Black Hole Rain

At the center of nearly every massive galaxy sits at least one supermassive black hole. These black holes weigh in at millions or even billions of times the mass of the Sun. They’re gregarious beasts, surrounded by a swarm of stars and stellar-mass black holes in the galactic equivalent of a bustling metropolis’s downtown.

Occasionally, the small black holes (we’re talking masses of a few to a few dozen Suns) gravitationally nudge each one. Eventually, these nudges can shove some of the objects onto slingshot orbits around the supermassive black hole. But this is an unsteady arrangement, and the little black holes spiral closer to the big one. As the objects swirl down the gravitational drain toward their doom, gravitational waves ripple out and away, carrying away the orbital energy until plink, the little black hole merges with the big one, like a drop falling into a bucket.

These extreme mass-ratio inspirals (EMRIs) are one of the main targets for the upcoming Laser Interferometer Space Antenna (LISA), a European-led project set to launch in the mid-2030s. LISA will detect gravitational waves with frequencies between 0.1 millihertz and 1 Hz, much lower than those that LIGO and its cohorts sense. Tuning into this frequency band will give astronomers access to mergers involving supermassive black holes, which current ground-based detectors can’t hear.

Estimates vary widely as to how many of these little-black-holes-diving-into-big-ones LISA will detect over its planned four-year mission — it might be none to several hundred. A conservative ballpark is a few dozen.  

But this forecasted sprinkle might instead prove to be a steady rain, Smadar Naoz (University of California, Los Angeles) and Zoltán Haiman (Columbia University) write in the October 1st Astrophysical Journal Letters.

Last year, Naoz and her colleagues explored what would happen if a second supermassive black hole entered the picture. Many colossi won’t be alone in their galactic centers: When galaxies merge, the central black hole of the smaller galaxy can sink to the center of the larger galaxy, dragging its cluster of stars and little black holes with it. There, it forms a binary with the other giant.

A pair of supermassive black holes creates a gravitational melee. The cluster around the smaller of the two titans is particularly vulnerable, because general-relativistic effects tend to protect the swarm around the larger one. The giants’ combined influence on the small black holes, and the little black holes’ nudges to each other, kick the stellar-mass objects onto kamikaze orbits, causing a downpour — well, on astronomical time scales — that clears out the cluster in 100 million years. For a billion-solar-mass beast, that might mean a deluge of some 100,000 black holes.

If little black holes continue to form in the cluster (because stars keep being born and dying) or migrate in from elsewhere, the rain could continue even longer.

Stars in the cluster would suffer the same effect. Tossed toward their supermassive black hole, they’ll be shredded, their debris lighting up in a tidal disruption event (TDE). Astronomers have seen around 100 of these events and use them to try to estimate the mass of the black hole that ate the star. The heightened number of TDEs that would arise from the second giant’s effect actually matches the number of TDEs seen in galaxies coming off a star-forming high, many of which have recently merged with another galaxy — and which, therefore, would be exactly where we’d expect to find two supermassive black holes instead of one.

In the new paper, Naoz and Haiman look at what this all means for LISA. They estimate that the combined gravitational effects could produce thousands of EMRI sources, a few hundred of which should ring out loud and clear for LISA during its four-year primary mission. The quieter sources will be a constant pitter-patter on LISA’s cosmic window, some 10 times louder than the weakest signal LISA could detect.

If astronomers can pinpoint the host galaxies of individual EMRI events, then they could check to see if the mass of the supermassive black hole that’s implied by the gravitational-wave signal matches the mass predicted from the galaxy’s properties. If the black hole is smaller than expected, then that would indicate we’ve detected a merger involving the smaller leviathan in a binary.

Even detecting the drum of the black hole drizzle at the predicted level might lend credence to the idea that many of these events are coming from post-starburst galaxies, just as TDEs do.

References:

S. Naoz and Z. Haiman. “The Enhanced Population of Extreme Mass-ratio Inspirals in the LISA Band from Supermassive Black Hole Binaries.” Astrophysical Journal Letters. October 1, 2023.

S. Naoz et al. “The Combined Effects of Two-body Relaxation Processes and the Eccentric Kozai–Lidov Mechanism on the Extreme-mass-ratio Inspirals Rate.” Astrophysical Journal Letters. March 1, 2022.

B. Mockler et al. “Uncovering Hidden Massive Black Hole Companions with Tidal Disruption Events.” arXiv.org. June 12, 2023.

D. Melchor et al. “Tidal Disruption Events from the Combined Effects of Two-Body Relaxation and the Eccentric Kozai-Lidov Mechanism.” arXiv.org. June 13, 2023.