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1. Non-Kolmogorov turbulence in multiphase intracluster medium driven by cold gas precipitation and AGN jets

   https://doi.org/10.1093/mnras/stab966  

   Wang, C ; Ruszkowski, M ; Pfrommer, C ; Oh, S Peng ; Yang, H-Y K ( April 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Active galactic nuclei (AGNs) feedback is responsible for maintaining plasma in global thermal balance in extended haloes of elliptical galaxies and galaxy clusters. Local thermal instability in the hot gas leads to the formation of precipitating cold gas clouds that feed the central supermassive black holes, thus heating the hot gas and maintaining global thermal equilibrium. We perform 3D magnetohydrodynamical (MHD) simulations of self-regulated AGNs feedback in a Perseus-like galaxy cluster with the aim of understanding the impact of the feedback physics on the turbulence properties of the hot and cold phases of the intracluster medium (ICM). We find that, in general, the cold phase velocity structure function (VSF) is steeper than the prediction from Kolmogorov’s theory. We attribute the physical origin of the steeper slope of the cold phase VSF to the driving of turbulent motions primarily by the gravitational acceleration acting on the ballistic clouds. We demonstrate that, in the pure hydrodynamical case, the precipitating cold filaments may be the dominant agent driving turbulence in the hot ICM. The arguments in favour of this hypothesis are that: (i) the cold phase mass dominates over hot gas mass in the inner cool core; (ii) hot and cold gas velocities are spatially correlated; (iii) both the cold and hot phase velocity distributions are radially biased. We show that, in the MHD case, the turbulence in the ambient hot medium (excluding the jet cone regions) can also be driven by the AGN jets. The driving is then facilitated by enhanced coupling due to magnetic fields of the ambient gas and the AGN jets. In the MHD case, turbulence may thus be driven by a combination of AGN jet stirring and filament motions. We conclude that future observations, including those from high spatial and spectral resolution X-ray missions, may help to constrain self-regulated AGN feedback by quantifying the multitemperature VSF in the ICM. 

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2. The nature of the motions of multiphase filaments in the centers of galaxy clusters

   https://doi.org/10.3389/fspas.2023.1138613  

   Ganguly, Shalini ; Li, Yuan ; Olivares, Valeria ; Su, Yuanyuan ; Combes, Francoise ; Prakash, Sampadaa ; Hamer, Stephen ; Guillard, Pierre ; Ha, Trung ( May 2023 , Frontiers in Astronomy and Space Sciences)

   The intracluster medium (ICM) in the centers of galaxy clusters is heavily influenced by the “feedback” from supermassive black holes (SMBHs). Feedback can drive turbulence in the ICM and turbulent dissipation can potentially be an important source of heating. Due to the limited spatial and spectral resolutions of X-ray telescopes, direct observations of turbulence in the hot ICM have been challenging. Recently, we developed a new method to measure turbulence in the ICM using multiphase filaments as tracers. These filaments are ubiquitous in cluster centers and can be observed at very high resolution using optical and radio telescopes. We study the kinematics of the filaments by measuring their velocity structure functions (VSFs) over a wide range of scales in the centers of ∼ 10 galaxy clusters. We find features of the VSFs that correlate with the SMBHs activities, suggesting that SMBHs are the main driver of gas motions in the centers of galaxy clusters. In all systems, the VSF is steeper than the classical Kolmogorov expectation and the slopes vary from system to system. One theoretical explanation is that the VSFs we have measured so far mostly reflect the motion of the driver (jets and bubbles) rather than the cascade of turbulence. We show that in Abell 1795, the VSF of the outer filaments far from the SMBH flattens on small scales to a Kolmogorov slope, suggesting that the cascade is only detectable farther out with the current telescope resolution. The level of turbulent heating computed at small scales is typically an order of magnitude lower than that estimated at the driving scale. Even though SMBH feedback heavily influences the kinematics of the ICM in cluster centers, the level of turbulence it drives is rather low, and turbulent heating can only offset ≲ 10% of the cooling loss, consistent with the findings of numerical simulations. 

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3. Gas Accretion Can Drive Turbulence in Galaxies

   https://doi.org/10.3847/1538-4357/acb53e  

   Forbes, John C. ; Emami, Razieh ; Somerville, Rachel S. ; Genel, Shy ; Nelson, Dylan ; Pillepich, Annalisa ; Burkhart, Blakesley ; Bryan, Greg L. ; Krumholz, Mark R. ; Hernquist, Lars ; et al ( May 2023 , The Astrophysical Journal)

   The driving of turbulence in galaxies is deeply connected with the physics of feedback, star formation, outflows, accretion, and radial transport in disks. The velocity dispersion of gas in galaxies therefore offers a promising observational window into these processes. However, the relative importance of each of these mechanisms remains controversial. In this work we revisit the possibility that turbulence on galactic scales is driven by the direct impact of accreting gaseous material on the disk. We measure this effect in a disk-like star-forming galaxy in IllustrisTNG, using the high-resolution cosmological magnetohydrodynamical simulation TNG50. We employ Lagrangian tracer particles with a high time cadence of only a few million years to identify accretion and other events. The energies of particles are measured by stacking the events in bins of time around the event. The average effect of each event is measured by fitting explicit models for the kinetic and turbulent energies as a function of time. These measurements are corroborated by cross-correlating the turbulent energy with other time series and searching for signals of causality, i.e., asymmetries across zero time lag. We find that accretion contributes to the large-scale turbulent kinetic energy even if it does not dominate in this ∼5 × 10⁹M_⊙stellar mass galaxy. Extrapolating this finding to a range of galaxy masses, we find that there are regimes where energy from direct accretion may dominate the turbulent energy budget, particularly in disk outskirts, galaxies less massive than the Milky Way, and at redshift ∼2.

    

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4. X-Ray Cavity Dynamics and Their Role in the Gas Precipitation in Planck Sunyaev–Zeldovich (SZ) Selected Clusters

   https://doi.org/10.3847/1538-4357/ace359  

   Olivares, V. ; Su, Y. ; Forman, W. ; Gaspari, M. ; Andrade-Santos, F. ; Salome, P. ; Nulsen, P. ; Edge, A. ; Combes, F. ; Jones, C. ( August 2023 , The Astrophysical Journal)

   We study active galactic nucleus (AGN) feedback in nearby (z< 0.35) galaxy clusters from the Planck Sunyaev–Zeldovich sample using Chandra observations. This nearly unbiased mass-selected sample includes both relaxed and disturbed clusters and may reflect the entire AGN feedback cycle. We find that relaxed clusters better follow the one-to-one relation of cavity power versus cooling luminosity, while disturbed clusters display higher cavity power for a given cooling luminosity, likely reflecting a difference in cooling and feedback efficiency. Disturbed clusters are also found to contain asymmetric cavities when compared to relaxed clusters, hinting toward the influence of the intracluster medium (ICM) “weather” on the distribution and morphology of the cavities. Disturbed clusters do not have fewer cavities than relaxed clusters, suggesting that cavities are difficult to disrupt. Thus, multiple cavities are a natural outcome of recurrent AGN outbursts. As in previous studies, we confirm that clusters with short central cooling times,t_cool, and low central entropy values,K₀, contain warm ionized (10,000 K) or cold molecular (<100 K) gas, consistent with ICM cooling and a precipitation/chaotic cold accretion scenario. We analyzed archival Multi-Unit Spectroscopic Explorer observations that are available for 18 clusters. In 11/18 of the cases, the projected optical line emission filaments appear to be located beneath or around the cavity rims, indicating that AGN feedback plays an important role in forming the warm filaments by likely enhancing turbulence or uplift. In the remaining cases (7/18), the clusters either lack cavities or their association of filaments with cavities is vague, suggesting alternative turbulence-driven mechanisms (sloshing/mergers) or physical time delays are involved.

    

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5. Turbulence in compact to giant H  ii regions

   https://doi.org/10.1093/mnras/stad1631  

   García-Vázquez, J. ; Henney, William J. ; Castañeda, H. O. ( June 2023 , Monthly Notices of the Royal Astronomical Society)

   Radial velocity fluctuations on the plane of the sky are a powerful tool for studying the turbulent dynamics of emission line regions. We conduct a systematic statistical analysis of the H α velocity field for a diverse sample of nine ${\rm H\, \small{II}}$ regions, spanning two orders of magnitude in size and luminosity, located in the Milky Way and other Local Group galaxies. By fitting a simple model to the second-order spatial structure function of velocity fluctuations, we extract three fundamental parameters: the velocity dispersion, the correlation length, and the power-law slope. We determine credibility limits for these parameters in each region, accounting for observational limitations of noise, atmospheric seeing, and the finite map size. The plane-of-sky velocity dispersion is found to be a better diagnostic of turbulent motions than the line width, especially for lower luminosity regions where the turbulence is subsonic. The correlation length of velocity fluctuations is found to be always roughly 2 per cent of the ${\rm H\, \small{II}}$ region diameter, implying that turbulence is driven on relatively small scales. No evidence is found for any steepening of the structure function in the transition from subsonic to supersonic turbulence, possibly due to the countervailing effect of projection smoothing. Ionized density fluctuations are too large to be explained by the action of the turbulence in any but the highest luminosity sources. A variety of behaviours are seen on scales larger than the correlation length, with only a minority of sources showing evidence for homogeneity on the largest scales.

    

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