The jets of a supermassive black hole are one of the cosmos’ greatest spectacles — and also one of its greatest mysteries. These beams of ionized matter burst forth from the cores of galaxies at speeds approaching that of light. How those black holes harness and focus that energy remains hotly contested.
In recent years, astronomers have succeeded in finding galaxies with central black holes that have just recently turned on, in hopes of finding leftover clues to what triggered them. And at this month’s American Astronomical Society (AAS) winter meeting in National Harbor, Maryland, researchers reported an intriguing example — a massive supermassive black hole launching a jet from the center of a galaxy with visible spiral structure.
The object, named J0742+2704, is classified as a quasar, a type of active galaxy with jets so brilliant that they outshine the rest of the galaxy, causing it to resemble a point-like star in images. (The name “quasar” comes from the descriptive term “quasi-stellar object.”) Galaxies with such powerful jets are typically old, elliptical, and featureless, having lost all their youthful spiral structure in messy mergers with other galaxies. Yet, when astronomer Olivia Achenbach analyzed data from the Hubble Space Telescope, her processing clearly revealed spiral arms winding around it — evidence that it has not yet been through the gravitational turmoil of a major merger.
“At first, I thought I completely messed up,” said Achenbach, a midshipman at the U.S. Naval Academy, during a Jan. 13 press conference. She was advised on the research by astronomer Kristina Nyland of the Naval Research Laboratory. “We both predicted [that] with such a large supermassive black hole at the center, we’d see an elliptical galaxy.”
The find is a new entry in an ongoing debate over how supermassive black holes and their host galaxies grow, evolve, and interact. The dominant theory — that black holes start feeding and their jets turn on when galaxies merge — is called the major merger model. “To have such a clear, unambiguous result that seems to defy the major merger model is extremely exciting,” says Rachel Cionitti, an astronomer and graduate student at the University of Missouri in Kansas City, who was not part of the study.
A cosmic dance
In the major merger model, most galaxies start out with spiral forms, but eventually lose them as they grow and merge. A major merger triggers star formation, but also churns the galaxies into large elliptical galaxies. This process also causes stars, gas, and dust to fall into the central black holes, which are also growing and merging. Then, when the black holes begin to feast, they trigger activity like jets and bright radio emission from just outside their event horizons.
J0742+2704 complicates that picture. It has a large central black hole with the mass of 400 million Suns. But its jets have only turned on recently — they did not appear in similar observations 20 years ago. And the spiral arms found by Achenbach are a clear indication that it has not been through a major merger. “No galaxy coming out of a merger would be an undisturbed spiral,” says Cionitti.
Instead, the jets may be triggered by interactions with a smaller companion galaxy. In the Hubble image, a ring-shaped galaxy appears to the right of J0742+2704, with the smaller companion lying between them. There are also hints of a potential tidal tail — a stream of stars being ripped away as J0742+2704 and the companion interact.
Cionitti alsothinks interactions are the key to what is lighting up the jets. She says the “little baby galaxy” is probably “shaking up the gas” in the galaxy, causing it to lose momentum and fall toward the center. Still, she notes, “there are still a lot of open questions about that mechanism.”
Strengthening the case, says Achenbach, are observations from the Low-Frequency Array (LOFAR), a radio telescope network that spans Europe. They revealed evidence of previous jet activity — large lobes of radio emission on either side of the galaxy. These may be remnants of previous jets that plowed into intergalactic material; these jets eventually faded away and shut off before turning back on within the last two decades.
The picture that Achenbach and Nyland propose is that the jets of J0742+2704 are being triggered repeatedly as this companion orbits its larger neighbor — what Achenbach dubbed a “cosmic dance” — with no major merger required.
Major merger questions
Cionitti presented work at the AAS meeting that also calls into question the major merger model.
When galaxies begin to merge, they trigger bursts of star formation. These young hot stars emit mostly blue light, but the major merger model suggests they are initially obscured by kicked-up dust. Seen through this dusty veil, the galaxy appears redder. But after a galaxy’s jets turn on, the winds from the galactic core clear out that dust, and the galaxy gradually becomes bluer as more light from the newborn stars are revealed.
But Cionitti and her colleagues found that for galaxies at the same distance, galaxies brighter in redder light tend to be more massive than those brighter in bluer light. This is the opposite of what would be expected if galaxies evolve from red to blue over the course of a merger.
Overall, Cionitti says there is “just a huge wealth of confusion” among astronomers trying to determine how active galaxies and their central holes co-evolve (or not). To Cionitti, Achenbach’s work is exciting because it shows that there are multiple “evolutionary paths, that there’s not just one” for how galaxies’ central black holes become active — what astronomers call an active galactic nucleus (AGN).
“I just think there’s a lot more going on under the hood of the AGN-quasar umbrella than we’re currently aware of,” says Cionitti.
