We investigate the effects of AGN feedback on the cosmological evolution of an isolated elliptical galaxy by performing two-dimensional high-resolution hydrodynamical numerical simulations. The inner boundary of the simulation is chosen so that the Bondi radius is resolved. Compared to previous works, the two accretion modes—namely, hot and cold, which correspond to different accretion rates and have different radiation and wind outputs—are carefully discriminated, and the feedback effects by radiation and wind in each mode are taken into account. The most updated AGN physics, including the descriptions of radiation and wind from the hot accretion flows and wind from cold accretion disks, are adopted. Physical processes like star formation and SNe Ia and II are taken into account. We study the AGN light curve, typical AGN lifetime, growth of the black hole mass, AGN duty cycle, star formation, and X-ray surface brightness of the galaxy. We compare our simulation results with observations and find general consistency. Comparisons with previous simulation works find significant differences, indicating the importance of AGN physics. The respective roles of radiation and wind feedback are examined, and it is found that they are different for different problems of interest, such as AGN luminosity and star formation. We find that it is hard to neglect any of them, so we suggest using the names “cold feedback mode” and “hot feedback mode” to replace the currently used ones.

We perform a series of high-resolution 2D hydrodynamical simulations of equal-mass binary black holes (BBHs) embedded in active galactic nucleus (AGN) accretion disks to study whether these binaries can be driven to merger by the surrounding gas. We find that the gravitational softening adopted for the BBH has a profound impact on this result. When the softening is less than 10% of the binary separation, we show that, in agreement with recent simulations of isolated equal-mass binaries, prograde BBHs expand in time rather than contract. Eventually, however, the binary separation becomes large enough that the tidal force of the central AGN disrupts them. Only when the softening is relatively large do we find that prograde BBHs harden. We determine through detailed analysis of the binary torque, that this dichotomy is due to a loss of spiral structure in the circum-single disks orbiting each black hole when the softening is a significant fraction of the binary separation. Properly resolving these spirals—both with high resolution and small softening—results in a significant source of binary angular momentum. Only for retrograde BBHs do we find consistent hardening, regardless of softening, as these BBHs lack the important spiral structure in their circum-single disks. This suggests that the gas-driven inspiral of retrograde binaries can produce a population of compact BBHs in the gravitational-wave-emitting regime in AGN disks, which may contribute a large fraction to the observed BBH mergers.

Binary black hole (BBH) mergers, particularly those with component masses in the pair-instability gap, may be produced by hierarchical mergers in the disks surrounding Active Galactic Nuclei (AGNs). While the interaction of an embedded BBH with an AGN disk is typically assumed to facilitate a merger, recent high-resolution hydrodynamical simulations challenge this assumption. However, these simulations often have simplified treatments for gas thermodynamics. In this work, we model the possible consequence of various feedback from an embedded BBH with a simple model that maintains an enhanced temperature profile around each binary component. We show that when the minidisks around each BH become hotter than the background by a factor of three, the BBH orbital evolution switches from expansion to contraction. By analyzing the gravitational torque profile, we find that this change in direction is driven by a weakening of the minidisk spirals and their positive torque on the binary. Our results highlight the important role of thermodynamics around BBHs and its effect on their orbital evolution, suggesting that AGN disks could be efficient factories for BBH mergers.

Quasiperiodic eruptions (QPEs) are intense, repeating, soft x-ray bursts with recurrence times of about a few to ten hours from nearby galactic nuclei. The origin of QPEs is still unclear. In this work, we investigate the extreme mass ratio inspiral (EMRI) + accretion disk model, where the disk is formed from a previous tidal disruption event (TDE). In this $\mathrm{EMRI}+\mathrm{TDE}$ disk model, the QPEs are the result of collisions between a TDE disk and a stellar mass object (a stellar mass black hole or a main sequence star) orbiting around a supermassive black hole (SMBH) in galactic nuclei. If this interpretation is correct, QPEs will be invaluable in probing the orbits of stellar mass objects in the vicinity of SMBHs and further inferring the formation of EMRIs, which are one of the primary targets of spaceborne gravitational wave missions. Taking GSN 069 as an example, we find the EMRI that is of low eccentricity ($e<0.1$ at the $3\text{−}\sigma$ confidence level) and a semimajor axis about $O({10}^2)$ gravitational radii of the central SMBH, which is consistent with the prediction of the wet EMRI formation channel while incompatible with alternatives.

How galaxies maintain the inefficiency of star formation with physically self-consistent models is a central problem in understanding galaxy evolution. Although numerous theoretical models have been proposed in recent decades, the debate continues. By means of high-resolution two-dimensional hydrodynamical simulations, we study the three feedback effects (the stellar wind heating, supernova (SN) feedback, and active galactic nucleus (AGN) feedback) in suppressing star formation activities on the evolution of early-type galaxies with different stellar masses. The AGN feedback models are updated from work by Yuan et al. The gas sources arise exclusively from the mass losses of dying low-mass stars for most of our models. We find that SN feedback can keep star formation at a significantly low level for low-mass elliptical galaxies for a cosmological evolution time. For high-mass galaxies, AGN feedback can efficiently offset radiative cooling and thus regulate star formation activity. Such a suppression of star formation is extremely efficient in the inner regions of galaxies. Asymptotic giant branch heating cannot account for this suppression for low- or high-mass galaxies. The X-ray temperature T _X and luminosity L _X of hot plasma can be in agreement with the observed data with the inclusion of effective feedback processes. These results thus suggest that we can use T _X and L _X to probe the role of different feedback processes. The inclusion of additional gas sources can cause the mass scale between SN and AGN feedback to dominate in suppressing star formation decrease to an observationally inferred value of a few 10¹⁰ M _⊙.