February 27, 2014
Elliptical galaxies are filled with old, red stars because they have no cold gas to make new stars from. New study suggests that the supermassive black holes at cores of these galaxies may be the reason the gas is too hot.
NGC 1399, a giant elliptical galaxy. Blueish hue around the centre is hot, ionized hydrogen gas. It reaches temperatures up to tens of millions Kelvins. Credit: Digitised Sky Survey/NASA Chandra/Very Large Array
There are three types of galaxies in the universe – spiral, elliptical and irregular. Spiral galaxies, like our Milky Way, are rich with gas, blue and home to many newly-formed stars. Unlike them, elliptical galaxies are dubbed "red and dead" – they lack cold gas needed for star formation, so they are filled with old, small red stars that give these galaxies their colour.
Cold gas is crucial for star formation because only then is it dense enough to come together and have gravitational pull that strong enough to attract more gas and eventually form a star. The fact that elliptical galaxies don't have cold gas has been puzzling us for some time now.
A new study led by Norbert Werner, Stanford University, Standford, USA, suggests that supermassive black holes, believed to be at the centre of all galaxies, are the reason for gas not being able to cool enough for new stars to be born. "These galaxies are red, but with the giant black holes pumping in their hearts, they are definitely not dead," said Werner.
By observing eight elliptical galaxies with ESA's Herschel Space Observatory, Werner and his team have found out that although two of these galaxies have no cold gas at all, six of them in fact are rich with it. Problem is, this cold gas wasn't cold enough.
The cold gas has been detected with Herschel in far-infrared, emitted by carbon ions and oxygen atoms. For hot gas observation, optical and X-ray data has been used.
"The contrasting behavior of these galaxies may have a common explanation: the central supermassive black hole," says study's co-author Raymond Oonk, ASTRON Institute for Radio Astronomy, Dwingeloo, the Netherlands.
Two galaxies without any cold gas have very active, supermassive black holes at their cores. They are accreting the surrounding gas which then accelerates to tremendous speeds and heats up in the process. As it reaches the black hole, some of it is ejected into space in form of energetic jets that reheat the cold gas or push it out of the galaxy. These black holes have to be enormous in order to heat the gas in an entire galaxy.
In other six galaxies, study authors believe that black holes are also heating up the gas, just not as much as in the other two because they are not as active. So, in the end, they do have stockpiles of cold gas, but it is not cold enough for star formation.