The origin of the spectral versus dynamical age discrepancy in radio galaxies

TL;DR

This work uses 3D RMHD simulations of FR-II-like radio lobes to test whether turbulent lobe magnetic fields resolve the mismatch between spectral and dynamical ages. By comparing mixing-only, pressure-turbulence, and magnetic-turbulence emission models, the study shows that particle-age mixing, not magnetic-field structure, is the primary driver of underestimating lobe ages, though magnetic fields enhance spectral steepening and affect morphology, especially in equatorial regions and cluster environments. Tribble-based CI/JP modeling fails to reproduce true ages due to non-uniform magnetic fields and mixed particle populations, implying that standard spectral-age methods systematically underestimate AGN energetics by at least a factor of two. The results highlight the need for aging models that incorporate realistic magnetic-field distributions and particle mixing to accurately interpret radio galaxy spectra and feedback energetics, with implications for galaxy evolution and cluster heating estimates.

Abstract

We investigate the effect of turbulent magnetic fields on the observed spectral properties of synchrotron radio emission in large-scale radio galaxy lobes. We use three-dimensional relativistic magnetohydrodynamic simulations of fast, high-powered jets to study the structure of the lobe magnetic fields and how this structure affects the radio spectrum of the lobes. It has previously been argued that lobe ages inferred from radio spectra underestimate the true ages of radio galaxies due to re-acceleration of electrons in the lobe, mixing of electron populations, or the presence of turbulent magnetic fields in the lobes. We find that the spectral ages with and without accounting for the lobe magnetic field structure are consistent with each other, suggesting that mixing of radiating populations of different ages is the primary cause of the underestimation of radio lobe ages. By accounting for the structure of lobe magnetic fields, we find greater spectral steepening in the equatorial regions of the lobe. We demonstrate that the assumptions of the continuous injection, Jaffe-Perola, and Tribble models for radio lobe spectra do not hold in our simulations, and we show that young particles with high magnetic field strengths are the dominant contributors to the overall radio lobe spectrum.

Figures (14)

• Figure 1: Midplane slices of the $z$-velocity at $y = 0$ for each simulation, with increasing environment density from left to right. Simulation RAG-B327 is shown at $8$ Myr and simulation RAC-B327 is shown at $17$ Myr. The backflow is shown in pink for the upper jets and green for the lower jets.
• Figure 2: 3D illustration of the particle magnetic energy density in simulation RAG-B327 at $8$ Myr, demonstrating the dispersion of magnetic energy in the jet head. Particles with a fluid tracer value $> 0.2$ are plotted with full opacity; particles with lower fluid tracer values are plotted with low opacity to show the lobe shape.
• Figure 3: Top row: maximum absolute z-velocities of particles within $\pm 1$ kpc of the z-axis along the z-axis for each simulation. Bottom row: distance between the disruption point and the end of the lobe over time. Simulations RAG-B16, RAG-B327, and RAC-B327 are shown from left to right. The dashed vertical lines in the top row correspond to the disruption point at each time shown. The crosses in the bottom row correspond to the times shown in the top row.
• Figure 4: Distributions of the field curving length scale for simulations RAG-B16, RAG-B327, and RAC-B327, plotted at times corresponding to a total source length of roughly $160$ kpc ($20$ Myr/$8$ Myr/$17$ Myr respectively). This length scale has been normalised by the cube root of the volume to account for differences in morphology. The jet has been removed from this calculation using a tracer threshold $< 0.1$, and the lobe is defined using a tracer threshold of $> 10^{-4}$.
• Figure 5: Average magnetic field strength in the lobes for simulations RAG-B327 and RAC-B327 over time. The average is a volume-weighted average of the logarithm of the magnetic field strength in the radiating particles in the lobe (i.e, $10^{\sum log(B_{\rm rad}) dV / \sum dV}$).