At the very core of almost every galaxy, there is a supermassive black hole. These cosmic giants grow by feeding on the material that gets too close to them, although this doesn't explain how some of them grow so huge. Now, observations using ultra-sensitive radio telescopes have revealed that the manner of feeding is very different from one black hole to the other.
Astronomers found that some galaxies have very gluttonous supermassive black holes. These experience feeding frenzies, accreting as much material as they possibly can very quickly. Others would appear to be more sophisticated, slowly munching on what’s available to them. A third group appears to be on some kind of strict diet, nearly starving of hunger.
The systematic study of supermassive black holes, reported in two papers (here and here) in Astronomy & Astrophysics, highlights both things that we know and what the next generation of observatories will help us understand.
"We are getting more and more indications that all galaxies have enormously massive black holes in their centres. Of course, these must have grown to their current mass. It seems that, thanks to our observations, we now have these growth processes in view and are slowly but surely starting to understand them," co-author Peter Barthel, from the University of Groningen, said in a statement.
The team also saw that some supermassive black holes produce jets during feeding but sometimes they don’t, and this doesn’t seem to depend on the speed of ingestion of material. Another discovery is related to the wider galaxy. Sometimes the accretion phase happens while the host galaxy is experiencing a fresh burst of star formation. Other times there’s a starburst but no feeding black hole, although the team admits that it is difficult to spot due to the star-formation going on all around it.
The study combined several space observatories looking at the sky in visible, infrared, and X-ray wavelengths with some of the current and most impressive networks of radio telescopes such as the Very Large Array, e-MERLIN, and the European VLBI Network.
The use of radio telescopes allowed the researchers to find the quieter supermassive black holes, indicating that the behavior of these gargantuan objects is not a simple binary of feeding or not feeding. Upcoming observatories like the Square Kilometer Array (SKA) will push the envelope of radio observations of black holes, which has proven useful for studying the eating habits of black holes in the distant universe.
"That's good news, because the SKA radio telescopes are coming and they will allow us to look deeper into the universe with even more detail," added lead author Jack Radcliffe from the University of Pretoria.