Investigating the spectral age problem with powerful radio galaxies

TL;DR

This study investigates the longstanding spectral age problem in powerful radio galaxies by combining high-resolution radio and X-ray observations with analytic models, revealing that spectral ages are significantly underestimated due to magnetic field assumptions and turbulent mixing.

Contribution

The paper provides a comprehensive observational and modeling approach to quantify the spectral age discrepancy, highlighting the impact of magnetic field assumptions and turbulence in radio galaxy lobes.

Findings

Spectral ages are underestimated by at least a factor of two.

Equipartition magnetic fields can underestimate spectral ages by up to ~20.

Turbulent mixing likely causes the remaining age discrepancy.

Abstract

The 'spectral age problem' is our systematic inability to reconcile the maximum cooling time of radiating electrons in the lobes of a radio galaxy with its age as modelled by the dynamical evolution of the lobes. While there are known uncertainties in the models that produce both age estimates, `spectral' ages are commonly underestimated relative to dynamical ages, consequently leading to unreliable estimates of the time-averaged kinetic feedback of a powerful radio galaxy. In this work we attempt to solve the spectral age problem by observing two cluster-centre powerful radio galaxies; 3C320 and 3C444. With high-resolution broad-band Karl G. Jansky Very Large Array observations of the radio sources and deep XMM-Newton and Chandra observations of their hot intra-cluster media, coupled with the use of an analytic model, we robustly determine their spectral and dynamical ages. After finding self-consistent dynamical models that agree with our observational constraints, and accounting for sub-equipartition magnetic fields, we find that our spectral ages are still underestimated by a factor of two at least. Equipartition magnetic fields will underestimate the spectral age by factors of up to ~20. The turbulent mixing of electron populations in the radio lobes is likely to be the main remaining factor in the spectral age/dynamical age discrepancy, and must be accounted for in the study of large samples of powerful radio galaxies.