XRISM mock observations of simulated AGN jets in the core of a galaxy cluster

TL;DR

The paper addresses how AGN jets interact with the ICM and how these interactions manifest in high-resolution XRISM spectra. It employs two-dimensional hydrodynamic simulations of Cygnus A–like jets and constructs XRISM/Resolve mock spectra to explore viewing-angle effects on velocity dispersion. The key finding is that the smallest apparent line broadening occurs when viewing along the jet axis due to the hot, low-density jet head contributing minimally to X-ray emission, while larger dispersions arise at larger angles when the shocked cocoon dominates emission; including unshocked ICM reduces angular sensitivity and increases total spectral normalization. These results guide the interpretation of XRISM data and advance understanding of AGN feedback in cluster cores.

Abstract

Jets from active galactic nuclei (AGNs) are expected to heat the surrounding intracluster medium (ICM). We investigate how the interaction between jets and the ICM appears in high-resolution X-ray observations using mock X-ray observations based on two-dimensional hydrodynamic simulations. We constructed a model of an active galactic nucleus (AGN) similar to Cygnus A (Cyg A), a powerful FR II radio galaxy. Our simulations model bipolar jets propagating into a stratified ICM, forming forward shocks and low-density cocoons. Based on these results, we generate synthetic spectra that incorporate both shocked and unshocked ICM components. Then, we perform mock observations using the XRISM/Resolve X-ray spectrometer. We focus particularly on viewing angle effects. Our mock observations revealed that the smallest line broadening, observed as velocity dispersion, associated with the cocoon's bulk expansion occurs when observing along the jet direction, where the expansion velocity is highest. Although this may appear counterintuitive, it occurs because the rapidly expanding jet head contributes little to X-ray emission due to its high temperature and low density. Our results highlight the importance of considering the temperature and density structure of AGN-driven shocks and cocoons when interpreting XRISM data. These findings lay the groundwork for XRISM's observations of AGN jets and will improve our understanding of AGN feedback processes in galaxy clusters.

Figures (4)

• Figure 1: Simulation results at $t = 13.5~\mathrm{Myr}$. From left to right: density, pressure, temperature, and velocity maps. The velocity map shows the magnitude of the velocity vector in color and its direction with arrows. The assumed axisymmetry yields $v_y = 0~\mathrm{km~s^{-1}}$. The semi-minor and semi-major axes of the forward shock are $x_{\mathrm{sh}} = 34~\mathrm{kpc}$ and $z_{\mathrm{sh}} = 85~\mathrm{kpc}$, respectively. Alt text: Simulation results in the computational domain.
• Figure 2: Schematic illustration of the mock observation setup. The outermost rectangular prism indicates the full observation volume, which has an observation area of $120~\mathrm{kpc} \times 180~\mathrm{kpc}$ on the $XZ$ plane. The $Y$-axis is in the direction of the LOS. The blue cylinder shows the simulated region when the jet direction ($z$-axis) is parallel to the $Z$-axis. The viewing angle is defined by the angle between the $z$-axis and $Y$-axis. The red dashed ellipse represents the shocked ICM (cocoon) at $\theta = 90^\circ$, while the red solid ellipse corresponds to the shocked ICM at $\theta \neq 90^\circ$. Alt text: Schematic illustration. The cylinder has a height of 180 kpc, with its top and bottom faces coincident with the top and bottom $XY$ planes of the rectangular prism. The cylinder’s central axis passes through the origin $O$.
• Figure 3: XRISM mock spectra for the shocked ICM alone. Panels show results for viewing angles $\theta = 0^\circ$ (left), $53^\circ$ (middle), and $90^\circ$ (right). The upper part of each spectrum represents the normalized counts and best-fit bapec model curve, while the lower one represents the residuals divided by errors. Alt text: Tree line graphs. Horizontal axis shows the energy from 6.26 to 6.37 kilo electron volt.
• Figure 4: XRISM mock spectra including both shocked and unshocked ICM. From left to right, the panels correspond to viewing angles $\theta = 0^\circ$, $53^\circ$, and $90^\circ$. The top column shows the full band ($2-10~\mathrm{keV}$), the center focuses on He-like Fe emission ($6.26-6.37~\mathrm{keV}$), and the bottom focuses on H-like Fe emission ($6.56-6.63~\mathrm{keV}$). The upper part of each spectrum represents the normalized counts and best-fit bapec model curve, while the lower one represents the residuals divided by errors. The emission lines broaden as the viewing angle increases, indicating larger velocity dispersions. Alt text: Nine line graphs.