The Quiescent Intracluster Medium in the Core of the Perseus Cluster (1607.04487v1)

Abstract: Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. It can enable new insights into mechanical energy injection by the central supermassive black hole and the use of hydrostatic equilibrium for the determination of cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50 million K diffuse hot plasma filling its gravitational potential well. The Active Galactic Nucleus of the central galaxy NGC1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These likely induce motions in the intracluster medium and heat the inner gas preventing runaway radiative cooling; a process known as Active Galactic Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus cluster core, which reveal a remarkably quiescent atmosphere where the gas has a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s is found across the 60 kpc image of the cluster core. Turbulent pressure support in the gas is 4% or less of the thermodynamic pressure, with large scale shear at most doubling that estimate. We infer that total cluster masses determined from hydrostatic equilibrium in the central regions need little correction for turbulent pressure.

• The paper presents a novel measurement of a 164 ± 10 km/s velocity dispersion in the Perseus core, highlighting the quiescent nature of its ICM.
• It employs Hitomi’s high-resolution X-ray spectrometer to identify minimal turbulence, with turbulent pressure comprising only 4% of the thermodynamic pressure.
• The findings challenge prevailing AGN feedback models and support the reliability of hydrostatic mass estimates in central galaxy clusters.

An Analysis of the Quiescent Intracluster Medium in the Perseus Cluster Core

The recent Hitomi X-ray observations provide pivotal insights into the dynamics of the intracluster medium (ICM) within the Perseus cluster. As the most massive gravitationally bound entities in the Universe, galaxy clusters offer valuable opportunities to probe cosmological parameters and various astrophysical mechanisms. Central to these investigations is an understanding of the ICM dynamics, which is primarily dominated in mass by hot plasma, significantly more than the stellar content within clusters.

This research prominently documents the surprising quiescent nature of the ICM in the Perseus cluster core, challenging previous assumptions of pervasive turbulence. Achieving a line-of-sight velocity dispersion of 164 ± 10 km/s, the Hitomi data suggest that turbulent pressure support in this core is remarkably low—at just 4% of the thermodynamic pressure. With a gradient in line-of-sight velocity detected at 150 ± 70 km/s over a 60 kpc area, the turbulence levels are lower than expected, demonstrating minimal large-scale shear.

The Hitomi space observatory's Soft X-ray Spectrometer (SXS) has afforded unprecedented accuracy in these measurements. Its non-dispersive calorimetric capabilities, operating within an energy range of 0.3-12 keV with a 4.9 eV energy resolution, have allowed for precise detection of Doppler shifts and broadening of emission lines, key to discerning both bulk and turbulent ICM motions. The instrumental resolution significantly surpasses previous data constraints, notably those from the XMM-Newton Reflection Grating Spectrometer.

This paper implies that the mass estimates derived from hydrostatic equilibrium in central cluster regions may require minimal adjustments for turbulence, thereby reinforcing their reliability, both for individual clusters and cosmological applications. Moreover, these results suggest that current AGN feedback models may need revisions to account for the subdued turbulence, despite the continuous stirring from AGN activities. The association of low-velocity turbulence with the generation of ultrarelativistic electron populations supporting radio synchrotron emission hints at complex energy transfer and dissipation processes in the ICM.

The implications of these findings extend to almost all astrophysical models related to turbulence and energy distribution in galaxy clusters. Importantly, the acknowledgment of low turbulence and shear across a region disturbed by central AGN motions posits constraints on the generation and propagation of ICM turbulence. This opens new hypotheses regarding energy replenishment mechanisms essential to counteract radiative cooling while maintaining low turbulence levels.

Looking ahead, these observations beckon further investigation into the interplay between AGN feedback and ICM dynamics. Subsequent missions or analysis could enhance understanding of the discrepancies in model predictions of turbulence and its supporting role in energy dissipation, contributing to our broader understanding of cluster evolution dynamics. As Hitomi's unfortunate loss restricts continued direct observation, future X-ray observatories must leverage and expand upon these groundbreaking advancements in our exploration of cosmic phenomena.