Astronomers from MIT, Columbia University and other institutions used NASA's James Webb Space Telescope (JWST) to peer through thick layers of dust in nearby galaxies to study the consequences of black holes devouring stars.Unlike active galaxies, which are constantly devouring nearby material, these black holes are dormant, active only briefly to devour stars unlucky enough to pass by.
Astronomers from MIT, Columbia University and other institutions used NASA's James Webb Space Telescope to observe the scene after the black hole's star feast through the dust of nearby galaxies. Image source: NRAO/AUI/NSF/NASA
For the first time, researchers have detected multiple tidal disruption events using the James Webb Space Telescope, according to a new study published July 24 in The Astrophysical Journal Letters. These rare cosmic phenomena occur when a black hole at the center of a galaxy sucks in nearby stars and tears them apart with powerful tidal forces, releasing huge bursts of energy.
Since the 1990s, scientists have documented about 100 tidal disruption events (TDEs), mostly in galaxies with less dust around them, making the resulting X-rays, or visible light, easier to observe. However, recent findings from MIT researchers suggest that there may be many more similar star-destroying events that are simply obscured by thick clouds of gas and dust, making them invisible to conventional telescopes.
In previous work, the team found that most of the X-rays and visible light emitted by tidal disruption events are obscured by galactic dust and therefore unobservable with conventional X-ray and optical telescopes. But that same light can heat surrounding dust and generate a new signal in the form of infrared light.
Now, the same researchers have used the James Webb Space Telescope (JWST)—the world's most powerful infrared detector—to study signals from four dusty galaxies that they suspect have undergone TDEs. In the dust, JWST detected clear traces of black hole accretion. Accretion is the process by which material, such as fragments of a star, orbits and eventually falls into a black hole. The telescope also detected distinct patterns from the dust surrounding active galaxies, where the central black hole continuously absorbs surrounding material.
Taken together, these observations confirm that tidal disruption events are indeed occurring in these four galaxies. Furthermore, the researchers concluded that the four events were not caused by active black holes, but dormant black holes that showed little activity until a star happened to pass by.
The new results highlight the James Webb Space Telescope's potential to delve deeper into otherwise elusive tidal disruption events. They also help scientists reveal key differences in the environments surrounding active and dormant black holes.
"This is the first time that the James Webb Space Telescope has observed tidal disruption events, and they look completely different from anything we've seen before," said the study's lead author Megan Masterson, a graduate student at MIT's Kavli Institute for Astrophysics and Space Studies. "We learned that these events are indeed driven by black hole accretion, and they don't look like the environment around a normal active black hole. It's exciting that we can now study what a dormant black hole environment really looks like."
The study's MIT authors include Christos Panagiotou, Erin Kara, and Anna-Christina Eilers, along with Columbia University's Kishalay De and collaborators from multiple other institutions.
The new study expands on previous research the team conducted using another infrared detector, NASA's Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission. Using an algorithm developed by co-author Kishalay De of Columbia University, the team scoured a decade of data from the telescope, looking for infrared "transients," brief spikes in infrared activity from otherwise quiet galaxies that could signal a black hole briefly waking up and swallowing a passing star. The search turned up about a dozen signals, which the team determined were likely produced by tidal disruption events.
"Through this study, we identified 12 sources that look like tidal disruption events," Masterson said. "We made a lot of arguments that these signals were very energetic and these galaxies didn't appear to be active before, so these signals must have been caused by a sudden tidal disruption event. But other than these small fragments, there was no direct evidence."
Using the James Webb Space Telescope's increased sensitivity, researchers hope to be able to discern key "spectral lines," or specific wavelengths of infrared light, that could clearly identify conditions associated with tidal disruption events.
"With NEOWISE, it's like our eyes can only see red or blue light, whereas with JWST we can see the full rainbow," Masterson said.
In the new study, the team looked specifically for infrared wave peaks that could only be produced by black hole accretion - the process by which matter is drawn toward a black hole in a circulating disk of gas. Black hole accretion produces large amounts of radiation strong enough to liberate electrons from individual atoms. Specifically, this accretion process can release multiple electrons from neon atoms, and the resulting ions can jump, releasing specific wavelengths of infrared radiation that can be detected by the James Webb Space Telescope.
"Nothing else in the universe other than black hole accretion can excite this gas to such high energies," Masterson said.
The researchers looked for this confirming signal in four of the 12 previously identified candidates for tidal disruption events. The four signals include: the closest TDE detected so far, located in a galaxy about 130 million light-years away from us; a TDE accompanied by an X-ray burst; a signal that may be produced by gas rotating at extremely high speeds around the central black hole; and a signal accompanied by an optical flash, which scientists previously suspected was a supernova explosion or the collapse of a dying star rather than a TDE.
"These four signals are the closest we can find to a definite event," Masterson said. "But data from the James Webb Space Telescope help us firmly assert that these are true tidal disturbance events."
When the team used a specially designed program to point the James Webb Space Telescope at the four signaling galaxies, they observed distinct spectral lines from all four sources. These measurements confirm that black hole accretion occurs in all four galaxies. But the question remains: Is this accretion a temporary phenomenon triggered by tidal disruption and a black hole that briefly wakes up and swallows a passing star? Or is this accretion a more permanent feature of "active" black holes, which are always active? If the latter is the case, a tidal disruption event is less likely to occur.
To distinguish between the two possibilities, the team used data from the James Webb Space Telescope to detect another wavelength of infrared light, which suggests the presence of silicates or dust in the galaxy. They then created images of this dust in four galaxies and compared them with images of active galaxies. Active galaxies are known to have clump-like, donut-shaped dust clouds surrounding their central black holes. Masterson observed that these four dust sources showed very different patterns compared to typical active galaxies, suggesting that the black hole at the center of each galaxy is usually not active but dormant. If an accretion disk formed around such a black hole, the researchers concluded, it must be the result of a tidal disruption event.
"Taken together, these observations suggest that the only possibility for these flares is tidal disruption events," Masterson said.
She and her colleagues plan to use NEOWISE, JWST and other infrared telescopes to discover more previously undetected tidal disruption events. They say that if enough of them are detected, TDEs could serve as effective probes of black hole properties. For example, how much a star is torn apart, and how quickly its fragments are accreted and consumed, can reveal fundamental properties of the black hole, such as its mass and rotational speed.
"The actual process of a black hole swallowing all the star's material takes a very long time," Masterson said. "This is not an instantaneous process. Hopefully we can start to explore how long this process takes and what the environment was like at that time. No one knows because we are just starting to discover and study these events."
Compiled from /scitechdaily