Swift and the Star-Eaters of the Cosmos: A New X-ray Census Reveals Secrets of Supermassive Black Holes Burning, Burning Brightly
I once was blind, but now I see. Astronomers who study the supermassive black holes beaming brightly at the centers of galaxies will be singing this line from “Amazing Grace” now.
Researchers using the Swift orbiting observatory demonstrated a way to detect virtually every supermassive black hole actively feeding on gas in nearby galaxies.
These galactic grazers are known in astrogeekspeak as “active galactic nuclei.” Active indeed! Imagine a mob of King Henry the 8ths tearing into railcars full of mutton, spewing gristle and gnawed leg bones in all directions. Active galactic nuclei — let’s just call them AGNs — are messy, voracious eaters, too, but they spew energy instead of table scraps. They can radiate more energy than all the billions of stars in the galaxy combined.
Blogolicious Active Galactic Nuclei facts
- Large galaxies contain supermassive black holes, with a million to a billion times the sun’s mass.
- About 1% of the black holes are active galactic nuclei (AGNs), feeding on gas and emitting vast energy.
- A survey by NASA’s Swift satellite finds that a quarter of AGNs are within merging galaxies or close pairs.
- This is strong evidence for the theory that mergers trigger active galactic nuclei.
But I digress: back to Swift.
A team of scientists observed the local universe with Swift’s Burst Alert Telescope (BAT), which sees in so-called hard X-rays. Those are the energetic rays that zip through your body during a medical scan. And what the scientists observed is that about a quarter of AGNs are in merging galaxies or close pairs of galaxies grabbing gravitationally at each other.
[Imagine loud "ah hah!" sound emanating collectively from the world's galactic astronomers.]
Theorists have always said that most AGNs are probably powered by mergers. As the galaxies come together, it stirs up gas, which feeds the black holes. Now we have the “hard” (X-ray) evidence, and 6 years worth.
Once upon a time, many astronomers would have said that AGNs were fueled by stars being torn part near the supermassive black hole. This provides years worth of fuel. The shredded star spirals down into the hole to near-light speed, releasing gobs and gobs of energy.
This hypothesis is not off the royal banquet table just yet. Some AGNs may, in fact, be star gobblers. But the Swift result sure makes it look like many — maybe most? — AGNs trace to mergers.
With the Swift survey, astronomers have the cosmic equivalent of a well-done national census. Like good census data, it allows us to spot statistical trends and convince ourselves they are real. In this case, the trend is that many galaxies with AGNs are merging or closely interacting.
In contrast, observing galaxies at energies lower than hard X-rays can throw off a census. That’s because lower-energy light can be absorbed by all the gas and stuff tossed around by a merger. As a result, you may miss some of the AGNs. Also, the AGNs bright optical emission can get lost in the overall glow of stars in the galaxy.
Look for the findings in the June 20 issue of The Astrophysical Journal Letters, if you care to graze on some real astrophysics.
ROLL THE CREDITS . . . Gogblog gratefully tips his supermassive hat to the study’s lead author, Michael Koss, a graduate student at the University of Maryland in College Park. He explained the science to Gogblog and reviewed the post for accuracy. Other members of the team include Richard Mushotzky and Sylvain Veilleux at the University of Maryland, College Park, and Lisa Winter at the Center for Astrophysics and Space Astronomy at the University of Colorado in Boulder.
OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center.