How Black Holes Bring an Abrupt End to Galactic Starbirth

Galaxies die. Sort of. Undisturbed, a galaxy will eventually run out of the gas and dust it needs to produce new stars and planets and stuff in general. It will lose structure, evolving from the wild twirl of a spiral galaxy to a composition more diffuse and generic. Its stars will usually be darker, redder "population II" category stars.

Our own Milky Way galaxy will end somewhat early as it merges with the Andromeda galaxy, with the result being an elliptical galaxy and the slow, lingering death described above: ​"an old population of stars rolling around in a halo of dark matter."

A somewhat elusive factor in the development and eventual end-stages of galaxies are black holes. A ​new paper in the Monthly Notices of the Royal Astronomical Society offers a rare front-row seat to the galactic transition from star-forming blue galaxies (blue being hotter and more energetic) to red galaxies (cool, death-state galaxies).

What the astronomers behind the new paper found is that galactic death can be an abrupt, "premature" end rather than the slow exhaustion one might expect, with no galactic merger required. The killer? Supermassive black holes blasting away all of the galaxies' stellar raw materials before they can go toward actual new stars.

​ This result, according to the astronomers, "is unexpected, [a] quick truncation of star formation in these systems, rather than a simple exhaustion of gas supply."

The mechanism is a bit counterintuitive. When we think of black holes we more usually think of giant cosmic vacuum cleaners, but that's not quite right. A black hole is a two-way street: swallowing up new material is answered by belches of radiation. A black hole of sufficient size and given enough material to consume will kick out radiation with enough force to blast away all of the surrounding gas and dust, leaving scant raw materials. This is what was observed.

"If the gas reservoir fuelling star formation is destroyed effectively instantaneously, then star formation will cease and the galaxy will redden rapidly within 1 [gigayear]," the paper explains. "We conclude that the rapid quenching of star formation in low-redshift [early-type galaxies] is due to the physical expulsion of the entire gas reservoir."

It's a spectacular way to go, at least. The very black hole that's robbing you of your stars is at the same time blasting away the stuff you need to make more. A tragic and poetic end.