March 29, 2014
By finding unusually large black holes in dwarf galaxies, astronomers question the theory where supermassive black holes start as small and merge to become supermassive.
The dwarf galaxy NGC 4214 is located 10 million light years away and is rich with young stars. Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration
Supermassive black holes are thought to be at the very centre of each galaxy. When we say "supermassive", we are talking about objects that are billion times more massive than our Sun. Astronomers don't know how a black hole can get so big. We know that when a massive star uses up its fuel and stops burning, it collapses under its own weight in an event called supernova, leaving behind a black hole.
Those black holes, called stellar black holes, are mostly up to 100 solar masses. Supermassive black holes are much heavier, but it is thought they too start as their stellar cousins and grow with time, possibly through merger with other black holes.
You can imagine two galaxies colliding. Although it sounds dramatic, it is actually a slow and harmless process because galaxies are mostly empty space. If we assume that both galaxies have a black hole at their core, these two black holes could merge into a bigger black hole. With time, as galaxies become bigger and more massive, so do black holes at their hearts.
A new study published in Astrophysical Journal used data from the Wide-field Infrared Survey Explorer (WISE) space telescope and focused on a couple of hundreds of small galaxies also called dwarf galaxies. If supermassive black holes form when galaxies merge, dwarf galaxies with only a couple of billion stars should have stellar black holes at their cores. In a way, they should be similar to early galaxies from our Universe's past galaxies that haven't undergone many mergers.
So, what did the astronomers find? A surprise! These black holes that the team observed turn out to be about 1,000 to 10,000 times the mass of the Sun. This is more than expected. If these numbers are confirmed, they represent the elusive third type of black holes – the intermediate-mass black hole.
"Our findings suggest the original seeds of supermassive black holes are quite massive themselves," says team leader Shobita Satyapal of George Mason University, Fairfax, USA. "We still don't know how the monstrous black holes that reside in galaxy centres formed. But finding big black holes in tiny galaxies shows us that big black holes must somehow have been created in the early universe, before galaxies collided with other galaxies."
While this finding doesn't invalidate theory that supermassive black holes form through a merger of galaxies, it suggests that such mechanism isn't necessary. If they are huge from early days, black holes could become supermassive by feeding off the surrounding gas during billions of years.
Even so, the reason these black holes are so massive even in their infancy remains unclear.