Supermassive black hole controls star birth

In a galaxy far, far away there is a maternity ward for stars.

At the centre of the ‘Phoenix Cluster’ of galaxies some 5.7 billion light years from earth, stars are forming at an unusually stupendous rate.

Every year between 500 and 800 stars are born whereas in our own galaxy, the Milky Way, only a couple of new stars form each year.

And what is more odd, the stars are being born despite there being a ‘supermassive’ black hole at the centre of the cluster that would normally prevent stars from growing. That’s because black holes shoot out hot gas that disperses the cold clouds that help form stars.

But University of Melbourne observational cosmologist Dr Christian Reichardt says the Phoenix Cluster’s supermassive black hole is acting like a cosmic thermostat to regulate the growth of stars and galaxies.

“It could also explain how the most massive black holes were able to both suppress run-away starbursts and regulate the growth of their host galaxies over the past six billion years or so of cosmic history.”

Dr Christian Reichardt has been fascinated by the Phoenix Cluster for the last six years after being part of the team that discovered it in 2009. He studies the origin, evolution, and eventual fate of the universe using the most powerful telescopes in the world.

“The Phoenix Cluster is located 5.7 billion light-years from Earth, and derives its name from its location in the constellation of the Phoenix, a minor constellation in the Southern skies,” Dr Reichardt says.

“A super-massive black hole lives inside the galaxy at the centre of the Phoenix Cluster.

“While these black holes are common, the situation in the Phoenix cluster is intriguing because it is undergoing an extraordinary burst of new stars. In other clusters, the black holes usually prevent new stars from being born.”

Dr Reichardt is part of an international research team led by the University of Cambridge that have just revealed more about the special relationship between the black hole and star production in a paper published in the Astrophysical Journal. They have been using the world’s most powerful radio telescope, the Atacama Large Millimetre Array (ALMA) that is located in the Chilean mountains.

“As the black hole devours nearby gas, it fuels a pair of powerful jets that erupt from the black hole in opposite directions into intergalactic space,” he says.

Regulating galaxy growth

“Astronomers refer to this type of black-hole powered system as an active galactic nucleus, or AGN,” explains Dr Reichardt.

“These jets create expanding bubbles of hot gas, blowing apart the cold gas clouds that can collapse to form the next generation of stars.

“But our latest ALMA observations have revealed long filaments of cold molecular gas condensing around the outer edges of the radio bubbles.

These filaments extend up to 82,000 light-years from either side of the AGN. They collectively contain enough material to make about 10 billion suns.

The results surprised the research team because it appears that the supermassive black hole is regulating the growth of the galaxy by blowing bubbles and heating the gases around it. Remarkably, it also is cooling enough gas to feed itself.

“These results help us understand the workings of the cosmic “thermostat” that controls the launching of radio jets from the supermassive black hole.’’

The team hopes to discover whether this newly identified supply of cold, dense gas could eventually fuel future star birth as well as feed the black hole itself.

“Whatever happens, the Phoenix cluster is 5.7 billion light years away so it will not affect Earth, but will be an amazing phenomenon for us to observe and understand.”

Banner Image: Still from an animation showings how large numbers of stars form in the Phoenix Cluster. It begins by showing several galaxies in the cluster and hot gas (in red). This hot gas contains more normal matter than all of the galaxies in the cluster combined, and can only be detected with X-ray telescopes like Chandra. The camera then flies in towards the large elliptical galaxy at the centre of the cluster. The hot gas near this galaxy is giving off copious amounts of X-rays and cooling quickly over time, as shown by the change to a blue colour. This cooling causes gas to flow inwards along filaments and form huge numbers of stars when it continues to cool. Picture: NASA/CXC/A. Hobart