Astronomers from MIT, Columbia University, and other institutions have utilized NASA’s James Webb Space Telescope (JWST) to examine the obscured regions of nearby galaxies and delve into the aftermath of a black hole’s stellar consumption.
In a study published today in Astrophysical Journal Letters, the scientists announce that, for the first time, JWST has detected several tidal disruption events — occasions when a galaxy’s central black hole pulls in a nearby star, unleashing tidal forces that tear the star apart, resulting in a massive release of energy.
Since the 1990s, researchers have identified around 100 tidal disruption events (TDEs), primarily detected as X-ray or optical emissions disseminating across relatively dust-free galaxies. However, as MIT researchers recently reported, numerous star-disrupting occurrences may still exist within the cosmos, potentially “concealed” in dust-laden, gas-shrouded galaxies.
In their earlier research, the team discovered that a significant portion of the X-ray and optical emissions produced by a TDE can be obscured by a galaxy’s dust, making them undetectable by conventional X-ray and optical telescopes. Yet, this same burst of light can warm the surrounding dust and create a new signal in the form of infrared light.
The same team has now employed JWST — the most powerful infrared detector available — to analyze signals from four dusty galaxies, where they suspect tidal disruption events may have transpired. Amidst the dust, JWST uncovered definitive signatures of black hole accretion, a phenomenon where material, like stellar remnants, encircles and ultimately falls into a black hole. The telescope also identified patterns that starkly contrast with the dust surrounding active galaxies, where the central black hole continuously draws in adjacent material.
Collectively, the findings affirm that a tidal disruption event truly occurred in each of the four galaxies. Furthermore, the researchers deduce that these four events were results of not active black holes but rather dormant ones, which experienced minimal to no activity until a star happened to approach.
The new findings underscore JWST’s capability to investigate in detail previously obscured tidal disruption events. They also assist scientists in revealing significant differences between the surroundings of active versus dormant black holes.
“These are the inaugural JWST observations of tidal disruption events, and they appear completely different from anything we’ve observed previously,” says lead author Megan Masterson, a graduate student at MIT’s Kavli Institute for Astrophysics and Space Research. “We’ve discovered that these are indeed driven by black hole accretion, and they don’t resemble the environments around ordinary active black holes. The ability to explore what a dormant black hole environment actually looks like is an exhilarating aspect.”
The study’s MIT contributors include Christos Panagiotou, Erin Kara, Anna-Christina Eilers, along with Kishalay De from Columbia University and collaborators from several additional institutions.
Observing the illumination
The new investigation builds on the team’s prior efforts using another infrared detector — NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission. Employing an algorithm devised by co-author Kishalay De from Columbia University, the team sifted through a decade’s worth of data from the telescope, searching for infrared “transients,” or brief surges of infrared activity from otherwise quiescent galaxies that could indicate a black hole momentarily reawakening to consume a passing star. This exploration uncovered approximately a dozen signals that the group concluded were likely generated by a tidal disruption event.
“In that study, we identified these 12 sources that closely resemble TDEs,” Masterson explains. “We made substantial arguments regarding the energetic nature of the signals, and the galaxies previously appeared inactive, thus these signals must stem from a sudden TDE. However, apart from these minor indications, there was no direct proof.”
With the significantly more sensitive capabilities of JWST, the researchers aimed to identify key “spectral lines,” or infrared light at distinct wavelengths, that would serve as clear markers of conditions linked to a tidal disruption event.
“With NEOWISE, it’s as if our eyes could only perceive red light or blue light, whereas with JWST, we’re recognizing the entire spectrum,” Masterson remarks.
A Genuine signal
In their latest work, the group specifically targeted a spike in infrared, which could exclusively arise from black hole accretion — a process by which material is pulled towards a black hole in a swirling disc of gas. This disc generates an immense quantity of radiation that is so intense that it can displace electrons from individual atoms. Particularly, such accretion processes can expel multiple electrons from neon atoms, and the resulting ion can transition, emitting infrared radiation at a precise wavelength detectable by JWST.
“Nothing else in existence can energize this gas to these energies, other than black hole accretion,” Masterson asserts.
The researchers sought this telltale signal in four of the 12 TDE candidates they had previously identified. The four signals consist of: the closest tidal disruption event detected to date, situated in a galaxy roughly 130 million light-years away; a TDE that also exhibits a flash of X-ray light; a signal potentially generated by gas swirling at incredibly high velocities around a central black hole; and a signal that included an optical flash, which scientists had initially believed to be a supernova, or the collapse of a dying star, rather than a tidal disruption event.
“These four signals were as close to a guaranteed discovery as we could attain,” Masterson states. “But the JWST data enabled us to confirm that these are indeed authentic TDEs.”
When the team directed JWST towards the galaxies with each of the four signals, in a program designed by De, they found that the distinct spectral lines emerged in all four sources. These measurements validated that black hole accretion transpired in all four galaxies. However, the question lingered: Was this accretion a temporary occurrence, triggered by a tidal disruption and a black hole that briefly awakened to consume a passing star? Or was this accretion a more stable characteristic of “active” black holes that are perpetually operating? In the latter scenario, it would be less plausible that a tidal disruption event occurred.
To distinguish between the two scenarios, the team utilized the JWST data to detect an additional wavelength of infrared light, indicating the presence of silicates, or dust within the galaxy. They then mapped this dust in each of the four galaxies and compared the patterns with those of active galaxies, known to contain clumpy, donut-shaped dust formations surrounding the central black hole. Masterson noted that all four sources exhibited notably different patterns compared to typical active galaxies, implying that the black hole at each galaxy’s center is not normally active, but dormant. The researchers concluded that if an accretion disk formed around such a black hole, it must have resulted from a tidal disruption event.
“Altogether, these observations suggest that the only interpretation for these flares is TDEs,” Masterson concludes.
She and her collaborators aim to uncover numerous more previously concealed tidal disruption events, using NEOWISE, JWST, and other infrared telescopes. With sufficient detections, they propose TDEs could serve as effective tools for probing black hole characteristics. For example, details regarding how much of a star is shattered, and the speed at which its debris is accreted and consumed, can disclose fundamental attributes of a black hole, such as its mass and spin rate.
“The actual mechanism of a black hole consuming all that stellar material takes considerable time,” Masterson elaborates. “It’s not an instantaneous event. And hopefully, we can begin to explore the duration of this process and what that environment appears like. No one knows yet because we are only beginning to discover and analyze these occurrences.”
This research received partial support from NASA.