Black Holes Stamp Out Rampant Star Birth with a Gassy Solution

Astronomers may have uncovered a unique process for how the universe's largest elliptical galaxies continue making stars long after their peak years of star birth. Researchers have discovered a brilliant group of blue stars forming along the jets of active black holes.

"Think of the gas surrounding a galaxy as an atmosphere," said Megan Donahue, lead author of the study, in a news release. "That atmosphere can contain material in different states, just like our own atmosphere has gas, clouds and rain. What we are seeing is the process like a thunderstorm. As the jets propel gas outward from the center of the galaxy, some of that gas cools and precipitates into cold clumps that fall back toward the galaxy's center like raindrops."

These "raindrops" eventually cool enough to become star-forming clouds of gas. With the help of Hubble, the researchers were able to directly see these showers of star formation.

Interestingly, though, the raining gas is reduced by the black hole. While some outwardly flowing gas will cool, the black hole heats the rest of the gas around a galaxy. This prevents the whole gaseous envelope from cooling more quickly. The entire cycle is a self-regulating feedback mechanism, like the thermostat on a house's heating and cooling system, because the "puddle" of gas around the black hole provides the fuel that powers the jets. If too much cooling happens, the jets become more powerful and add more heat. If the jets add too much heat, they reduce their fuel supply and weaken.

This particular discovery explains the mystery of why many elliptical galaxies in the present-day universe don't have a higher rate of star birth. Now, researchers know that their development is partially halted by a cycle of heating and cooling that takes place.

The findings are published in The Astrophysical Journal and Monthly Notices of the Royal Astronomical Society.

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