Radio signatures of AGN-wind-driven shocks in elliptical galaxies: From simulations to observations

TL;DR

AGN wind feedback is expected to drive shocks that accelerate electrons and produce synchrotron radio emission in elliptical galaxies. Using MACER-based hydrodynamic simulations plus shock-detection and diffusive shock acceleration post-processing, the study predicts spatially evolving radio signatures and tests the framework on M32, finding a radio component consistent with wind-driven shocks. Key findings include radio luminosities around $10^{29}$ erg s$^{-1}$ Hz$^{-1}$ in the 0.5–5 GHz band, spectra evolving from $\alpha \sim -0.4$ to $-1.4$ with age, and a morphology that starts compact then becomes diffuse; M32 shows a central, compact component matching the R1 source during a modest past wind event. These predictions offer observable diagnostics for AGN feedback with future facilities like SKA, FAST Core Array, and ngVLA, while acknowledging limitations from magnetic-field modeling, particle-injection uncertainties, 2D geometry, and CR transport.

Abstract

We investigate the synchrotron emission signatures of shocks driven by active galactic nucleus (AGN) wind in elliptical galaxies based on our two-dimensional axisymmetric hydrodynamic MACER numerical simulations. Using these simulation data, we calculate the synchrotron radiation produced by nonthermal electrons accelerated at shocks, adopting reasonable assumptions for the magnetic field and relativistic electron distribution (derived from diffusive shock acceleration theory), and predict the resulting observational signatures. In our fiducial model, shocks driven by AGN winds produce synchrotron emission with luminosities of approximately $10^{29}\,\mathrm{erg\,s^{-1}\,Hz^{-1}}$ in the radio band (0.5-5 GHz), with spectral indices of $α\approx -0.4$ to $-0.6$ during the strongest shock phases, gradually steepening to about $-0.8$ to $-1.4$ as the electron population ages. Spatially, the emission is initially concentrated in regions of strong shocks, later expanding into more extended, diffuse structures. We also apply our model to the dwarf elliptical galaxy Messier 32 (M32), and find remarkable consistency between our simulated emission and the observed nuclear radio source, suggesting that this radio component likely originates from hot-wind-driven shocks. Our results indicate that AGN winds not only influence galaxy gas dynamics through mechanical energy input but also yield direct observational evidence via nonthermal radiation. With the advent of next-generation radio facilities such as the FAST Core Array, SKA, and ngVLA, these emission signatures serve as important probes for detecting and characterizing AGN feedback.

Figures (12)

• Figure 1: Shock detection in fiducial model. Color map shows pressure distribution at $t=5.33\,\mathrm{Myr}$ and $r=6.50\,\mathrm{kpc}$. Grayscale regions present detected shock fronts. Black regions mark post-shock zones with nonthermal electron injection.
• Figure 2: AGN winds and shocks evolution in fiducial model. Snapshots are shown at $t=4.69$, $4.75$, $5.50$, $7.01$, and $9.99\,\mathrm{Myr}$ (left to right). Top to bottom panels show density ($n$), pressure ($P$), and radial velocity ($v_r$) in the $r$-$z$ plane. Black regions mark strong shock fronts.
• Figure 3: Synchrotron emission spectra in fiducial model. Colored lines show different times from $t=5.0\,\mathrm{Myr}$ (purple) to $t=9.0\,\mathrm{Myr}$ (yellow). Spectra show broken power-law shapes with high-frequency cutoffs.
• Figure 4: Synchrotron light curves in fiducial model. The blue, orange and green lines present radio emissions in $0.5$, $1.5$ and $5.0\,\mathrm{GHz}$ respectively. The red line show the optical emission in $725.0\,\mathrm{THz}$. The purple line show the X-ray emission in $242.0\,\mathrm{PHz}$. Vertical dashed line at $t=5.5\,\mathrm{Myr}$ marks shock weakening onset.
• Figure 5: Spatial distribution of synchrotron emission properties in fiducial model. Top panels show $5.0\,\mathrm{GHz}$ emissivity ($\mathrm{erg\,s^{-1}\,cm^{-3}\,Hz^{-1}}$). Bottom panels show spectral index $\alpha$ ($L_\nu \propto \nu^\alpha$ between $0.5-5.0\,\mathrm{GHz}$). Times are $t=5.0$, $5.5$, $6.0$, $7.0$, $8.0$, $9.0\,\mathrm{Myr}$ (left to right).