Astro2020 Science White Paper: A Unification of the Micro and Macro Physics in the Intracluster Medium of Nearby Clusters

TL;DR

This white paper discusses the importance of understanding the microphysical properties of intracluster plasma in galaxy clusters to improve cosmological models and simulations, highlighting recent progress and future prospects.

Contribution

It emphasizes the need to measure microphysical properties of intracluster plasma and reviews recent advances and future X-ray missions for this purpose.

Findings

High-resolution X-ray imaging has advanced understanding of plasma properties.

Future X-ray calorimeter missions will enable direct measurements of transport coefficients.

Current simulations often assume inviscid plasma, which may oversimplify real physics.

Abstract

Clusters of galaxies are the most massive virialized structures in the Universe. The microphysical properties of the intracluster plasma can affect dynamical processes over an enormous range: from the feedback of active galactic nuclei to shock acceleration in merging clusters. All the major cosmological simulations assume the astrophysical plasma to be inviscid. It is critical to measure microphysical properties of the intracluster plasma to truly understand the physical processes that drive the cosmic evolution. Tremendous progress has been made by comparing high spatial resolution X-ray images to (magneto-)hydrodynamic simulations. Future X-ray missions with calorimeters promise a direct measurement of transport coefficients and gas motions, providing a more realistic benchmark for cosmological simulations.

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Full text

Astro2020 Science White Paper

A Unification of the Micro and Macro Physics in the Intracluster Medium of Nearby Clusters

Thematic Areas:                    $\square$ Planetary Systems     $\square$ Star and Planet Formation       $\square$ Formation and Evolution of Compact Objects            $\square$ $\checkmark$ Cosmology and Fundamental Physics $\square$ Stars and Stellar Evolution   $\square$ Resolved Stellar Populations and their Environments              $\square$ $\checkmark$ Galaxy Evolution                $\square$ Multi-Messenger Astronomy and Astrophysics

Principal Author:

Name: Yuanyuan Su Institution: University of Kentucky Email: ysu262@uky.edu Phone: (859)-257-1397

Co-authors: R. Kraft (Smithsonian Astrophysical Observatory), E. Roediger (University of Hull), P. Nulsen (Smithsonian Astrophysical Observatory), A. Sheardown (University of Hull), T. Fish (University of Hull), J. ZuHone (Smithsonian Astrophysical Observatory), E. Churazo (Max Planck Institute for Astrophysics), W. Forman (Smithsonian Astrophysical Observatory), C. Jones (Smithsonian Astrophysical Observatory), J. Irwin (University of Alabama), S. Randall (Smithsonian Astrophysical Observatory) \justifyAbstract:

Clusters of galaxies are the most massive virialized structures in the Universe. The microphysical properties of the intracluster plasma can affect dynamical processes over an enormous range: from the feedback of active galactic nuclei to shock acceleration in merging clusters. All the major cosmological simulations assume the astrophysical plasma to be inviscid. It is critical to measure microphysical properties of the intracluster plasma to truly understand the physical processes that drive the cosmic evolution. Tremendous progress has been made by comparing high spatial resolution X-ray images to (magneto-)hydrodynamic simulations. Future X-ray missions with calorimeters promise a direct measurement of transport coefficients and gas motions, providing a more realistic benchmark for cosmological simulations.

1 The intracluster medium as a magnetized plasma

The space between cluster member galaxies is filled with hot and diffuse ionized plasma that emits X-rays through thermal bremsstrahlung and atomic lines. We often treat it as a fluid with transport coefficients, where the particle-magnetic field interactions and plasma instabilities determine the effective transport coefficients. It is critical to understand the transport processes in the ICM on a variety of physical scales: microscopic plasma physics impacts many macroscopic processes, e.g., the dissipation and redistribution of the kinetic energy released by mergers or AGN outbursts (Fabian et al. 2005), the cooling and heating in cluster cores, and gas stripping from galaxies (Nulsen 1982). Furthermore, large cosmological simulation is our primary way to understand the baryonic processes of the Universe. However, all the current major cosmological simulations assume the interstellar medium, intracluster medium, and large scale gas to be inviscid (Figure 1) i.e., illustris project (Vogelsberger et al. 2014), the EAGLE project (Schaye et al. 2014) (although the numerical viscosity is non-zero and depends on the resolution of the simulation). To fully understand feedback, galaxy evolution, and structure formation etc., it is crucial to constrain the microphysical properties of the astrophysical plasma including viscosity, thermal conductivity, the strength and topology of the magnetic field, and small-scale turbulence.

X-ray observations have been used to probe the physical conditions in the ICM. For example, turbulent velocity has been determined indirectly via resonance scattering and surface brightness fluctuations (Ogorzalek et al. 2017; Gu et al. 2018; Zhuravleva et al. 2015). The microcalorimeter on board Hitomi has measured the level of turbulence directly using line broadening (Hitomi collaboration 2016). The Chandra X-ray Observatory (half arcsec spatial resolution) has revealed the ubiquity of “cold fronts” in the ICM (see Markevitch & Vikhlinin 2007 for a review), which can be used to constrain the viscosity and magnetic field. Cold fronts are sharp interfaces between cooler, denser, hence brighter, gas and hotter, lower density, hence fainter, medium, where the pressure is continuous. They result from merger activity, either created directly by the infall of a low entropy subcluster (merging cold front), or from gas motions induced by the gravity of an infalling subcluster (sloshing cold fronts). For purely hydrodynamic interactions, Kelvin-Helmholtz Instability (KHI) is expected to develop at shearing interfaces. However, magnetic fields or viscosity can suppress the KHI (Chandrasekhar 1961; Lamb 1932). State of the art simulations have demonstrated that cold fronts appear smoother in the presence of either magnetic field or viscosity at the levels expected in the ICM (see ZuHone & Roediger 2016 for a review).

Recent deep Chandra observations have been dedicated to revisiting cold fronts identified by previous observations, leading to a deeper understanding of the microphysics of the ICM. Multiple-edge structures in X-ray surface brightness have been identified at the cold fronts in a growing number of systems (Werner et al. 2016; Su et al. 2017a; Ichinohe et al. 2017; Ichinohe et al. 2018). These features are consistent with the presence of KHI eddies and an inviscid ICM (Figure 2-top). Walker et al. (2017) studied the sloshing cold fronts in a number of clusters (Perseus, Centaurus and Abell 1795) and identified concave ‘bay’ substructures from X-ray and radio imaging. These features resemble the giant KHI rolls expected when the ratio of thermal to magnetic pressure is $\beta=200$ (Figure 2-middle).

In addition to cold fronts, stripped tails are another product of ongoing infall in galaxy clusters and can be used to constrain micro physics of the ICM (Roediger et al. 2015a,b). For example, a temperature gradient was detected in the stripped tail in NGC 1404, indicating a well mixed plasma of a low viscosity (Su et al. 2017a; Sheardown et al. 2018). The temperature map derived with the deep Chandra observations is remarkably similar to that derived from an inviscid simulation specifically tailored to the infall of NGC 1404 (Figure 2-bottom). Results from deep X-ray observations of the nearby early-type galaxy NGC 4552 were compared with the viscous and inviscid hydrodynamic simulations specifically tailored to the stripping of this galaxy (Roediger et al. 2015a,b; Kraft et al. 2017). Inviscid stripping was favored by the study. Based on the survival of the stripped low-entropy infalling group in the hot cluster Abell 2142, Eckert et al. (2014, 2017) find thermal conduction to be strongly suppressed. Note that the properties of cold fronts and stripped tails also depend on the merger history (Su et al. 2017d; Kraft et al. 2017). It is important to understand the entire dynamic state of the system before any conclusions can be drawn from the immediate observables.

2 Measuring gas motions in galaxy clusters

The ICM is always in a dynamically active state. Forming at the knots of the cosmic filaments, galaxy clusters grow continually via mergers of subclusters and accretions of galaxies. In addition, AGN periodically release mechanical energy at cluster centers. Both effects are expected to cause shocks, bulk motions, and turbulence in the ICM over a large span of physical scales. To date, the most practical way of probing the ICM velocity is based on the Rankine-Hugoniot jump conditions: the gas properties on both sides of a shock wave can be used to infer the infalling speed of a substructure (e.g, Su et al. 2016; Vikhlinin et al. 2001; Markevitch et al. 2002; Zhang et al. 2019). For infalling substructures, Su et al. (2017b) has developed an analytical method combining the jump conditions and the Bernoulli equation, which allows us to derive a general speed, the complete velocity field, and the distance of the substructure from the pressure distribution. The motion of gas sloshing can also be determined via the pressure distributio. Ueda et al. (2018) analyzed the cool core cluster RX J1347.5-1145. They found that while the residual X-ray image derived from Chandra shows a clear spiral pattern characteristic of gas sloshing, no significant variation is shown in the Sunyaev-Zel’dovich effect image derived from ALMA. This study has confirmed the subsonic nature of sloshing gas predicted by simulations (e.g., Roediger et al. 2011; ZuHone et al. 2010).

The line-of-sight gas motion can be measured directly from line centroids based on the Doppler effect. The line centroids measurements have been performed in a number of galaxy clusters with CCDs (Dupke & Bregman 2006; Ota et al. 2007; Tamura et al. 2011; Ueda et al. 2019). However, these results are mostly upper limits or of low significance due to the poor spectral resolution of CCDs ($\Delta E\sim 150{\rm eV}$). In contrast, the microcalorimeter on board Hitomi provides a spectral resolution of 5 eV. Based on the FeXXV K$\alpha$ line at 6.7 keV, Hitomi accurately measures a bulk motion of 150 km/s for the gas at the center of the Perseus Cluster. Although this is the only ICM measurement made by Hitomi due to its untimely loss, it has demonstrated that calorimeter science can revolutionize our view of the gas dynamics of the ICM.

3 What can we learn with future X-ray telescopes?

The biggest change promised by future missions is to be able to measure the kinematics of the intracluster medium directly with calorimeters at high spatial resolution. Thanks to the hierarchical formation process, massive clusters are the rare breed in the Universe. Galaxy groups and low mass clusters are much more common, for which the measurement of FeXXV K$\alpha$ line at 6.7 KeV is not available. Instead, the centroid and broadening of the O VIII line at $18.967\mathring{A}$ (0.654 keV), which is bright and isolated for the hot atmospheres of low mass clusters and galaxies, can be used to map the bulk speed and turbulence of the gas. Using the FLASH simulation (Sheardown et al. 2018), sixte 1.3.6 and SOXS 2.2.0, we simulated a 200 ksec observation of a nearby X-ray bright galaxy like NGC 1404 using an instrument with an effective area of 1.4 $m^{2}$ and with a superb spectral resolution, similar to the X-ray Integral Field Unit (X-IFU) on board Athena (Barret et al. 2018). A simulated image and an example spectrum is shown in Figure 3-left, demonstrating its 2.5 eV spectral resolution with $5^{\prime\prime}$ pixels.

We have also simulated a $3\times 3$ mosaic of 30 ksec observations for an instrument with a large field of view and low and stable background resembling the wide field X-ray imager (WFI) on board Athena as shown in Figure 3-right. We can capture the entire ICM of a cluster like Fornax out to (and even beyond) the virial radius and detect fainter sloshing cold fronts at larger radii, which is not feasible with existing missions for a reasonable exposure time. We have selected regions of interest to directly measure their gas motions and turbulence with X-IFU simulations We can further identify uneven edges at the cold front and turbulent regions in the path of the infalling object. The details of these faint structures match the resolution of the simulations, which will transform our understanding of the microphysics over a large span of radii the ICM. Gas distribution at large radii will also pin down the initial conditions for the simulation, maximizing the science return from the tailored simulations.

4 Concluding Remarks

The vast bulk of the hot baryons in the Universe is in the form of the intracluster medium, a hot diffuse plasma emitting X-ray via thermal bremsstrahlung and atomic lines. With Chandra and XMM-Newton observations and (magneto-)hydrodynamic simulations, tremendous progress has been made in understanding how clusters are assembled and how energy is transported in the ICM. Future missions such as XARM, Athena, and Lynx will be equipped with calorimeters, allowing us to measure gas motions and turbulence directly and to put quantitative constraints on the transport coefficients in their ICM. By the end of the 2020’s, hundreds of thousands galaxy clusters will be detected by various cluster surveys such as eROSITA (Pillepich et al. 2018). Most of them are expected to be low mass clusters. The O VIII line is relatively isolated and sufficiently bright, which can be used to constrain gas dynamics in low mass clusters and galaxies. Through a connection between the micro and macro scale astrophysics in the ICM, our knowledge of hydrodynamics in galaxy clusters will be revolutionized.