Investigating the spectral age problem with powerful radio galaxies. (arXiv:1912.01028v1 [astro-ph.HE])

Investigating the spectral age problem with powerful radio galaxies. (arXiv:1912.01028v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Mahatma_V/0/1/0/all/0/1">Vijay H. Mahatma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hardcastle_M/0/1/0/all/0/1">Martin J. Hardcastle</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Croston_J/0/1/0/all/0/1">Judith H. Croston</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Harwood_J/0/1/0/all/0/1">Jeremy Harwood</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ineson_J/0/1/0/all/0/1">Judith Ineson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Moldon_J/0/1/0/all/0/1">Javier Moldon</a>

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.

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.

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