The Halo Rings

The Halos from which this series receives its title are creations of the Forerunner. There are in fact seven Halos scattered throughout the galaxy. They are both research centers as well as the ultimate weapon versus the Flood. These giant ring-like creations contain simulated atmosphere and earth like surfaces.
However, they are also very deadly creations. Activating all seven Halos (which can only be done from The Ark) will cause explosions that wipe out all life in the galaxy. The reason for these creations were to allow for a victory over the Flood. If the Flood had spread to a point of little to no control then by wiping the galaxy of all life the flood would starve and die, of course this would only be used as a last resort.
The Forerunners are said to have initiated this sequence as their final stand against the Flood before transcending. However, I would like to save further analysis of this specific topic for the discussion concerning The Ark.
For that reason I will now move on to detail the importance of using Halos as the shape of these constructs. According to E. H. Ramsden, while the Halo appears in Christian art, it would seem to have no particular significance in Christianity.1 Despite this, it is obvious that many Christians have come to accept Halos as being symbolic of angels and saints.
It is for this reason that I think we see these constructions taking the shape of Halos. If the Forerunners are to be viewed as angelic, it seems to me that it is only fitting for the symbols of their existence and protection to be in the shape of Halos.