Black Hole Bullies Shut Down Star Formation in Their Galaxies

A supermassive black hole in the heart of a galaxy is the ultimate 800-pound gorilla of astrophysics. Not only do the most active ones suck in material and hide it away, but their accretion disks also blast strong quasar winds out to space. Those winds push things around, and in the process, they sometimes shut down star formation.

There’s one monster out there—SBS 1408+544—that astronomers observed for over eight years. It was part of a long-term survey called the Black Hole Mapper Reverberation Mapping Project. That’s a Sloan Digital Sky Survey program that tracks activity at the hearts of these active black-hole-powered quasars. For SBS 1408+544, a team led by Wisconsin astronomers Catherine Grier and Robert Wheatley studied the wind-accelerated gas flowing from the quasar’s heart.

A wind like the one they studied could influence a galaxy’s evolution. It all depends on how massive the black hole is, and the nearby galactic environment. “If they’re energetic enough, the winds may travel all the way out into the host galaxy, where they could have a significant impact,” Wheatley said.

It’s long been known that there’s a correlation with how dense a galaxy’s central region is and star formation dropoff. It appears there’s some sort of “feedback” from the core to the outlying regions and winds are likely part of that. They could spur bursts of new star formation. Or, if they’re too strong, they’ll bully the gas clouds away, choking off star formation. Not only does that change the stellar populations, but it also changes the future evolution of the galaxy.

Charting Winds from the Quasar

Supersonic winds blasting out from a supermassive black hole indicate something energetic is going on there. That makes sense, since the black hole actively eats whatever crosses its event horizon from the accretion disk, according to Grier. “The material in that disk is always falling into the black hole, and the friction of that pulling and pulling heats up the disk and makes it very, very hot and very, very bright,” she said. “These quasars are really luminous, and because there’s a large range of temperatures from the interior to the far parts of the disk, their emission covers almost all of the electromagnetic spectrum.”

Astronomers take advantage of the spectrum to study the environment around the black hole, as well as the temperatures, velocities, and chemistry of the region. Grier and Wheatley’s observations used the Black Hole Mapper project to track the winds. Those gusts contain superheated streams of gaseous carbon. The fingerprint of the carbon shows up in the spectra of the quasar as black lines. They indicate the element is absorbing light from the quasar. The position of the lines tells astronomers what element is doing the absorbing.

However, here’s what’s interesting—the lines appear “shifted” to a different spot on the spectrum in each observation. That indicates motion. “That shift tells us the gas is moving fast, and faster all the time,” says Wheatley. “The wind is accelerating because it’s being pushed by radiation that is blasted off of the accretion disk.”

The Long-term Effects of Black Hole Bullying

One side-effect of the high wind gusts from the supermassive black hole is on the star-forming regions of a galaxy. Those regions are generally molecular clouds that need to clump together to form stars. Winds from supernovae (for example) or even those from the heart of the galaxy could be enough to jump-start the star birth process. However, if they’re too strong, they’ll blow the gas clouds apart or even destroy them. That effectively shuts down the stellar creches before they get started. The long-term effect is to cut down the amount of star formation. That also means the contributions those future stars would have made to the galaxy’s chemical enrichment never occurred.

Stellar formation shutoff or enhancement aren’t the only things astronomers study in these active galactic nuclei. The Black Hole Mapper project uses optical spectroscopy to understand galactic supermassive black holes and the winds that flow from their accretion disks. Astronomers need to know their masses, how they accrete and grow, and how they change over time. Quasars offer the perfect targets since they’re incredibly bright. That allows good spectral mapping to take place.

The BHM project lets astronomers study these objects over specific periods (called time-domain studies), to see how they change. This includes the winds that flow from these regions and their effects on surrounding objects such as molecular clouds. Characterizing and understanding the winds and their effects on the larger environment around the active galactic nucleus gives astronomers more insight into the big picture of how galaxies evolve over time.

For More Information

Wind From Black Holes May Influence Development of Surrounding Galaxies
The SDSS-V Black Hole Mapper Reverberation Mapping Project: C iv Broad Absorption Line Acceleration in the Quasar SBS 1408+544
The Sloan Digital Survey and About the Black Hole Mapper