Just as a thermostat controls the temperature in your house, it's been discovered that black holes can help to control the thermal activity of the galaxy in which they live. Their regulating properties explain a perplexing mystery around a number of elliptical galaxies. These galaxies are awash with gas and dust, which is perfect for star formation, but the rate of creation of new stars is lower than expected. The reason why had puzzled scientists.

Two new studies, though, may have an answer. One, published in The Astrophysical Journal, examined elliptical galaxies in the early universe. The other, published in the Monthly Notices of the Royal Astronomical Society, examined elliptical galaxies in the nearby universe. They found that supermassive black holes can regulate the temperature of gas in a galaxy, preventing stars from forming, despite an abundance of fuel being available.

A layer of cool gas, perfect for star formation, surrounds a central black hole of an elliptical galaxy. As stars are created, they also serve as fuel for the black hole, which sucks up dust and gas from the stars. Eventually, the greedy black hole has over-gorged on star dust and it spits out a hot, energetic jet of particles. This jet heats up the surrounding, star-forming puddle so that it is too hot to form new stars. The star birth halts while the gas cools down. When it's cool enough, star formation can restart and the process starts all over again.

The temperature fluctuations of the star-forming gas dictates how much energy the black hole releases, much like a thermostat. This give-and-take relationship the black hole has with the surrounding, cool gas is what regulates star formation and gives these galaxies a lower rate of star birth than expected. 

These theories were used to compare a computer simulation of the gas flow in a galaxy, such as the one described, to actual photographs. The models produced galaxies that look remarkably similar to the real images. This gives scientists a fantastic resource to predict how the galactic structures were formed.

^{Photographs of elliptical galaxies in the ultraviolet range. Hubble/NASA.}

^{Computer simulation of gas formation in elliptical galaxies. M. Donahue and Y. Li/NASA/ESA.}

The process of star formation being stunted begins when an active black hole jet blasts through the galaxy and propels out some gas from the atmosphere. This gas, once separated from the main galaxy, starts to cool down and form cold clumps of gas. These clumps then begin to rain back down into the galaxy; a downpour of star fuel. 

These falling droplets of gas eventually cool down enough to form stars. The clouds of gas require extremely low temperatures of only a few degrees above absolute zero to form stars. If they were hotter, then the vibrations of the atoms would overcome the attractive force of gravity, and the atoms could never start to fuse and power a star. The star formation within these clumps of gas was visible using the Hubble telescope's ultraviolet eye.

This puddle of star-forming gas around the black hole can now serve as fuel for the black hole. This is where the temperature regulation comes in. If lots of stars are born then there's an increase in the amount of fuel for the black hole to guzzle into its accretion disk: a swirling disk of accelerating matter surrounding a black hole before it is sucked in. An increase in fuel leads to an increase in energy, which is released as a jet, which heats up the surrounding star-forming dust. This delays star birth until the gas can cool down and the whole process restarts. 

Lead author of the second study Grant Tremblay, from Yale University, concluded in a statement: “We know that these showers are linked to the jets because they’re found in filaments and tendrils that wrap around the jets or hug the edges of giant bubbles that the jets have inflated.”