Spectrally Resolved Gas Kinematics in Cygnus A: XRISM Detects AGN Jet-induced Velocity Dispersion in Multi-temperature Gas

Abstract

We report spectral analysis on a 170 ks XRISM \textit{Resolve} exposure of the core of Cygnus A. Analyzing the full field of view spectrum in the $1.7-12.0$ keV band, we find evidence for two-temperature cluster gas. The hotter ($kT=5.53\pm 0.13$ keV) gas has a velocity dispersion of $261\pm 13$ km s${}^{-1}$ and a bulk velocity of $120\pm 20$ km s${}^{-1}$ with respect to the central galaxy. The cooler gas ($kT=2.0_{-0.3}^{+0.4}$ keV) has an even broader velocity dispersion of $440\pm 130$ km s${}^{-1}$, with a systematic uncertainty of $120$ km s${}^{-1}$. The relative line-of-sight velocity between the hotter and cooler gas can be as high as $450\pm 140$ km s${}^{-1}$. We interpret the high velocity dispersions as a combination of turbulence and bulk motion due to the cocoon shock. The upper limit on the non-thermal pressure fraction for the hotter gas is $7.7\pm 0.7\%$. We associate the cooler gas with the central region ($<35$ kpc) and the hotter phase with the gas surrounding it ($35-100$ kpc). The total energy due to the kinetic motion is $5.1\times 10^{60}$ erg, consistent with the energy associated with the central radio source. The kinetic energy injection rate is $6.9\times 10^{44}-7.4\times 10^{45}$ erg s${}^{-1}$ under varying assumptions of injection timescales. The range of injection power is higher than the cooling luminosity, and thus the heating and cooling rates in Cygnus A are unbalanced.

Accepted for publication in ApJ