Astro2020 Science White Paper Physics of cosmic plasmas from high angular resolution X-ray imaging of galaxy clusters Thematic Areas: Planetary Systems Star and Planet Formation Formation and Evolution of Compact Objects 3Cosmology and Fundamental Physics Stars and Stellar Evolution Resolved Stellar Populations and their Environments 3Galaxy Evolution Multi-Messenger Astronomy and Astrophysics Principal Author: Name: Maxim Markevitch Institution: NASA GSFC Email: [email protected] Phone: 301-286-5947 Co-authors: Esra Bulbul (CfA), Eugene Churazov (MPA, IKI), Simona Giacintucci (NRL), Ralph Kraft (CfA), Matthew Kunz (Princeton), Daisuke Nagai (Yale), Elke Roediger (Hull), Mateusz Ruszkowski (Michigan), Alex Schekochihin (Oxford), Reinout van Weeren (Leiden), Alexey Vikhlinin (CfA), Stephen A. Walker (NASA GSFC), Qian Wang (Maryland), Norbert Werner (ELTE, Masaryk), Daniel Wik (Utah), Irina Zhuravleva (Chicago), John ZuHone (CfA) Abstract: Galaxy clusters are massive dark matter-dominated systems filled with X-ray emitting, optically thin plasma. Their large size and relative simplicity (at least as astrophysical objects go) make them a unique laboratory to measure some of the interesting plasma properties that are inaccessible by other means but fundamentally important for understanding and modeling many astrophysical phenomena — from solar flares to black hole accretion to galaxy formation and the emergence of the cosmological Large Scale Structure. While every cluster astrophysicist is eagerly anticipating the direct gas velocity measurements from the forthcoming microcalorimeters onboard XRISM, Athena and future missions such as Lynx, a number of those plasma properties can best be probed by high-resolution X-ray imaging of galaxy clusters. Chandra has obtained some trailblazing arXiv:1903.06356v1 [astro-ph.HE] 15 Mar 2019 results, but only grazed the surface of such studies. In this white paper, we discuss why we need arcsecond-resolution, high collecting area, low relative background X-ray imagers (with modest spectral resolution), such as the proposed AXIS and the imaging detector of Lynx. March 11, 2019 v3 i MODERN astrophysics relies on computer sim- ing Doppler shifts of the X-ray emission lines. ulations to help us understand complex phe- Several important measurements can be done nomena in the Universe, from solar flares to using high-resolution X-ray imaging. Shock supernova explosions, black hole accretion, fronts, discovered by Chandra thanks to its galaxy formation and the emergence of Large sharp mirror, let us study heat conductivity, Scale Structure. As supercomputers advance, the electron-ion temperature equilibration and the benefits of numeric simulations will grow. the physics of cosmic ray acceleration1. An- However, for systems that include plasma, other interesting plasma probe is provided by there is a fundamental limitation — we can’t the ubiquitous, sharp contact discontinuities, or simultaneously model all the relevant linear “cold fronts”1. While Chandra has obtained scales from first principles. For example, tur- tantalizing results, it has only scratched the sur- bulence in the cosmological volume is driven face of what can be learned from detailed imag- by structure formation on the galaxy cluster ing of these and some other cluster phenomena. scale (1024 cm), but can cascade down to scales as small as the ion gyroradius (108-9 cm), a dy- PLASMA EQUIPARTITION TIMES namic range that is impossible to implement The common assumption that all particles in a in codes. To model such systems, we have to plasma have the same local temperature may rely on observed plasma properties and encode not be true if the electron-ion equilibration them at the “subgrid” level. However, many timescale is longer than heating timescales3,4. properties that affect large-scale phenomena — This timescale is fundamental for such pro- viscosity, heat conductivity, energy exchange cesses as accretion onto black holes and X-ray between the particle populations and the mag- emission from the intergalactic medium. It can netic field — are still unmeasured and their the- be directly measured using cluster shocks. oretical estimates uncertain by orders of mag- At a low-Mach shock, ions are dissipatively nitude because of the complexity of the plasma heated to a temperature Ti, while electrons are physics. Of course, apart from being “under the adiabatically compressed to a lower Te. The hood” of many astrophysical systems, plasma two species then equilibrate to the mean post- physics is interesting on its own. shock temperature5 (Fig. 1). From the X-ray Mircoscale phenomena in β ∼ 1 plasmas brightness and spectra, we can measure the (where β is the ratio of thermal to magnetic plasma density and Te across the shock (this re- pressure) can be studied in situ in our space quires only a modest spectral resolution). For neighborhood. Larger scales, including the the typical low sonic Mach numbers in clus- transition from “kinetic” to “fluid” regime, can ters (M = 2 - 3), the mean post-shock tem- be probed in another natural laboratory that is perature can be accurately predicted from the galaxy clusters. Clusters are Megaparsec-size shock density jump. If the equilibration is via clouds of X-ray emitting, optically thin plasma Coulomb collisions, the region over which the (ICM), permeated by tangled magnetic fields electron temperature Te increases is tens of kpc and ultrarelativistic particles, with typical β > wide — resolvable with a Chandra-like tele- 100. This regime is directly relevant for many scope at distances of z < 2. This direct test is astrophysical systems, among them SNR, ac- unique to cluster shocks because of the fortu- cretion disks and the intergalactic medium. itous combination of the linear scales and rela- Several phenomena observed in clusters are tively low Mach numbers; it cannot be done for sensitive to plasma physics. Turbulence is the solar wind or SNR shocks. one, and it will be characterized by the fu- A Chandra measurement for the Bullet ture microcalorimeters (XRISM and Athena) us- cluster shock (Fig. 1) suggests that Te - Ti equi- 1 Shock front in electron-ion plasma Shock front in Bullet cluster Bullet cluster Chandra X-ray image resolvable in clusters shock front τ Coulomb ei ≪ τei = Coulomb cold front 1600 km/s 500 kpc Fig. 1 —(a) X-ray image of the Bullet cluster, the textbook example of a bow shock. The shock is driven by a moving subcluster, whose front boundary is a “cold front.” (b) Expected electron and ion temperature profiles across a shock front. Temperatures are unequal immediately after the shock and then equalize. If electron heat conduction is not suppressed, a temperature precursor is also expected. (c) Chandra deprojected electron temperature profile immediately behind the Bullet shock (crosses; errors are 1σ) with models for Coulomb collisional and instant equipartition2. This measurement favors fast electron-proton equilibration, but uncertainties are large. libration is quicker than Coulomb2, although should connect the post-shock and pre-shock with a systematic uncertainty that arises from regions (unlike for the magnetically-insulated the assumption of symmetry and requires av- cold fronts), though the field structure in the eraging over a sample of shocks. With Chan- narrow shock layer can be chaotic. Electron- dra, this measurement is limited to only three dominated conduction may result in an observ- 2,6,7 shocks, and the results are contradictory .A able Te precursor (Fig. 1). more sensitive imager is needed to find many The magnetic field can be stretched and more shocks (most of them in the cluster out- untangled in a predictable way in the cluster skirts), select a sample of suitable ones, and ro- sloshing cool cores. The characteristic spi- bustly determine this basic plasma property. ral temperature structure that forms there20 can also be used to constrain parallel conductivity. HEAT CONDUCTIVITY A telescope with a bigger mirror than Chan- Heat conduction erases temperature gradients dra’s could look for temperature precursors in and competes with radiative cooling, and is shocks and obtain detailed maps of tempera- of utmost importance for galaxy and clus- ture gradients along the field filaments in many ter formation. The effective heat conductiv- cluster cores to measure the conductivity. ity in a plasma with tangled magnetic fields is unknown, with a large uncertainty for the VISCOSITY component parallel to the field, which recent Plasma viscosity is a fundamental quantity theoretical works predict to be reduced11–15. that governs damping of turbulence and sound The existence of cold fronts in clusters con- waves, suppression of hydrodynamic instabil- firms that conduction across the field lines is ities and mixing of different gas phases, and very low16–18, but constraints for the average thus relevant to such important processes as or parallel conductivity are poor18,19. Shock heating the gas, spreading metals ejected from fronts are locations where the parallel compo- galaxies, and amplification of magnetic fields. nent can be constrained, because the field lines At present it is largely unknown. Isotropic 2 Simulation of galaxy infall and stripping X-ray brightness Fornax cluster A2142 cluster Chandra image XMM image no viscosity infalling group infalling 0.1 Spitzer galaxy 20 kpc 500 kpc tail of unmixed stripped gas Fig. 2 — Plasma viscosity determines how the gas is stripped from the infalling groups and galax- ies. Left: If viscosity is not strongly suppressed,