Supermassive Black Holes Stop Galaxies From Making New Stars

The galaxies of the universe grow up kind of like we do. When they’re young, they’re very active, producing new stars out of clouds of dust at a rapid rate. As they age, the churning slows down and eventually stops. No more new stars. The galaxies settle into some relative peace and quiet.

Astronomers have long sought to figure out what exactly leads to this halt in star formation, a phenomenon known as “quenching.” Most simulations show that supermassive black holes, the mysterious objects at the center of most galaxies, must play a major part. The only way the simulations work—the only way astronomers can explain what they see in galaxies through their telescopes—is if black holes somehow contribute to the quenching.

Black holes are not picky eaters. They gobble up any material—cosmic dust or even stars—that wades into their gravitational grasp. When they feed, the material they devour heats up and glows brightly. Scientists give this stage in the life cycle of a black hole the very Star Trek–y name of active galactic nucleus, or AGN. The AGN, scientists believe, releases a bunch of energy, heating up gas in the galaxy and preventing it from cooling enough to condense into individual, brand-new stars. Eventually, this process extinguishes any new star formation.

But scientists haven’t yet nailed down observational evidence for this effect. “There’s a long battle within the community to try to understand the connection between black holes and star formation,” said Ignacio Martín-Navarro, a postdoctoral researcher at the University of California at Santa Cruz.

To investigate that connection, Martín-Navarro and his colleagues recently analyzed massive galaxies and the supermassive black holes that reside in their centers. They found that black holes are indeed responsible for the quenching of galaxies, and that the mass of a black hole influences how quickly that quenching occurs.

Galaxies with more massive black holes became quenched earlier and faster than did galaxies with less massive black holes. The galaxies with bigger black holes experienced more intense rates of star formation in the beginning, during the early universe, than did the ones with smaller black holes. But galaxies with smaller black holes kept producing stars longer, presumably because their black holes weren’t powerful enough to blow away star-making gas and dust. For this reason, galaxies with smaller black holes tend to have a younger population of stars.

The findings were published Monday in a paper in Nature. Martín-Navarro and his team studied the light from a sample of galaxies by splitting it into different wavelengths—a frequently used method in astrophysics that can reveal important properties of astrophysical objects, like the ages of stars. This allowed them to trace the history of star formation in each galaxy, then compare this information with the masses of the black holes at the galaxies’ centers. The mass of a black hole served as a proxy for the amount of energy that gets spewed into the galaxy. The bigger the black hole, the more energy unleashed, the quicker the quenching.

The findings leave some mysteries untouched. Scientists still don’t know how a hungry, feasting black hole spits out star-quenching energy, a process known as feedback. The new research “does seem to give a firm indication of what the effect of black-hole feedback is, even if it doesn’t fill in all the gaps in our knowledge of exactly how the quenching process works,” said Caleb Scharf, the director of the Columbia Astrobiology Center in New York, in an email.

“Bottom line is that the relationship of black-hole mass to galactic stellar populations has been a problem staring us in the face for well over a decade, so any progress toward better quantifying the phenomenon is extremely welcome,” Scharf said.

Martín-Navarro said other research has attempted to find a connection between star formation and the brightness of an AGN, but with little success, perhaps because star formation occurs over longer periods of time than do AGN bursts, which can turn on and off. “It’s really hard to compare things in really different timescales,” Martín-Navarro said.

Although supermassive black holes measure more than 1 million times more massive than the sun, they are practically tiny compared to the galaxies they live in, Martín-Navarro said. Despite their “small” size, black holes influence how galaxies grow and evolve in big ways. It’s as if a single cell in the human body determined the direction of an entire life. That’s what fascinates Martín-Navarro the most—that something so small can dictate the future of something so much bigger.