient, slowing down the gas inflow. They found three criteria to successfully quench a halo (which are summarized in Fig. 1 below):
They also found that width of the jet cocoon determines how efficiently it quenches. Given that the effects of the kinetic jets are limited to a relatively small solid angle, the value of the solid angle can determine how effective the jet is at quenching. Keeping a similar solid angle but increasing the energy (or momentum) flux, on the other hand, has much smaller effects. To produce a kinetic-jet-inflated cocoon with a wide enough solid angle to suppress the cooling flows, they either need a jet with vJ >> 2 × 10⁴ km s-¹, or a jet that is initialized with a wide opening-angle. But guys, can we able to achieve effective quenching by increasing kinetic energy input, if the solid angle is not enlarged?
Well, not necessarily. The jet drives shocks which inflate a cocoon, directly impacting gas within an opening angle that depends on the form of the jet. Beyond this effective opening-angle, the cooling flows are much less affected. As a result, increasing the kinetic energy input does not necessarily mean more effective quenching if the affected solid angle is not enlarged because the maximum effect is expelling all the gas in that cone.
Why the CR jet quenches more efficiently? They found that cosmic ray (CR)-dominated jets quench more efficiently, and potentially more stably, than thermal, kinetic, and magnetic jets due to three factors: (i) CR pressure support, (ii) modification of the thermal instability, and (iii) CR propagation. Injected CRs provide pressure support to the gas and have long cooling times, which leads to the formation of a CR pressure-dominated cocoon (allowing CR jets to quench at order-of-magnitude lower energetics). Because the CR energy density is much larger than kinetic energy density, the CR jet cocoon covers a wider angle (as expected), and can therefore more efficiently suppress inflow.
They didn’t stop here. They also tested the effects of magnetic fields and found that magnetic fields usually only have limited effects (factor ≤ 2) in quenching the galaxy or suppressing cooling flows. Furthermore, they tested how the cooling flows and SFRs differ, when using the same total energy in a given form but with different mass flux and energy loading and found that given a fixed total energy, the lower the mass flux (i.e., the higher the specific energy of the jet), the more effective the quenching is.
They also experimented with different precession angles and precession periods and found that the dominant effect of the precessing kinetic jet is still shock heating the surrounding gas and suppressing the inflows or pushing the gas outward within a specific solid angle. Thus, making an otherwise narrow cocoon “efficient” requires precession angles & 30°– 45° so that the cocoon can become effectively quasi-isotropic.
In conclusion, their study supports the idea that quenching – at least of observed z ∼ 0 massive halos – can be accomplished within the viable parameter space of AGN jets. With their last study and this one, they showed that the viable parameter space which produces successful quenching and does not violate observational constraints is rather narrow, and points to specific jet/cocoon processes and quite possibly a role for CRs. Many caveats remain, alongside more detailed comparisons with observations, which they will explore in their future work.
Reference: Kung-Yi Su, Philip F. Hopkins, Greg L. Bryan, Rachel S. Somerville, Christopher C. Hayward, Daniel Anglés-Alcázar, Claude-André Faucher-Giguère, Sarah Wellons, Jonathan Stern, Bryan A. Terrazas, T. K. Chan, Matthew E. Orr, Cameron Hummels, Robert Feldmann, Dušan Kereš, “Which AGN Jets Quench Star Formation in Massive Galaxies?”, ArXiv, pp. 1-29, 2021. https://arxiv.org/abs/2102.02206
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