The giant radio galaxy NGC 6251 (above) stretches for more than a degree across the sky, or around 6 million light years in diameter. Seen here by LOFAR at 150 MHz, some of the faintest, oldest emission is revealed for the first time.
Giant radio galaxies are believed to be formed from relativistic jets of matter and energy, emanating from the central regions of active galactic nuclei. Synchrotron radiation, often polarized, from the radio jets and lobes of these objects will be studied with LOFAR. The low-energy electrons responsible for the low-frequency emission can propagate large distances from their origins in the central core or lobe hotspots, and large radio cocoons are expected to surround many objects. Using high angular resolution observations with the full international LOFAR array should help us to learn more about the low-energy electron population in these objects, and hopefully to better understand the acceleration mechanisms which produce the relativistic electrons responsible for synchrotron emission. The high degrees of polarization of giant radio galaxies makes them ideal polarization calibrators for the observation of weaker sources.
A supermassive black hole (SMBH) sits at the centre of this elliptical galaxy 243.4 Mpc (79.4 million light years) away from us. Whilst only a handful of pixels in the Sloan Digital Sky Survey, the elliptical galaxy hosting this SMBH is responsible for the enormous structure seen by the LOFAR telescope (seen in blue). Jets of relativistic particles produce radio emission as they travel outwards in the magnetic fields produced by the SMBH, projecting giant radio lobes on the sky with an end-to-end distance of 8.7 million light years from our vantage point on Earth. We call these objects Giant Radio Galaxies. Its shape is rather curious - whilst the main lobes travel out in straight lines, off shoots appear to occur closer to the host galaxy. These could be regions of lower pressure, where it becomes easier for the lobes to expand into.
The inset shows a zoom in of the host system, where a handful of galaxies are gravitationally interacting to produced a disrupted stellar distribution. The blue colour shows what the LOFAR telescope sees at higher resolution in this insert. Whilst all the emission seen by LOFAR is at 150 MHz in these images, the red colour is what we can see with the GMRT at 610 MHz. This gives us a slightly better resolved picture of the jet originating from the host galaxy. However, at these higher frequencies, we recover much less of the extended emission due to the fact that the synchrotron radiation becomes weaker.
NGC 6251 is a giant radio galaxy stretching across a degree of the sky. The supermassive black hole at the centre of the host galaxy is is powering huge jets that push out into the intra galactic medium terminating in large lobed structures. New telescopes such as LOFAR allow us to see the older particles and reveal structures in the radio galaxy that were previously undetectable. The image shows the LOFAR image NGC 6251. The counter jet is clearly visible as is a large extension to the northern lobe.