Cool core cycles: Cold gas and AGN jet feedback in cluster cores

Using high-resolution 3-D and 2-D (axisymmetric) hydrodynamic simulations in spherical geometry, we study the evolution of cool cluster cores heated by feedback-driven bipolar active galactic nuclei (AGN) jets. Condensation of cold gas, and the consequent enhanced accretion, is required for AGN feedback to balance radiative cooling with reasonable efficiencies, and to match the observed cool core properties. A feedback efficiency (mechanical luminosity $\approx \epsilon \dot{M}_{\rm acc} c^2$; where $\dot{M}_{\rm acc}$ is the mass accretion rate at 1 kpc) as small as $5 \times 10^{-5}$ is sufficient to reduce the cooling/accretion rate by $\sim 10$ compared to a pure cooling flow. This value is smaller compared to the ones considered earlier, and is consistent with the jet efficiency and the fact that only a small fraction of gas at 1 kpc is accreted on to the supermassive black hole (SMBH). We find hysteresis cycles in all our simulations with cold mode feedback: {\em condensation} of cold gas when the ratio of the cooling-time to the free-fall time ($t_{\rm cool}/t_{\rm ff}$) is $\lesssim 10$ leads to a sudden enhancement in the accretion rate; a large accretion rate causes strong jets and {\em overheating} of the hot ICM such that $t_{\rm cool}/t_{\rm ff} > 10$; further condensation of cold gas is suppressed and the accretion rate falls, leading to slow cooling of the core and condensation of cold gas, restarting the cycle. Therefore, there is a spread in core properties, such as the jet power, accretion rate, for the same value of core entropy or $t_{\rm cool}/t_{\rm ff}$. A fewer number of cycles are observed for higher efficiencies and for lower mass halos because the core is overheated to a longer cooling time. The 3-D simulations show the formation of a few-kpc scale, rotationally-supported, massive ($\sim 10^{11} M_\odot$) cold gas torus. (abstract abridged)Comment: 22 pages, 15 figures; ApJ accepted version (figures downgraded to smaller size, as required for arxiv submission

Cool-Core Clusters : Role of BCG, Star Formation & AGN-Driven Turbulence

Recent analysis shows that it is important to explicitly include the gravitational potential of the central brightest central galaxy (BCG) to infer the acceleration due to gravity ($g$) and the free-fall time ($t_{\rm ff} \equiv [2r/g]^{1/2}$) in cool cluster cores. Accurately measuring $t_{\rm ff}$ is crucial because according to numerical simulations cold gas condensation and strong feedback occur in cluster cores with min($t_{\rm cool}/t_{\rm ff}$) below a threshold value close to 10. Recent observations which include the BCG gravity show that the observed threshold in min($t_{\rm cool}/t_{\rm ff}$) lies at a somewhat higher value, close to 10-30; there are only a few clusters in which this ratio falls much below 10. In this paper we compare numerical simulations of feedback AGN (Active Galactic Nuclei) jets interacting with the intracluster medium (ICM), with and without a BCG potential. We find that, for a fixed feedback efficiency, the presence of a BCG does not significantly affect the temperature but increases (decreases) the core density (entropy) on average. Most importantly, min($t_{\rm cool}/t_{\rm ff}$) is only affected slightly by the inclusion of the BCG gravity. Also notable is that the lowest value of min($t_{\rm cool}/t_{\rm ff}$) in the NFW+BCG runs are about twice larger than in the NFW runs. We also look at the role of depletion of cold gas due to star formation and show that it only affects the rotationally dominant component (torus), while the radially dominant component (which regulates the feedback cycle) remains largely unaffected. The distribution of metals due to AGN jets in our simulations is predominantly along the jet direction and the radial spread of metals is less. We also show that the turbulence in cool core clusters is weak, consistent with recent Hitomi results on Perseus cluster.Comment: Submitted to ApJ; Suggestions and comments are welcome; Substantially updated from the previous version; Abstract abridged; 12 pages, 11 figure

Circumgalactic Gas and the Precipitation Limit

During the last decade, numerous and varied observations, along with increasingly sophisticated numerical simulations, have awakened astronomers to the central role the circumgalactic medium (CGM) plays in regulating galaxy evolution. It contains the majority of the baryonic matter associated with a galaxy, along with most of the metals, and must continually replenish the star forming gas in galaxies that continue to sustain star formation. And while the CGM is complex, containing gas ranging over orders of magnitude in temperature and density, a simple emergent property may be governing its structure and role. Observations increasingly suggest that the ambient CGM pressure cannot exceed the limit at which cold clouds start to condense out and precipitate toward the center of the potential well. If feedback fueled by those clouds then heats the CGM and causes it to expand, the pressure will drop and the "rain" will diminish. Such a feedback loop tends to suspend the CGM at the threshold pressure for precipitation. The coming decade will offer many opportunities to test this potentially fundamental principle of galaxy evolution.Comment: Astro2020 White Paper, 8 pages, 2 figures (differences from Astro2020 version: some typos fixed, some references added

AGN jet feedback on a moving mesh: cocoon inflation, gas flows and turbulence

In many observed galaxy clusters, jets launched by the accretion process onto supermassive black holes, inflate large scale cavities filled with energetic, relativistic plasma. This process is thought to be responsible for regulating cooling losses, thus moderating the inflow of gas onto the central galaxy, quenching further star formation and maintaining the galaxy in a red and dead state. In this paper, we implement a new jet feedback scheme into the moving mesh-code AREPO, contrast different jet injection techniques and demonstrate the validity of our implementation by comparing against simple analytical models. We find that jets can significantly affect the intracluster medium (ICM), offset the overcooling through a number of heating mechanisms, as well as drive turbulence, albeit within the jet lobes only. Jet-driven turbulence is, however, a largely ineffective heating source and is unlikely to dominate the ICM heating budget even if the jet lobes efficiently fill the cooling region, as it contains at most only a few percent of the total injected energy. We instead show that the ICM gas motions, generated by orbiting substructures, while inefficient at heating the ICM, drive large scale turbulence and when combined with jet feedback, result in line-of-sight velocities and velocity dispersions consistent with the Hitomi observations of the Perseus cluster.MAB and DS acknowledge support by the ERC starting grant 638707 “BHs and their host galaxies: co-evolution across cosmic time.” DS further acknowledges support from the STFC. This research used: The DiRAC Darwin Supercomputer hosted by the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council; The COSMA Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility. This equipment was funded by a BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/K00087X/1, DiRAC Operations grant ST/K003267/1 and Durham University. The DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the UK National EInfrastructure

AGN jet-driven stochastic cold accretion in cluster cores

International audienceSeveral arguments suggest that stochastic condensation of cold gas and its accretion on to the central supermassive black hole (SMBH) is essential for active galactic nuclei (AGNs) feedback to work in the most massive galaxies that lie at the centres of galaxy clusters. Our 3-D hydrodynamic AGN jet-ICM (intracluster medium) simulations, looking at the detailed angular momentum distribution of cold gas and its time variability for the first time, show that the angular momentum of the cold gas crossing ≲1 kpc is essentially isotropic. With almost equal mass in clockwise and counterclockwise orientations, we expect a cancellation of the angular momentum on roughly the dynamical time. This means that a compact accretion flow with a short viscous time ought to form, through which enough accretion power can be channeled into jet mechanical energy sufficiently quickly to prevent a cooling flow. The inherent stochasticity, expected in feedback cycles driven by cold gas condensation, gives rise to a large variation in the cold gas mass at the centres of galaxy clusters, for similar cluster and SMBH masses, in agreement with the observations. Such correlations are expected to be much tighter for the smoother hot/Bondi accretion. The weak correlation between cavity power and Bondi power obtained from our simulations also matches observations

Cool-core Clusters: The Role of BCG, Star Formation, and AGN-driven Turbulence

Recent observations of cool cluster cores that include the BCG gravity claim that the observed threshold in min(t(cool)/t(ff)) (cooling time to free-fall time ratio) lies at a somewhat higher value, close to 10-30, compared with the threshold seen in numerical simulations. There are only a few clusters in which this ratio falls much below 10. In this paper, we compare 3D hydrodynamic simulations of feedback active galactic nuclei (AGNs) jets interacting with the intracluster medium, with and without a BCG potential. We find that, for a fixed feedback efficiency, the presence of a BCG does not significantly affect the temperature, but increases (decreases) the core density (entropy) on average. Most importantly, min(t(cool)/t(ff)) is only affected slightly by the inclusion of the BCG gravity. Also notable is that the lowest value of min(t(cool)/t(ff)) in the NFW+BCG runs is about twice as large as in the NFW runs. We also look at the role of depletion of cold gas due to star formation, and show that it only affects the rotationally dominant component, while the radially dominant component remains largely unaffected. Stellar gas depletion also increases the repetition rate of AGN jets. The distribution of metals due to AGN jets in our simulations is predominantly along the jet direction, and the equatorial spread of metals is less compared with the observations. We also show that the turbulence in cool-core clusters is weak, which is consistent with recent Hitomi results on the Perseus cluster

The Concussion Awareness Training Tool for Women’s Support Workers Improves Knowledge of Intimate Partner Violence-Caused Brain Injury

Women who experience physical intimate partner violence (IPV) are at high risk of suffering a brain injury (BI) due to head impacts and/or strangulation. Currently, most staff at women’s shelters tend not to be aware of IPV-caused BIs. The objective of this study was to address this by developing a new online module within the Concussion Awareness Training Tool (cattonline.com) specifically focused on IPV-caused BI, and measuring its effectiveness in increasing BI awareness and knowledge among staff members at women’s shelters. A mixed-methods approach was used which included (i) a survey to measure participant knowledge before and after completing the module; (ii) a 1-on-1 interview 6 months post-training to better understand participants’ perceptions of what effect the training had on how they worked with women in their job; and (iii) an evaluation of the content of the module using behavior change techniques. About 81 participants recruited from staff at women’s shelters completed the pre/post survey. The average BI knowledge score increased significantly from the pre-survey (M = 8.12/12, SD = 1.05) to the post-survey (M = 9.72/12, SD = 1.62), t(80) = 9.12, P < .001, d = 1.01). Analysis of the interviews with 9 participants highlighted 3 main themes arising from the module: knowledge, mindfulness, and advocacy. All participants felt their knowledge of IPV-caused BIs had increased and said they would recommend the training to their co-workers. Analysis of the module content revealed the most frequent behavior change techniques were related to instructions on how to perform screening and accommodation for IPV-caused BI. The results showed the module was effective in increasing knowledge of IPV-caused BIs amongst women’s shelter staff as well as improving how they advocate for, and are mindful of, their clients with BIs. This online training may help improve the care women with IPV-caused BIs receive, and ultimately improve their quality of life.Health and Social Development, Faculty of (Okanagan)Medicine, Faculty ofNon UBCHealth and Exercise Sciences, School of (Okanagan)Pediatrics, Department ofReviewedFacultyResearche