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 .