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Cooling Flow Problem     Which AGN Jet Quenches    CR in AGN Jet


How Satellites Populate the Cold Phase in CGM


Microphysics & Galaxy Evloution    Stellar FB Alters Magnetic Fields


Discrete Effects in Stellar FB


Bondi Radius Scale


AGN Jet & BH Accretion


Bridging Scales

Cooling Flow Problem

How to keep massive galaxies quenched and remained "red and dead" over a large portion of cosmic time?

We investigate this in halos with masses ∼1012−1014 M⊙, using non-cosmological high-resolution hydrodynamic simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. We show that various proposed "non-AGN" solution mechanisms in the literature, including Type Ia supernovae, shocked AGB winds, other forms of stellar feedback (e.g. cosmic rays), magnetic fields, Spitzer-Braginskii conduction, or "morphological quenching" do not halt or substantially reduce cooling flows nor maintain "quenched" galaxies in this mass range.
The results are published in: MNRAS Vol. 487, Issue 3, p.4393-4408
Before jumping right into a specific, potentially more realistic AGN feedback model, we tested verious simplified toy energy-injection models, where we arbitrarily vary the strength, injection scale, and physical form of the energy. Different scenarios include thermal ``heating,'' direct wind or momentum injection, cosmic ray heating or pressure support, or turbulent ``stirring'' of the intra-cluster medium (ICM). We show that turbulent stirring in the central ~100 kpc, or cosmic-ray injection, can both maintain a stable low-SFR halo for >Gyr timescales with modest energy input, by providing a non-thermal pressure which stably lowers the core density and cooling rates.In both cases, associated thermal-heating processes are negligible. Turbulent stirring preserves cool-core features while mixing condensed core gas into the hotter halo. Pure thermal heating or nuclear isotropic momentum injection require vastly larger energy, are less efficient in lower-mass halos, easily over-heat cores, and require fine-tuning to avoid driving unphysical temperature gradients or gas expulsion from the halo center.
The results are published in: MNRAS Vol. 491, Issue 1, p.1190-1212
Fig.1   The star formation rate (SFR) for a 1014 M⊙, halo. Stellar feedback, magnetic fields, conduction, cosmic ray from supernovae do not quench the galaxy.  AGN toy models with (a) thermal heating with a 30kpc kernel, (b) turbulent stirring confined within 100kpc, and (c) CR injection can stable suppress the SFR.

What Types of AGN Jet Quench?

CR jets require less fine tuning
Our previous work suggested that AGN jets are likely required, but the form of jet energy required to quench remains unclear. This is particularly challenging for galaxy simulations, in which the resolution is orders of magnitude coarser than necessary to form and evolve the jet. On such scales, the uncertain parameters include: jet energy form (kinetic, thermal, and cosmic ray (CR) energy), energy, momentum, and mass flux, magnetic field strength and geometry, jet precession angle and period, opening-angle, and duty cycle. We investigate all of these parameters in a 1014M⊙ halo using high-resolution non-cosmological MHD simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model, conduction, and viscosity. We explore which scenarios match observational constraints and show that CR-dominated jets can most efficiently quench the central galaxy through a combination of CR pressure support and a modification of the thermal instability. Jets with most energy in mildly relativistic (∼ MeV or ∼1010 K) thermal plasma work, but require a factor ∼10 larger energy input. For a fixed energy flux, jets with higher specific energy (longer cooling times) quench more effectively. For this halo size, kinetic jets are less efficient in quenching unless they have wide opening or precession angles. Magnetic fields play a minor role except when the magnetic flux reaches ≳1044 erg s−1 in a kinetic jet model, which causes the jet cocoon to significantly widen, and the quenching to become explosive. We conclude that the criteria for a successful jet model are an optimal energy flux and a sufficiently wide jet cocoon with long enough cooling time at the cooling radius.
The results are published in: MNRAS Vol. 507, Issue 1, p.175-204
In this paper, we investigate three jet modes with constant fluxes satisfying the criteria above, including high-temperature thermal jets, cosmic ray (CR)-dominant jets, and widely precessing kinetic jets in 1012 - 1015 M⊙ halos using high-resolution, non-cosmological MHD simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model, conduction, and viscosity. We find that scaling the jet energy according to the free-fall energy at the cooling radius can successfully suppress the cooling flows and quench galaxies without obviously violating observational constraints. We investigate an alternative scaling method in which we adjust the energy flux based on the total cooling rate within the cooling radius. However, we observe that the strong interstellar medium (ISM) cooling dominates the total cooling rate in this scaling approach, resulting in a jet flux that exceeds the amount needed to suppress the cooling flows. With the same energy flux, the CR-dominant jet is most effective in suppressing the cooling flow across all the surveyed halo masses due to the enhanced CR pressure support. We confirm that the criteria for a successful jet model, which we proposed in Su et al. (2021), work across a much wider range, encompassing halo masses of 1012 - 1015 M⊙.
The results are published in: arXiv:2310.17692
Fig. 2   Active galactic nucleus (AGN) jets can suppress cooling flows (and therefore quench star formation) in massive galaxies if the following three criteria are met.

1. Moderate jet energy flux Enough energy for cocoon expansion to balance gas cooling, but not so much energy as to exceed escape velocity at Rcool.

2. Long cooling time within jet cocoon Longer than time to reach Rcool. Can be achieved by thermal or kinetic jets with high specific energy or CR jets.

3. Wide jet cocoon Width of cocoon at Rcool is enough to suppress the cooling flow over a wide solid angle. Can be achieved by jets with a high non-kinetic component, very light kinetic jets, or jets with initially wide angles.


Cosmic Rays in AGN Jet

Cosmic rays at jet cocoon shock fronts is very effective
Cosmic rays associated with AGN jets have the potential to efficiently suppress cooling flows and inhibit star formation. However, the specific locations where cosmic rays are produced and coupled to the system play a crucial role in the overall self-regulation process. To investigate this, we conducted high-resolution non-cosmological MHD simulations of a $10^{14}$ halo using the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. We explored a variety of AGN jet feedback scenarios with cosmic rays, examining different fractions of cosmic ray energy, as well as cosmic ray coupling sites (in the vicinity of the black hole versus at the shock fronts of large-scale jet cocoons), and the effects of jet precession. Our findings indicate that when cosmic rays are injected near the vicinity of the black hole, they efficiently inhibit black hole accretion by suppressing the density before sufficient jet production can occur. As a result, this leads to episodic black hole accretion histories, with the jet lacking the necessary energy flux to reach large radii and impact cooling flows. Conversely, if the jet is primarily kinetic and cosmic rays are injected at the shock front of the jet cocoon, it not only sustains a higher overall jet energy flux over an extended period but also disperses cosmic rays to larger radii, effectively suppressing the cooling flow. Furthermore, the period and angle of jet precession can influence the position of shock fronts. We have identified an optimal range of jet precession parameters that generates shocks precisely at the inner circumgalactic medium (CGM), where cooling flows are most severe. This configuration offers the most effective scenario for cosmic rays to quench cooling flows if injected at the shock fronts.
Fig. 3   Different CR jet models exhibit distinct cosmic ray pressure distributions. All runs employ the same gravitation torque accretion model, feedback efficiency, and jet specific energy. The variations lie in the energy form and injection point. Injecting CR at shocks leads to a more extended CR distribution. .

How Satellites Populate the Cold Phase of MW Halos

Mixing layer induced cooling is very important
Leaded by Manami Roy, we stidied the origin of the cold phase in the CGM is a highly debated question. We investigate the contribution of satellite galaxies to the cold gas budget in the circumgalactic medium (CGM) of a Milky Way-like host galaxy. We perform controlled experiments with three different satellite mass distributions and identify several mechanisms by which satellites can add cold gas to the CGM, including ram pressure stripping and induced cooling in the mixing layer of the stripped cold gas. These two mechanisms contribute a comparable amount of cold gas to the host CGM. We find that the less massive satellites (≤109 M⊙) not only lose all of their cold gas in a short period (~ 0.5-1 Gyr), but their stripped cold clouds also mix with the hot CGM gas and get heated up quickly. However, stellar feedback from these less massive satellites can hugely alter the fate of their stripped gas. Feedback speeds up the destruction of the stripped cold clouds from these satellites by making them more diffuse with more surface area. On the other hand, the more massive satellites (LMC or SMC-like ~1010 M⊙) can add cold gas to the total gas budget of the host CGM for several Gyrs.

The results are published in: MNRAS Vol. 527, Issue 1, p.265-280
Fig. 4   The temperature map illustrates a simulation featuring two 1010 M⊙ satellites within a Milky Way (MW)-mass halo. Additionally, the plot is color-coded to represent the fraction of gas in each cell originating from the satellite. Our findings indicate that an order-of-unity cold gas within the circumgalactic medium (CGM) primarily results from the induced cooling of the mixing layer between ram pressure-stripped cold gas and the host CGM.

Fluid Microphysics & Galaxy Evolution

Stellar feedback has the dominating effects
Fig. 5   Images of the gas morphology of the isolated galaxies with feedback. The intensity encodes the projected density (log-weighted with ∼ 4 dex stretch); different temperatures are shown in red (> 105 K), green (8000 − 105 K), and magenta (< 8000 K). We show edge-on and face-on projections for our NoFB, Stellar FB and SFB+MHD+Cond+Vis. The morphologies and phase structure of the runs with the same stellar feedback but different additional physics show little difference.

Stellar Feedback Alters Magnetic Fields

Stellar feedback strongly alters the amplification and morphology of galactic magnetic fields

Fig. 6   Edge-on and face-on projections of the gas density. Arrows indicate the relative magnitudes and directions of the magnetic field. Different columns correspond to different baryonic physics models. In all maps, the magnetic fields in dense clumps are not only stronger but also more randomly distributed. No-feedback runs fragment most dramatically and therefore exhibit magnetic fields highly concentrated in dense clumps with random directions. Runs that employ the FIRE explicit stellar feedback model have irregular magnetic field distributions owing to supernova shocks, turbulence and outflows driven by stellar feedback, in addition to the greater fragmentation present in these runs compared with the Adiabatic and S&H sub-grid stellar feedback runs. The latter two types of runs generally have smooth, highly ordered gas and magnetic field morphologies.




Diecrete Effects in Stellar Feedback

Individual supernovae, hypernovae, and IMF sampling in dwarf galaxies
Fig. 7   Upper left: Stellar mass as a function of cosmic time in our simulations. m10q has ∼ 30 HNe randomly distributed among the SNe over its history. Lower left: SFR averaged over the preceding 100 Myr as a function of time. Upper right: The mass outflow rate as a function of time smoothed over 100 Myr. To estimate the mass outflow rate, we consider all gas particles between 0.08 and 0.1 rvir that have radial velocities greater than 30 km s−1 . Lower right: Outflow mass-loading factor, η ≡ outflow/ SFR, smoothed over 500 Myr. Treating SN feedback as continuous results in higher SFRs – and thus stellar masses – and lower outflow mass loading factors. The final stellar mass of m10q “Default” and “Default 2” runs differ by a factor of ∼ 2. Given such range of stochastic effect, the effect of IMF sampling or HNe is not obvious.



AGN Jets Regulates BH Accretion

The isotropic component of the jet cocoon momentum flux is regulated to match the inflowing momentum flux at the Bondi radius.
The early growth of black holes (BHs) in high-redshift galaxies is likely feedback regulated. While radiative feedback has been extensively studied, the role of mechanical feedback has received less scrutiny to date. Here, we use high-resolution parsec-scale hydrodynamical simulations to study jet propagation and its effect on 100 M⊙ BH accretion in the dense, low-metallicity gas expected in early protogalaxies. As the jet propagates, it shocks the surrounding gas forming a jet cocoon. The cocoon consists of a rapidly cooling cold phase at the interface with the background gas and an overpressured subsonic phase of reverse shock-heated gas filling the interior. We vary the background gas density and temperature, BH feedback efficiency, and the jet model. We found that the width of the jet cocoon roughly follows a scaling derived by assuming momentum conservation in the jet-propagation direction and energy conservation in the lateral directions. Depending on the assumed gas and jet properties, the cocoon either stays elongated to large radii or isotropizes before reaching the Bondi radius, forming a nearly spherical bubble. Lower jet velocities and higher background gas densities result in self-regulation to higher momentum fluxes and elongated cocoons. In all cases, the outward cocoon momentum flux balances the inward inflowing gas momentum flux near the Bondi radius, which ultimately regulates BH accretion. The time-averaged accretion rate always remains below the Bondi rate, and exceeds the Eddington rate only if the ambient medium is dense and cold, and/or the jet is weak (low velocity and mass loading).
The results are published in: MNRAS, Vol.520, Issue 3, p4256-4275
Fig. 8   A cartoon picture of the jet cocoon propagation and the two possible cocoon morphologies as a result of different jet parameters, background gas density, and temperature. The left-hand panel shows the isotropic ‘bubble’ case where riso < rBondi. The right-hand panel shows the elongated ‘cocoon’ case where riso > rBondi. The blue arrow represents the jet. Each half oval indicates the jet cocoon at a given time. The grey dashed line indicates the resulting overall cocoon shape of a continuous jet injection by ‘linking’ the cocoon shock-front at each time.

Bridging Scales for Galaxy to BH Horizon

With the bridging scale collaboration at Black Hole Initiative, leaded by Ramesh Narayan,, Priyamvada Natarajan,, and in collaboration with Hyerin Cho,, Ben Prather,, Angelo Ricarte,, and Koushik Chatterjee, we aim to bridge galaxy scale and BH horizon scale consistently in a single simulation.

The first publication of the project, led by Hyerin Cho, presented a novel multi-zone computational method. The method is based on the general relativistic magnetohydrodynamic (GRMHD) code KHARMA, allowing us to span seven orders of magnitude in spatial scale. This enables the simulation of magnetized accretion onto a non-spinning supermassive black hole (SMBH) from an external medium with a Bondi radius over 2x105 times larger than the gravitational radius. The energy feedback from the accretion flow to the external medium is at a level of approximately 10−2 dMBH/dt c2 ∼ 5×10−5 dMBondi/dt c2.

The results are published in: arXiv: 2310.19135

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