Astronomers find possible signs of black-hole kicks in quasars
A study of about 100,000 quasars reports a link between dust and inferred black-hole speeds, offering a way to test a predicted effect of mergers.
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A study of about 100,000 quasars reports a link between dust and inferred black-hole speeds, offering a way to test a predicted effect of mergers.
Astronomers studying about 100,000 quasars have found a pattern consistent with supermassive black holes receiving a kick after merging. The group with the largest inferred speeds had three times the fraction of dust-obscured quasars found in the slowest group, according to Universe Today’s October 22 report. The association is possible evidence for a population of recoiling black holes, not proof that any particular object has recoiled.
Earlier searches had identified individual candidates, but other explanations could account for their signals. The study, published in Monthly Notices of the Royal Astronomical Society, instead looks for a predicted relationship across a large population.
When two black holes merge, they release gravitational waves—ripples in space-time. Theory predicts that unequal emission in different directions can kick the black hole left by the merger away from the galaxy’s centre. Recoil describes the motion after a merger, not a failure to merge.
In the scenario tested here, the black hole stays within its galaxy and moves back and forth through the centre. It travels fastest near the centre and slows near the ends of its path. Because the centre is also rich in dust, the researchers expect higher speeds to be associated with stronger dust obscuration.
Quasars are exceptionally bright galactic centres powered by supermassive black holes drawing in material. Their brightness lets astronomers study the light from surrounding gas. The researchers used the quasar catalogue from the Sloan Digital Sky Survey’s 16th data release.
For the motion measurement, the team compared emission lines—features in the light produced by gas—from two regions. Gas close to the black hole forms the broad-line region; more distant gas forms the narrow-line region. The study assumes that the nearby gas travels with a recoiling black hole, while the more distant gas remains at the host galaxy’s velocity.
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Motion towards or away from an observer shifts the wavelengths of light, an effect called the Doppler shift. Comparing the shifts of the broad and narrow lines gives an inferred velocity difference along our line of sight. It is not a direct measurement of the black hole’s full speed through its galaxy.
The second clue is reddening. Dust dims and reddens quasar light, so greater reddening relative to similar objects indicates stronger dust extinction. The reported link between larger velocity differences and a higher obscured fraction matches the recoil prediction. The threefold comparison concerns the fraction of quasars obscured by dust, not three times as much dust around each object.
Analysis
The authors acknowledge that winds, viewing angle or other effects could also produce the association. The result therefore supports testing recoil across a population, but does not establish how many of these quasars contain recoiling black holes.
A possible route to stronger evidence is the Laser Interferometer Space Antenna, or LISA, a planned space observatory designed to detect gravitational waves. The researchers expect it to observe supermassive-black-hole mergers. Universe Today’s October 22 report gives LISA’s planned launch as 2035.
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Previously seen only through its infrared heat, the dusty feature is visible in reflected starlight, allowing researchers to test a possible collision origin.
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