A thermostat is a device that can regulate the temperature in your living room. If it becomes too cold, the thermostat fires up the central heating. If it becomes too hot, the heating is shut down. Now, using ALMA, astronomers have discovered something like a cosmic thermostat in a distant galaxy.
If the gas in a galaxy is very cold, it can easily collapse into small clouds from which new stars are born. In contrast, very hot gas cannot condense into stars. Without a cosmic thermostat, a galaxy would either produce stars at a huge rate, or no stars at all, depending on the temperature of the interstellar gas.
So how does the gas in a galaxy heats up? One way is through the very massive black hole that sits in the core of almost every galaxy. The black hole gobbles up material from its surroundings. In the process, it produces two powerful jets of particles that are blown into opposite directions. These jets heat up the gas in the outer parts of the galaxy, carving out two huge bubbles with very high temperatures.
Detailed observations of a remote galaxy with ALMA have now revealed filaments of cold molecular gas at the edges of these bubbles. Apparently, the jets also stimulate the production of this cold gas. In the future, it will fall back into the galaxy, where it can be converted into new stars. Part of the cold gas will also fuel the central black hole, making sure that the ‘heating mechanism’ won’t be turned off completely.
So, in effect, the black hole and its jets act like a giant thermostat. The galaxy will never become so hot that new stars can’t form anymore, since the jets also stimulate the production of cold gas. Meanwhile, the formation of new stars will never get completely out of hand: the cold gas fuels the black hole, and the resulting jets heat up the galaxy’s outer parts.
Astronomers think that other galaxies that harbor supermassive black holes have similar ‘thermostats’. So the new ALMA observations help them to better understand the workings of these active galaxies.
ALMA observed the central galaxy of the Phoenix Cluster – a huge assembly of thousands of galaxies at a distance of 5.7 billion light-years in the southern constellation Phoenix. The two jets from the central black hole in the galaxy have carved out bubbles of hot gas in the galaxy’s outer ‘halo’. ALMA has now detected the millimeter radiation from elongated filaments of cold gas, along the edges of the bubbles. The cold gas extends out to distances of some 80,000 light-years from the center, and it’s enough material for the formation of many billions of stars like our own Sun.
The ALMA observations of the central galaxy of the Phoenix Cluster were carried out in June 2014 by a team of astronomers led by Helen Russell of the University of Cambridge in the United Kingdom. Helen’s team consisted of astronomers from the United Kingdom, the United States, Canada, and Australia. The discovery of the filaments of cold gas was described in an article in The Astrophysical Journal, a renowned professional astronomy magazine.