[Paper Review] Electron Ageing and Polarization in Tailed Radio Galaxies

L. Feretti, G. Giovannini+4 more|arXiv (Cornell University)|Dec 3, 1997

Radio Astronomy Observations and Technology3 citations

TL;DR

This study investigates electron ageing and magnetic field ordering in tailed radio galaxies using multi-frequency radio observations from the Effelsberg 100-m telescope and Westerbork Synthesis Radio Telescope. It finds that spectra steepen with distance due to synchrotron energy losses, polarization increases along tails indicating improved magnetic field alignment, and electron lifetimes imply the need for reacceleration or bulk motions, with magnetic fields oriented longitudinally in the tails.

ABSTRACT

High-frequency observations of the tailed radio galaxies IC 310, NGC 1265, 3C 129, and 3C 465 have been performed with the Effelsberg 100-m telescope. For the radio galaxies IC 310, NGC 1265 and 3C 465, radio data obtained at low frequencies with the Westerbork Synthesis Radio Telescope are also available. These new radio data allow us to map the extended structure of the radio galaxies and obtain spectral and polarization information in the outermost regions. The multi-frequency spectra were used to study the synchrotron ageing of relativistic electrons with increasing distance from the active nucleus. We found that the spectrum in each radio galaxy progressively steepens with distance, and at each location it is steeper at high frequencies. The spectra are fitted by models involving synchrotron energy losses and the critical frequency is obtained at increasing distance from the core. Assuming that the magnetic field is the equipartition value, we obtain the radiating electron lifetimes and consequently their drift velocities. Our results imply the existence of reacceleration processes or bulk motions along the tails. The polarization data at 10.6 GHz give information on the intrinsic degree of polarized flux and the orientation of the magnetic field. We find that the polarization percentage increases along the tails, reflecting an increase of the degree of ordering of the magnetic field. The magnetic field in the tails is longitudinal.

Motivation & Objective

• To investigate the spectral evolution of relativistic electrons in the extended radio tails of radio galaxies due to synchrotron energy losses.
• To determine the degree of magnetic field ordering and its spatial variation along the tails using polarization measurements.
• To estimate electron lifetimes and infer the presence of reacceleration or bulk motions based on observed spectral steepening.
• To assess the role of equipartition magnetic fields in interpreting electron ageing and polarization in tailed radio galaxies.

Proposed method

• Performed high-frequency (10.6 GHz) observations with the Effelsberg 100-m telescope on IC 310, NGC 1265, 3C 129, and 3C 465.
• Combined with low-frequency data from the Westerbork Synthesis Radio Telescope to map extended radio structures and derive multi-frequency spectra.
• Fitted observed spectra with models incorporating synchrotron energy losses to determine the critical frequency and electron lifetimes.
• Used polarization data at 10.6 GHz to measure the intrinsic degree of polarized flux and infer magnetic field orientation.
• Assumed equipartition magnetic field strength to estimate electron radiative lifetimes and infer drift velocities.
• Analyzed spatial trends in spectral index and polarization to assess field ordering and tail dynamics.

Experimental results

Research questions

• RQ1 How does the spectral index evolve with distance from the nucleus in tailed radio galaxies, and what does this imply for electron ageing?
• RQ2 What is the spatial distribution of magnetic field ordering along the radio tails, as indicated by polarization measurements?
• RQ3 To what extent do observed electron lifetimes suggest the need for reacceleration or bulk motions in the tails?
• RQ4 How does the magnetic field geometry (e.g., direction and alignment) vary along the radio tails?
• RQ5 What is the relationship between spectral steepening and increasing polarization in the outer regions of the radio galaxies?

Key findings

• The spectra of IC 310, NGC 1265, and 3C 465 progressively steepen with increasing distance from the core, indicating synchrotron energy losses of relativistic electrons.
• The critical frequency shifts to lower values with distance, confirming that higher-energy electrons radiate away energy faster and are depleted in outer regions.
• Polarization percentages increase along the tails, indicating a growing degree of order in the magnetic field structure.
• The magnetic field in the tails is predominantly longitudinal, aligned with the direction of the radio lobes.
• Electron lifetimes derived under equipartition assumptions imply that electrons drift slowly, suggesting the need for reacceleration or bulk motions to sustain the observed emission.
• The observed spectral and polarization trends are consistent with a combination of electron ageing and magnetic field ordering in the extended radio structures.