# FR-II radio galaxies at low frequencies II: spectral ageing and source dynamics

## TL;DR

This study uses low-frequency radio observations to analyze the spectral ageing and dynamics of two FR II radio galaxies, providing new measurements of their physical extents, ages, and energetics, and revealing complex spectral features.

## Contribution

It offers improved spatial and energetic measurements of FR II galaxies and highlights the importance of considering absorption and non-homogeneous acceleration in spectral analysis.

## Key findings

- Spectral ages of ~77 and 85 Myr for the galaxies.
- Disparity between spectral and dynamical ages by a factor of ~2.
- Hotspot spectra show unexpected sharp discontinuities.

## Abstract

In this paper, the second in a series investigating FR II radio galaxies at low frequencies, we use LOFAR and VLA observations between 117 and 456 MHz in addition to archival data to determine the dynamics and energetics of two radio galaxies, 3C452 and 3C223, through fitting of spectral ageing models on small spatial scales. We provide improved measurements for the physical extent of the two sources, including a previously unknown low surface brightness extension to the northern lobe of 3C223, and revised energetics based on these values. We find spectral ages of $77.05^{+9.22}_{-8.74}$ and $84.96^{+15.02}_{-13.83}$ Myr for 3C452 and 3C223 respectively suggesting a characteristic advance speed for the lobes of around one per cent the speed of light. For 3C452 we show that, even for a magnetic field strength not assumed to be in equipartition, a disparity of factor of approximately 2 exists between the spectral age and that determined from a dynamical standpoint. We confirm that the injection index of both sources (as derived from the lobe emission) remains steeper than classically assumed values even when considered on well resolved scales at low frequencies, but find an unexpected sharp discontinuity between the spectrum of the hotspots and the surrounding lobe emission. We suggest that this discrepancy is due to the absorption of hotspot emission and/or non-homogeneous and additional acceleration mechanisms and, as such, hotspots should not be used in the determination of the underlying initial electron energy distribution.