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