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

TL;DR

The paper leverages a 170 ks XRISM Resolve observation to quantify jet-driven gas motions in Cygnus A’s core, revealing a two-temperature ICM where the cooler gas shows markedly broader velocity dispersion than the hotter component. Through single- and two-temperature spectral modeling, the authors find $\sigma_{1D,hot}=261\pm13$ km s$^{-1}$ and $\sigma_{1D,cool}=440\pm130$ km s$^{-1}$, with a hot–cool relative bulk motion of $\sim450\pm140$ km s$^{-1}$, and a hot bulk velocity of $\sim120\pm20$ km s$^{-1}$ with respect to the central galaxy. The analysis yields a total kinetic energy of $E_{kinetic}\approx5.1\times10^{60}$ erg and a kinetic power in the range $6.9\times10^{44}-7.4\times10^{45}$ erg s$^{-1}$, suggesting jet-driven energy injection dominates over radiative cooling and driving a heating imbalance. The lack of Fe-$w$ resonance scattering aligns with the high velocity dispersion, and the results are consistent with the cocoon-shock–driven turbulence and bulk motions expected from Cygnus A’s powerful radio jets, providing direct constraints on AGN feedback in a very energetic cluster core.

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 ($kT = 2.0^{+0.4}_{-0.3}$ 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.

Figures (6)

• Figure 1: Left: 2.2 Ms Chandra image of the core of Cygnus A in the $0.7-7.0$ keV band. The image was processed according to the description in majumder_decoding_2024. The Resolve field of view (FOV) has been overlaid in green. Right:Resolve view of the same region with pixels 12 and 27 removed. The image was created in the $1.7-12.0$ keV band. Only the Hp events were included.
• Figure 2: A single cie fit to the Full FOV Resolve spectrum of Cygnus A. Top: The AGN+ICM model fit to the $1.7-12.0$ keV band data along with the residuals. We also show the NXB model used for the fit. The spectrum has been binned by a factor of 5 in this energy range for visual purposes. Middle left: The same spectrum zoomed in the $1.7-2.5$ keV band showing Si and S lines along with the residuals. Middle right: The $2.5-3.5$ keV band showing Ar lines along with the residuals. Bottom left: The $3.5-4.5$ keV band showing Ca lines along with the residuals. Bottom right: The $5.9-7.6$ keV band showing Fe lines from AGN ($6.05$ keV redshfited) and from the ICM. The Middle and bottom panels have been binned by a factor of 2 for visual purposes.
• Figure 3: Two cie fit to the Full FOV Resolve spectrum of Cygnus A. Top: The AGN+ICM model fit to the $1.7-12.0$ keV band data along with the residuals. We also show the NXB model used for the fit. The spectrum has been binned by a factor of 5 in this energy range for visual purposes. Middle left: The same spectrum zoomed in the $1.7-2.5$ keV band showing Si and S lines along with the residuals. Middle right: The $2.5-3.5$ keV band showing Ar lines along with the residuals. Bottom left: The $3.5-4.5$ keV band showing Ca lines along with the residuals. Bottom right: The $5.9-7.6$ keV band showing Fe lines from AGN ($6.05$ keV redshfited) and from the ICM. The Middle and bottom panels have been binned by a factor of 2 for visual purposes.
• Figure 4: The various lines in the soft band ($1.7-4.5$ keV) causing the broad velocity of cooler component are shown. Left: One cie model fit. Wings and strength of these line complexes are fitted poorly. Right: Two cie model fit. The yellow line shows the hotter cie component and the green line shows the cooler cie component. The red line shows the total two cie model. The wings and strength of the lines are much better fitted. A shift between hotter and cooler cie component is evident. The cooler component is also much broader than the hotter component.
• Figure 5: Left: Unfitted spectrum including the Fe-$w$ line in the $6.2-6.4$ keV band with the two cie model from §\ref{['subsec:two_cie']} overlaid on top. Right:clus model simulation of the Fe-$w$ line for Cygnus A with and without resonance scattering (RS) for 170 ks.