|Description:|| On top is the image of one of the largest radio galaxies known, NGC 315, as obtained by the WSRT in 1979 (by Bridle, our own Richard Strom and their friends). The total size is about 1.5 Mpc - almost 5 million light years!|
We still don't know exactly how these huge structures form. Everything starts in the black hole (BH) sitting in the centre of the galaxy where the radio jets are originating. The role of gas in the regions around the black hole is crucial. On the one hand, it provides the "fuel" that makes the BH active. But the gas also has a "destructive" influence: the presence of the gas near the BH can affect, and sometimes even inhibit, the evolution of the radio jet by blocking its growth.
We have used the Global VLBI network (EVN+VLBA) to investigate the neutral hydrogen in the central regions of this huge radio galaxy. The bottom-right image covers a region that is just a few tens of light years in size! The white contours represent the distribution of the neutral hydrogen, that surprisingly is elongated in the same direction as the radio jet.
The figure bottom-left shows the central regions of this galaxy as seen by the Hubble Space Telescope. The dust (the dark elongated region) is distributed in a disk-like structure, as it is found in many radio galaxies. Dust and neutral hydrogen are therefore completely different structures.
The neutral hydrogen gas is in the process of falling into the black hole. We can see this from the velocities of the gas compared those of the centre of the galaxy - and therefore it may represent the fuel that makes the central engine going and that we have been looking for.