Ravenous black holes make some galaxies shine brightly
Many galaxies have a supermassive black hole lurking at their centre. These are monsters with masses that can reach up to 50 billion times the mass of our sun, and they pull galaxies together with their gravity.
Galaxies with these at their centres are called active active galaxies, because the black holes also emit powerful X-ray radiation.
There are two main types of active galaxies, type I and II. Both types have huge black holes at their centres, called active galactic nuclei (AGN), but the first type shines much more brightly, and this has been a mystery for many years.
It used to be thought the galaxies were generally the same, but type II galaxies are angled slightly differently towards Earth, so that their light is blocked by their own dust. This is called the ‘unified model’.
New research is challenging this idea, suggesting they are in fact fundamentally different. A paper published by the University of Maryland says that type I galaxies actually have much more ravenous black holes at their cores, so they consume matter and spit out radiation at a significantly higher rate.
“The unified model has been the prevailing wisdom for years. However, this idea does not fully explain the differences we observe in galaxies’ spectral fingerprints, and many have searched for an additional parameter that fills in the gaps,” said Richard Mushotzky, a professor of astronomy at UMD and a co-author of the study.
“Our new analysis of X-ray data from NASA’s Swift Burst Alert Telescope suggests that Type I galaxies are much more efficient at emitting energy.”
The researchers looked at 836 active galaxies that each emit high-energy X-rays through NASA’s space telescope. They measured the mass of each of the galaxies’ black holes, and how quickly they are growing using 12 ground-based telescopes.
When the researchers compared the X-rays coming from each, they discovered the black holes in type I galaxies emit much more radiation, regardless of the direction they face Earth.
“Our results suggest this has a lot to do with the amount of dust that sits close to the central black hole,” said Mushotzky, who is also a fellow of the Joint Space-Science Institute. “Type II galaxies have a lot more dust close to the black hole, and this dust pushes against the gas as it enters the black hole.”
The results might lead to the previously overlooked type I galaxies being studied in more detail. When astronomers thought type I and II were basically the same, they focused on type II because they are easier to see than the bright type I galaxies.
“But now, because our results suggest that the two types of galaxies are indeed fundamentally different, it is likely that a lot of researchers will re-evaluate their data and take another look at Type I galaxies,” Mushotzky said.
“By putting us on a path to better understand the differences between the galaxies that host Type I and Type II active nuclei, this work will help us better understand how supermassive black holes influence the evolution of their host galaxies.”