NGC 4869 in the Coma cluster: twist, wrap, overlap and bend

TL;DR

This study uses upgraded GMRT observations to analyze the complex radio morphology and spectral properties of NGC 4869 in the Coma cluster, revealing detailed jet structures, spectral steepening, and possible re-acceleration processes.

Contribution

First detailed multi-frequency radio imaging of NGC 4869 revealing complex morphology, spectral behavior, and jet dynamics in the Coma cluster.

Findings

Identification of five distinct radio regions with morphological features.

Detection of spectral steepening and evidence of re-acceleration.

Observation of a sheath-spine velocity structure and jet-environment interactions.

Abstract

The upgraded Giant Metrewave Radio Telescope (GMRT) has been used to image the head-tail radio galaxy NGC 4869 in the Coma cluster with an angular resolution of 6.26 arcsec at 250-500 MHz and 2.18 arcsec at the 1050-1450 MHz bands. The archival legacy GMRT data have also been used to image the source with angular resolutions from 4.9 to 21.8 arcsec at 610 MHz, 325 MHz, 240 MHz, and 150 MHz. We find that the ~200 kpc scale radio morphology consists of five distinct regions with the clear presence of a pinch at ~1.4 arcmin (= 38.8 kpc) and a ridge at ~3.4 arcmin (= 94.2 kpc) from the head. The sharp bend by ~70 deg at ~3.5 arcmin (= 97 kpc) from the head is possibly due to projection effects. The radio spectra show progressive spectral steepening as a function of distance from the head and there is possibly re-acceleration of the synchrotron electrons and perhaps also magnetic field re-generation in the 6-208 arcsec (= 2.8-96.1 kpc) region of the jet. We report a steep spectrum sheath layer enveloping a flat spectrum spine, hinting at a transverse velocity structure with a fast-moving spine surrounded by a slow-moving sheath layer. We also derive the lifetimes of the radiating electrons and equipartition parameters. A plausible explanation for the characteristic feature, a ridge of emission perpendicular to the direction of tail is the flaring of a straight, collimated radio jet as it crosses a surface brightness edge due to Kelvin-Helmholtz instabilities.