How Supermassive Black Holes Kill Off Old Galaxies

New observations shed light on the mysterious process of galaxy 'quenching.'

by Michael Byrne
May 25 2016, 5:00pm

Image: NOAU

Galaxies get old and die. This is natural. Existing stars within an aging galaxy will eventually become dim and cool, while the galaxy itself—and the other galaxies it's likely to merge with over time—will become diffuse and disordered. The dying galaxy will stop producing new stars in a process known as "quenching."

Galaxy quenching is a fairly mysterious subject. It happens when a particular galaxy has its supplies of cold gases shut off. No more cold gas, no more star formation. What's weird is that galaxies appear to quench much earlier than should be expected given their gas supplies, which means that they don't just run out. Something interferes instead—either the galaxy's cold gas reserves are blocked (a process known as "strangulation"), or those supplies are ejected from the galaxy by some awesome force.

This awesome force has been theorized to be the presence of a supermassive black hole, and computer simulations have supported this idea. Now, as described in a paper published Wednesday in Nature, astrophysicists led by the University of Tokyo's Edmond Cheung have used real-world spectroscopic observations to map the motions of ionized gas across a distant near-dormant galaxy nicknamed Akira.

What they found is that this gas is being pushed outward from Akira's center to the point that it's able to escape the galaxy's gravitational pull. In the process of being ejected, the gas is heated, with the result being the diffuse elliptical structure characteristic of old "early-type" galaxies (as in the image at the top of the post). This is in contrast to the spiral arms of a younger star-producing galaxy, which are the result of cold gases.

Image: Cheung et al

These extreme jets of cosmic wind have a great name: "red geyser." The redness has to do with the lack of blue, young stars within the galaxy being quenched.

"The researchers present observations for one prototypical example of this class of object, in which the activity of the central supermassive black hole seems to have been triggered by interaction with a nearby companion galaxy," explains the astrophysicist Marc Sarzi in an accompanying Nature commentary.

"In this example," Sarzi writes, "such activity is sufficient to sustain the kinetic power of the outflowing wind, which in turn balances the cooling of the warm, ionized gas. Even if the central activity is not enough to rid Akira of its gas, it would provide sufficient energy to stir it by causing turbulence and shocks, and therefore still prevent the gas cooling that leads to star formation."

Akira, which Cheung and co. were able to observe thanks to the Sloan Digital Sky Survey, is but a single example. Its observation, however, was part of a larger program tasked with surveying some 10,000 similar objects. While galaxies like Akira are expected to be in the minority of early-type galaxies, this could be just the tip of the iceberg: It's likely that black hole accretion activity (read: gas heating and barfing) recurs in galaxies as they merge with other galaxies.

"The observed outflows may also help to solve another riddle: the origin of gas in early-type galaxies," Sarzi notes. "One way to tell whether an early-type galaxy acquired gas from other galaxies or from recycled material lost from its stars is to compare the gas's angular momentum with that of the stars'. If the gas was internally produced, it should follow the motions of the stars, whereas if it was externally acquired it could just as well move in the opposite direction."

Finally, one troubling mystery persists. Why do a quarter of all early-type galaxies contain no gas at all? As stars fade and grow old within a galaxy, they should return some gas to its host, but this appears to be not always the case. Red geysers offer the start of an explanation for missing gases, but the picture is far from complete.