How do ORCs form?
The new radio observations also revealed a tiny dot in the very center of the nebulous ORC. That small source, which also pops out in images taken at other wavelengths, is a galaxy. Although not confirmed, scientists are fairly certain that this is the galaxy responsible for giving birth to the ORC.
Modeling of the ORC’s radio emissions also showed that the structure is nearly spherical. It only appears slightly ring-shaped because there is more material along our line of sight at the ORC’s edges. Moreover, thanks to the MeerKAT observations, scientists were able to reconstruct the magnetic fields within the object. As it turns out, those fields run along the ring's edges, as if a shock wave had compressed the expanding ORC.
Taken together, these findings have led astronomers to suspect that something happened to the ORC’s central galaxy about a billion years ago, causing it to send out a powerful shock wave through the tenuous intergalactic medium that surrounds it. As electrons wind around the magnetic fields threading through space, they emit radio waves, producing the shiny ring we see today.
"The image fidelity and its details really allow [us] to rule out a number of possibilities for the interpretation of these objects, which could not be ruled out before," Tiziana Venturi, director of the Radio Astronomy Institute in Italy, tells Astronomy. Venturi was not involved in the new study, but she was part of the team that discovered the first ORCs.
Because the history of the central galaxy seems to be the key unraveling the origin story of ORCs, the authors have come up with three plausible scenarios — though the list is not necessarily exhaustive.
First, a collision between two supermassive black holes inside the galaxy could have produced energetic shock waves. Such collisions have never been observed before, but theory predicts that the most hefty supermassive black holes grew via mergers of smaller black holes. "If that is what it turned out to be, that would be an important way of estimating the rates at which the supermassive black holes merge," says Norris.
Alternatively, the galaxy might have experienced a period of intense star formation. Energetic newborn stars, as well as exploding dying stars, would push away gas and particles in the form of galactic wind, resulting in an expanding shock wave.
The third option is that astronomers might be looking down the barrel of a jetted outflow from the active core of the galaxy.