LOFAR MSSS: Discovery of a 2.56 Mpc giant radio galaxy associated with a
  disturbed galaxy group
  We report on the discovery in the LOFAR Multifrequency Snapshot Sky Survey
(MSSS) of a giant radio galaxy (GRG) with a projected size of $2.56 \pm 0.07$
Mpc projected on the sky. It is associated with the galaxy triplet UGC 9555,
within which one is identified as a broad-line galaxy in the Sloan Digital Sky
Survey (SDSS) at a redshift of $0.05453 \pm 1 \times 10^{-5} $, and with a
velocity dispersion of $215.86 \pm 6.34$ km/s. From archival radio observations
we see that this galaxy hosts a compact flat-spectrum radio source, and we
conclude that it is the active galactic nucleus (AGN) responsible for
generating the radio lobes. The radio luminosity distribution of the jets, and
the broad-line classification of the host AGN, indicate this GRG is orientated
well out of the plane of the sky, making its physical size one of the largest
known for any GRG. Analysis of the infrared data suggests that the host is a
lenticular type galaxy with a large stellar mass
($\log~\mathrm{M}/\mathrm{M}_\odot = 11.56 \pm 0.12$), and a moderate star
formation rate ($1.2 \pm 0.3~\mathrm{M}_\odot/\mathrm{year}$). Spatially
smoothing the SDSS images shows the system around UGC 9555 to be significantly
disturbed, with a prominent extension to the south-east. Overall, the evidence
suggests this host galaxy has undergone one or more recent moderate merger
events and is also experiencing tidal interactions with surrounding galaxies,
which have caused the star formation and provided the supply of gas to trigger
and fuel the Mpc-scale radio lobes.
