Radiative age mapping of the remnant radio galaxy B2 0924+30: the LOFAR
  perspective
  We have observed the steep spectrum radio source B2 0924+30 using the LOw
Frequency ARray (LOFAR) telescope. Hosted by a z=0.026 elliptical galaxy, it
has a relatively large angular size of 12' (corresponding to 360 kpc projected
linear size) and a morphology reminiscent of a remnant Fanaroff-Riley type II
(FRII) radio galaxy. We combine LOFAR and archival images to study the spectral
properties at a spatial resolution of 1'. We derive low frequency spectral
index maps and use synchrotron ageing models to infer ages for different
regions of the source. Using radiative ageing model fitting we show that the
AGN activity ceased around 50 Myr ago. We note that the outer regions of the
lobes are younger than the inner regions which is interpreted as a sign that
those regions are remnant hotspots. We demonstrate the usefulness of maps of
AGN radio remnants taken at low frequencies and suggest caution over the
interpretation of spectral ages derived from integrated flux density
measurements versus age mapping. The spectral index properties as well as the
derived ages of B2 0924+30 are consistent with it being an FRII AGN radio
remnant. LOFAR data are proving to be instrumental in extending our studies to
the lowest radio frequencies and enabling analysis of the oldest source
regions.
