arXiv subjects

H. -Y. Karen Yang

Publications and source records attributed to H. -Y. Karen Yang.

17 recordsLinked to original sources

arXiv2026

Simulating AGN feedback in galaxy clusters with pre-existing turbulence

Feedback from active galactic nuclei (AGN) is believed to play a significant role in suppressing cooling flows in cool-core (CC) clusters. Turbulence in the intracluster medium (ICM), which may be induced by AGN activity or pre-existing motions, has been proposed as a potential heating mechanism based on analysis of Chandra X-ray surface brightness fluctuations. However, subsequent simulation results have found the subdominant role of turbulence in heating the ICM. To investigate this discrepancy, we perform three-dimensional hydrodynamic simulations of a Perseus-like cluster including both AGN feedback and pre-existing turbulence, which is stirred to the observationally constrained level in the Perseus cluster. Our results indicate that, although the velocity field is dominated by the pre-existing turbulence, AGN heating through bubbles and shocks remains significant. More importantly, analysis of the velocity structure function and the energy power spectrum shows that the turbulent heating rate is smaller than the radiative cooling rate, especially in the cluster core. Our results offer insights relevant for recent XRISM observations and indicate that turbulent heating alone cannot offset radiative cooling in CC clusters.

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arXiv2026

Effects of Viscosity on Sloshing Cold Fronts in Galaxy Clusters

The viscous properties of the intracluster medium (ICM) remain poorly constrained. Cold fronts-sharp discontinuities formed during cluster mergers-offer a potential avenue to probe the effective viscosity of the ICM. Velocity shear across these fronts should generate Kelvin-Helmholtz instabilities (KHI), unless viscosity or magnetic tension suppresses them. We perform cluster merger simulations incorporating four ICM viscosity models: (A) inviscid, (B) isotropic Spitzer viscosity, (C) anisotropic Braginskii viscosity, and (D) Braginskii viscosity limited by microinstabilities. The isotropic Spitzer viscosity (case B) strongly suppresses KHI, producing smooth cold front surfaces, while the inviscid (A) and microinstability-limited (D) cases show prominent ripples. The Braginskii case (C) yields intermediate suppression. We also vary the plasma $β$ parameter ($β\approx$ 100 and 1600) to examine how a changing magnetic field strength affects the results. Stronger magnetic fields further suppress KHI, leading to smoother fronts and reduced differences between different viscosity models, while also widening the range of permitted pressure anisotropies when microinstability-based limiters are present. These results indicate that both viscosity and magnetic fields play crucial roles in stabilising sloshing cold fronts in galaxy clusters.

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arXiv2025

Rotation Measure Analysis of Shocks and Sloshing Fronts in a Galaxy Cluster Merger Simulation

Recent observations of the Fornax cluster show depolarization signatures on megaparsec scales, which may be associated with shocks and/or sloshing motions during cluster merger and/or in-fall. To investigate the possible reasons behind the depolarization, we carry out analytical and full polarized radiative transfer (PRT) calculations of radio point sources behind a merging galaxy cluster simulated using the FLASH code. With uniform background light, we analyzed the rotation measure (RM) morphology near the shock front and the cluster center, where sloshing cold fronts appear. For the shock scenario, we find a local RM enhancement by $\sim56\%$ behind the shock front on megaparsec scales, arising from the compression of hot gas and magnetic field lines. Behind the sloshing cold front, the cluster center shows decrement in RM magnitude by $\sim23.3\%$, as a result of the cancellation effect of randomly-oriented magnetic fields induced by sloshing-driven turbulence. We find that beam depolarization increases behind shock fronts and across sloshing cold fronts, indicating enhanced magnetic field fluctuations across the plane of the sky in both scenarios. By fully accounting for all radiative transfer coefficients in the PRT calculations, the uniform background light becomes more depolarized near the cluster center, with the effect growing more pronounced as background intensity decreases. This suggests that synchrotron emission and Faraday rotation of the intracluster medium can significantly influence the polarization of background sources.

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arXiv2024

uGMRT sub-GHz view of the Sausage cluster diffuse radio sources

CIZA J2242.8+5301, or the Sausage cluster, is well studied over a range of frequencies. Since its first discovery, a lot of interesting features and unique characteristics have been uncovered. In this work, we report some more new morphological features using the uGMRT band-3 and band-4 data. In the north relic, we observe variation in spectral index profiles across the relic width from the east to west, which may indicate a decrease in downstream cooling rate in that direction. We re-confirm the presence of an additional ~ 930 kpc relic in the north. We classify the filamentary source in the downstream region to be a narrow angle tail (NAT) radio galaxy. The bright arc in the east relic shows sub-structure in the spectral index profile, which may indicate the presence of finer filaments. We further report the presence of a double-strand structure in the east relic similar to the 'Toothbrush' relic. We categorize the bright 'L' shaped structure in the southern relic to be a NAT radio galaxy, as well as trace the actual ~ 1.1 Mpc relic component. We re-confirm the existence of the faint southern extent, measuring the relic length to be ~ 1.8 Mpc. Furthermore, we suggest the southern relic to be a union of individual component relics rather than a single giant filamentary relic. Lastly, based on the morphological symmetry between northern and southern relics, we suggest a schematic shock structure associated with the merger event in an attempt to explain their formation scenario.

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arXiv2024

AGN jet-inflated bubbles as possible origin of odd radio circles

Odd radio circles (ORCs) are newly discovered extragalactic radio objects with unknown origin. In this work, we carry out three-dimensional cosmic-ray (CR) magnetohydrodynamic simulations using the FLASH code and predict the radio morphology of end-on active galactic nucleus (AGN) jet-inflated bubbles considering hadronic emission. We consider CR proton (CRp)-dominated jets as they tend to inflate oblate bubbles, promising to reproduce the large inferred sizes of the ORCs when viewed end-on. We find that powerful and long-duration CRp-dominated jets can create bubbles with similar sizes ($\sim 300-600$ kpc) and radio morphology (circular and edge-brightened) to the observed ORCs in low-mass ($M_{\rm vir}\sim 8\times 10^{12} - 8\times 10^{13}~M_\odot$) halos. Given the same amount of input jet energy, longer-duration (thus lower-power) jets tend to create larger bubbles since high-power jets generate strong shocks that carry away a significant portion of the jet energy. The edge-brightened feature of the observed ORCs is naturally reproduced due to efficient hadronic collisions at the interface between the bubbles and the ambient medium. We further discuss the radio luminosity, X-ray detectability, and the possible origin of such strong AGN jets in the context of galaxy evolution. We conclude that end-on CR-dominated AGN bubbles could be a plausible scenario for the formation of ORCs.

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arXiv2024

Can the symmetric Fermi and eROSITA bubbles be produced by tilted jets?

The Fermi Gamma-Ray Space Telescope reveals two large bubbles in the Galaxy, extending nearly symmetrically $\sim50^{\circ}$ above and below the Galactic center (GC). Previous simulations of bubble formation invoking active galactic nucleus (AGN) jets have assumed that the jets are vertical to the Galactic disk; however, in general, the jet orientation does not necessarily correlate with the rotational axis of the Galactic disk. Using three-dimensional special relativistic hydrodynamic simulations including cosmic rays (CRs) and thermal gas, we show that the dense clumpy gas within the Galactic disk disrupts jet collimation ("failed jets" hereafter), which causes the failed jets to form hot bubbles. Subsequent buoyancy in the stratified atmosphere renders them vertical to form the symmetric Fermi and eROSITA bubbles (collectively, Galactic bubbles). We find that (1) despite the relativistic jets emanated from the GC are at various angles $\le45^{\circ}$ with respect to the rotational axis of the Galaxy, the Galactic bubbles nonetheless appear aligned with the axis; (2) the edge of the eROSITA bubbles corresponds to a forward shock driven by the hot bubbles; (3) followed by the forward shock is a tangling contact discontinuity corresponding to the edge of the Fermi bubbles; (4) assuming a leptonic model we find that the observed gamma-ray bubbles and microwave haze can be reproduced with a best-fit CR power-law spectral index of 2.4; The agreements between the simulated and the observed multi-wavelength features suggest that forming the Galactic bubbles by oblique AGN failed jets is a plausible scenario.

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arXiv2023

Cosmic ray processes in galactic ecosystems

Galaxy evolution is an important topic, and our physical understanding must be complete to establish a correct picture. This includes a thorough treatment of feedback. The effects of thermal-mechanical and radiative feedback have been widely considered, however cosmic rays (CRs) are also powerful energy carriers in galactic ecosystems. Resolving the capability of CRs to operate as a feedback agent is therefore essential to advance our understanding of the processes regulating galaxies. The effects of CRs are yet to be fully understood, and their complex multi-channel feedback mechanisms operating across the hierarchy of galaxy structures pose a significant technical challenge. This review examines the role of CRs in galaxies, from the scale of molecular clouds to the circum-galactic medium. An overview of their interaction processes, their implications for galaxy evolution, and their observable signatures is provided and their capability to modify the thermal and hydrodynamic configuration of galactic ecosystems is discussed. We present recent advancements in our understanding of CR processes and interpretation of their signatures, and highlight where technical challenges and unresolved questions persist. We discuss how these may be addressed with upcoming opportunities.

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arXiv2023

Evolution and feedback of AGN Jets of different Cosmic-ray Composition

Jet feedback from active galactic nuclei (AGN) is one of the most promising mechanisms for suppressing cooling flows in cool-core clusters. However, the composition of AGN jets and bubbles remains uncertain; they could be thermally dominated, or dominated by cosmic-ray proton (CRp), cosmic-ray electron (CRe), or magnetic energy. In this work, we investigate the evolution and feedback effects of CRp and CRe dominated jets by conducting 3D magnetohydrodynamic simulations of AGN jet-inflated bubbles in the intracluster medium using the FLASH code. We present the evolution of their energies, dynamics and heating, and model their expected cavity-power versus radio-luminosity relation ($P_{\rm cav}-L_R$). We find that bubbles inflated by CRe dominated jets follow a very similar dynamical evolution to CRp dominated bubbles even though CRe within bubbles suffer significantly stronger synchrotron and inverse-Compton cooling. This is because, as CRe lose their energy, the jet-inflated bubbles quickly become thermally dominated within $\sim 30$ Myr. Their total energy stops decreasing with CR energy and evolves similarly to CRp dominated bubbles. The ability of CRe and CRp dominated bubbles to heat the intracluster medium is also comparable; the cold gas formed via local thermal instabilities is well suppressed in both cases. The CRp and CRe bubbles follow different evolutionary trajectories on the $P_{\rm cav}-L_R$ plane, but the values are broadly consistent with observed ranges for FRI sources. We also discuss observational techniques that have potential for constraining the composition of AGN jets and bubbles.

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arXiv2022

Emission from hadronic and leptonic processes in galactic jet-driven bubbles

We investigate the multiwavelength emission from hadronic and leptonic cosmic rays (CRs) in bubbles around galaxies, analogous to the Fermi bubbles of the Milky Way. The bubbles are modeled using 3D magnetohydrodynamical (MHD) simulations, and are driven by a 0.3 Myr intense explosive outburst from the nucleus of Milky Way-like galaxies. We compute their non-thermal emission properties at different stages throughout their evolution, up to 7 Myr, by post-processing the simulations. We compare the spectral and spatial signatures of bubbles with hadronic, leptonic and hybrid hadro-leptonic CR compositions. These each show broadly similar emission spectra, comprised of radio synchrotron, inverse Compton and non-thermal bremsstrahlung components. However, hadronic and hybrid bubbles were found to be brighter than leptonic bubbles in X-rays, and marginally less bright at radio frequencies, and in $γ$-rays between $\sim$ 0.1 and a few 10s of GeV, with a large part of their emission being driven by secondary electrons formed in hadronic interactions. Hadronic systems were also found to be slightly brighter in high-energy $γ$-rays than their leptonic counterparts, owing to the $π^0$ decay emission that dominates their emission between energies of 100s of GeV and a few TeV.

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arXiv2022

Production Efficiencies of Sound Waves in the Intracluster Medium Driven by AGN Jets

Feedback from active galactic nuclei (AGN) is believed to be the most promising solution to the cooling flow problem in cool-core clusters, though how exactly the jet energy is transformed into heat is a subject of debate. Dissipation of sound waves is considered as one of the possible heating mechanisms; however, its relative contribution to heating remains unclear. To estimate the energy budget for heating, we perform 3D hydrodynamic simulations of AGN jet injections in a Perseus-like cluster and quantify the amount of energy stored in the forms of weak shocks and waves. We find that, for a single jet injection with typical parameters in cool-core clusters, $\sim9\%$ of the total jet energy is stored in compressional waves (including both shocks and waves). However, due to the destructive effects among randomly phased waves as well as the dissipation of shock energies, in our simulations including self-regulated AGN feedback, no more than $3\%$ of the total injected energy goes into compressional waves. We further separate the energy contribution from shocks and waves and find that, for a single outburst, the shocks can only contribute to $\sim20-30\%$ of the total compressional energy in the inner radii, and they dissipate very quickly as they travel outward. However, because of the repeated generation of shocks by multiple AGN outbursts, in the self-regulated case shocks completely dominate over sound waves in the inner region and can still provide $\sim40-50\%$ of the total compressional energy even at outer radii. Our results suggest that the production of sound waves is not as efficient as what was found in previous single-outburst simulations, and thus sound wave dissipation may be a subdominant source of heating in cool-core clusters.

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arXiv2022

Fermi and eRosita bubbles as relics of the past activity of the Galactic black hole

The newly launched X-ray satellite, eRosita, has recently revealed two gigantic bubbles extending to ~80 degrees above and below the Galactic center. The morphology of these "eRosita bubbles" bears a remarkable resemblance to the Fermi bubbles previously discovered by the Fermi Gamma-ray Space Telescope and its counterpart, the microwave haze. The physical origin of these striking structures has been intensely debated; however, because of their symmetry about the Galactic center, they likely originate from some energetic outbursts from the Galactic center in the past. Here we propose a theoretical model in which the eRosita bubbles, Fermi bubbles, and the microwave haze could be simultaneously explained by a single event of jet activity from the central supermassive black hole a few million years ago. Using numerical simulations, we show that this model could successfully reproduce the morphology and multi-wavelength spectra of the observed bubbles and haze, which allows us to derive critical constraints on the energetics and timescales of the outburst. This study serves as an important step forward in our understanding of the past Galactic center activity of our Milky Way Galaxy, and may bring valuable insights into the broader picture of supermassive black hole-galaxy co-evolution in the context of galaxy formation.

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arXiv2022

Multi-wavelength emission from leptonic processes in ageing galaxy bubbles

The evolutionary behavior and multi-wavelength emission properties of bubbles around galaxies, such as the \textit{Fermi} bubbles of the Milky Way, is unsettled. We perform 3D magnetohydrodynamical simulations to investigate the evolution of leptonic galaxy bubbles driven by a 0.3 Myr intense explosive outburst from the nucleus of Milky Way-like galaxies. Adopting an ageing model for their leptonic cosmic rays, we post-process our simulations to compute the multi-wavelength emission properties of these bubbles. We calculate the resulting spectra emitted from the bubbles from radio frequencies to $γ$-rays, and construct emission maps in four energy bands to show the development of the spatial emission structure of the bubbles. The simulated bubbles show a progression in their spectral properties as they age. In particular, the TeV $γ$-ray emission is initially strong and dominated by inverse Compton scattering, but falls rapidly after $\sim$ 1 Myr. By contrast, the radio synchrotron emission remains relatively stable and fades slowly over the lifetime of the bubble. Based on the emission properties of our post-processed simulations, we demonstrate that $γ$-ray observations will be limited in their ability to detect galaxy bubbles, with only young bubbles around nearby galaxies being within reach. However, radio observations with e.g. the up-coming Square Kilometer Array, would be able to detect substantially older bubbles at much greater distances, and would be better placed to capture the evolutionary progression and diversity of galaxy bubble populations.

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arXiv2019

Chaotic cold accretion in giant elliptical galaxies heated by AGN cosmic rays

Black hole feedback plays a central role in shaping the circumgalactic medium (CGM) of elliptical galaxies. We systematically study the impact of plasma physics on the evolution of ellipticals by performing three-dimensional non-ideal magneto-hydrodynamic simulations of the interactions of active galactic nucleus (AGN) jets with the CGM including magnetic fields, and cosmic rays (CRs) and their transport processes. We find that the physics of feedback operating on large galactic scales depends very sensitively on plasma physics operating on small scales. Specifically, we demonstrate that: (i) in the purely hydrodynamical case, the AGN jets initially maintain the atmospheres in global thermal balance. However, local thermal instability generically leads to the formation of massive cold disks in the vicinity of the central black hole in disagreement with observations; (ii) including weak magnetic fields prevents the formation of the disks because local B-field amplification in the precipitating cold gas leads to strong magnetic breaking, which quickly extracts angular momentum from the accreting clouds. The magnetic fields transform the cold clouds into narrow filaments that do not fall ballistically; (iii) when plasma composition in the AGN jets is dominated by CRs, and CR transport is neglected, the atmospheres exhibit cooling catastrophes due to inefficient heat transfer from the AGN to CGM despite Coulomb/hadronic CR losses being present; (iv) including CR streaming and heating restores agreement with the observations, i.e., cooling catastrophes are prevented and massive cold central disks do not form. The AGN power is reduced as its energy is utilized efficiently.

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arXiv2018

Acoustic Disturbances in Galaxy Clusters

Galaxy cluster cores are pervaded by hot gas which radiates at far too high a rate to maintain any semblance of a steady state; this is referred to as the cooling flow problem. Of the many heating mechanisms that have been proposed to balance radiative cooling, one of the most attractive is dissipation of acoustic waves generated by Active Galactic Nuclei (AGN). Fabian (2005) showed that if the waves are nearly adiabatic, wave damping due to heat conduction and viscosity must be well below standard Coulomb rates in order to allow the waves to propagate throughout the core. Because of the importance of this result, we have revisited wave dissipation under galaxy cluster conditions in a way that accounts for the self limiting nature of dissipation by electron thermal conduction, allows the electron and ion temperature perturbations in the waves to evolve separately, and estimates kinetic effects by comparing to a semi-collisionless theory. While these effects considerably enlarge the toolkit for analyzing observations of wavelike structures and developing a quantitative theory for wave heating, the drastic reduction of transport coefficients proposed in Fabian (2005) remains the most viable path to acoustic wave heating of galaxy cluster cores.

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arXiv2018

Powering of H$α$ filaments by cosmic rays

Cluster cool cores possess networks of line-emitting filaments. These filaments are thought to originate via uplift of cold gas from cluster centers by buoyant active galactic nuclei (AGN) bubbles, or via local thermal instability in the hot intracluster medium (ICM). Therefore, the filaments are either the signatures of AGN feedback or feeding of supermassive black holes. Despite being characterized by very short cooling times, the filaments are significant H$α$ emitters, which suggests that some process continuously powers these structures. Many cool cores host diffuse radio mini halos and AGN injecting radio plasma, suggesting that cosmic rays (CRs) and magnetic fields are present in the ICM. We argue that the excitation of Alfvén waves by CR streaming, and the replenishment of CR energy via accretion onto the filaments of high plasma-$β$ ICM characterized by low CR pressure support, can provide the adequate amount of heating to power and sustain the emission from these filaments. This mechanism does not require the CRs to penetrate the filaments even if the filaments are magnetically isolated from the ambient ICM and it may operate irrespectively of whether the filaments are dredged up from the center or form in situ in the ICM. This picture is qualitatively consistent with non-thermal line ratios seen in the cold filaments. Future X-ray observations of the iron line complex with XARM, Lynx, or Athena could help to test this model by providing constraints on the amount of CRs in the hot plasma that is cooling and accreting onto the filaments.

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arXiv2016

Global simulations of galactic winds including cosmic ray streaming

Galactic outflows play an important role in galactic evolution. Despite their importance, a detailed understanding of the physical mechanisms responsible for the driving of these winds is lacking. In an effort to gain more insight into the nature of these flows, we perform global three-dimensional magneto-hydrodynamical simulations of an isolated Milky Way-size starburst galaxy. We focus on the dynamical role of cosmic rays injected by supernovae, and specifically on the impact of the streaming and anisotropic diffusion of cosmic rays along the magnetic fields. We find that these microphysical effects can have a significant effect on the wind launching and mass loading factors depending on the details of the plasma physics. Due to the cosmic ray streaming instability, cosmic rays propagating in the interstellar medium scatter on self-excited Alfven waves and couple to the gas. When the wave growth due to the streaming instability is inhibited by some damping process, such as the turbulent damping, the cosmic ray coupling to the gas is weaker and their effective propagation speed faster than the Alfven speed. Alternatively, cosmic rays could scatter from "extrinsic turbulence" that is driven by another mechanism. We demonstrate that the presence of moderately super-Alfvenic cosmic ray streaming enhances the efficiency of galactic wind driving. Cosmic rays stream away from denser regions near the galactic disk along partially ordered magnetic fields and, in the process, accelerate more tenuous gas away from the galaxy. For cosmic ray acceleration efficiencies broadly consistent with the observational constraints, cosmic rays reduce the galactic star formation rates and significantly aid in launching galactic winds.

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arXiv2010

The Impact of Cluster Structure and Dynamical State on Scatter in the Sunyaev-Zel'dovich Flux-Mass Relation

Cosmological constraints from cluster surveys rely on accurate mass estimates from the mass-observable relations. In order to avoid systematic biases and reduce uncertainties, we study the form and physical origin of the intrinsic scatter about the mean Sunyaev-Zel'dovich (SZ) flux-mass relation using a hydrodynamical simulation of galaxy cluster formation. We examine the assumption of lognormal scatter and detect non-negligible positive skewness and kurtosis (> 0.5) for a wide range of limiting masses and redshifts. These higher-order moments should be included in the parametrization of scatter in order not to bias cosmological constraints. We investigate the sources of the scatter by correlating it with measures of cluster morphology, halo concentration, and dynamical state, and we quantify the individual contribution from each source. We find that statistically the impact of dynamical state is weak, so the selection bias due to mergers is negligible. On the other hand, there is a strong correlation between the scatter and halo concentration, which can be used to reduce the scatter significantly (from 12.07% to 7.34% or by ~40% for clusters at z = 0). We also show that a cross-calibration by combining information from X-ray followups can be used to reduce the scatter in the flux-mass relation and also identify outliers in both X-ray and SZ cluster surveys.

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