Credit: NASA's Goddard Space Flight Center/Scott Noble; simulation data, d'Ascoli et al. 2018
Title: Uncovering Hidden Massive Black Hole Companions with Tidal Disruption Events
Authors: Brenna Mockler et al.
First Author’s Institution: The Observatories of the Carnegie Institution for Science & Department of Physics and Astronomy at the University of California, Los Angeles
Status: Accepted to ApJ
Two Is Company
Today, astronomers believe that nearly every galaxy hosts a supermassive black hole at its center. In addition, galaxies are thought to grow through mergers, in a process known as hierarchical growth.
Essentially, smaller galaxies smash together to form a larger galaxy,
and this process repeats many times as the universe evolves. When two
galaxies hosting supermassive black holes merge, the black holes should
sink to the center of the new galaxy rather rapidly, where they could
start orbiting each other as a supermassive black hole binary.
These binaries are therefore a natural consequence of this picture of
hierarchical galaxy evolution and should be a relatively common
occurrence in the universe.
However, finding supermassive black hole binaries has been rather difficult with current instrumentation and technology. A supermassive black hole makes itself known when it accretes gas from its surroundings, becoming a luminous active galactic nucleus. As two accreting black holes get closer and closer together, our telescopes become incapable of resolving them as two individual active galactic nuclei. There are other ways to infer that a binary exists when the black holes are close together, but these methods can be tricky — either the signals could also be produced by some other astrophysical phenomenon, or they take decades to confirm. The next generation of gravitational wave detectors, like the Laser Interferometer Space Antenna, will surely help, but we’d still like to be able to look for supermassive black hole binaries in the next decade or more before these detectors are built!
However, finding supermassive black hole binaries has been rather difficult with current instrumentation and technology. A supermassive black hole makes itself known when it accretes gas from its surroundings, becoming a luminous active galactic nucleus. As two accreting black holes get closer and closer together, our telescopes become incapable of resolving them as two individual active galactic nuclei. There are other ways to infer that a binary exists when the black holes are close together, but these methods can be tricky — either the signals could also be produced by some other astrophysical phenomenon, or they take decades to confirm. The next generation of gravitational wave detectors, like the Laser Interferometer Space Antenna, will surely help, but we’d still like to be able to look for supermassive black hole binaries in the next decade or more before these detectors are built!
Introducing the Star of the Show
One of the best ways to observe something we can’t see is by looking
for its interactions with things we can see. Today’s authors study the
interplay of a supermassive black hole binary with stars in the centers
of galaxies, highlighting this as a potential way to uncover these
binaries. To start, let’s consider just a single supermassive black hole
and throw a star at it. Most of the time, this star will orbit the
black hole, just like our planets orbit the Sun. However, in some cases,
when the orbit is eccentric
enough, the star can get just a bit too close to the supermassive black
hole, leading to the star’s demise. This measure of “too close” is set
by the distance at which the star’s self-gravity can no longer hold itself together against the tidal forces of the black hole, and the star gets ripped to shreds. We call this phenomenon a tidal disruption event, and these events release a huge amount of energy from a previously quiet black hole.
Okay, but how do we get stars onto these elliptical orbits so that they’re disrupted? And how often does this happen? Many research articles have investigated these questions (check out some of the many Astrobites written on tidal disruption events), both from a theoretical and observational perspective. It turns out that one way to get stars onto these highly elliptical orbits is to scatter them off of other stars (through a process called two-body relaxation). This process is relatively rare; both theory and observations agree that the rate for tidal disruption events around single black holes is somewhere around one every 104–105 years (per galaxy).
But what happens when we deposit these stars around a supermassive black hole binary? The authors of today’s article investigate this very question. In particular, they investigate the interaction of stars around the smaller of the two black holes (see Figure 1 for a schematic of this set up).
Okay, but how do we get stars onto these elliptical orbits so that they’re disrupted? And how often does this happen? Many research articles have investigated these questions (check out some of the many Astrobites written on tidal disruption events), both from a theoretical and observational perspective. It turns out that one way to get stars onto these highly elliptical orbits is to scatter them off of other stars (through a process called two-body relaxation). This process is relatively rare; both theory and observations agree that the rate for tidal disruption events around single black holes is somewhere around one every 104–105 years (per galaxy).
But what happens when we deposit these stars around a supermassive black hole binary? The authors of today’s article investigate this very question. In particular, they investigate the interaction of stars around the smaller of the two black holes (see Figure 1 for a schematic of this set up).
Figure 1: Cartoon schematic of the setup considered in today’s article. We have two supermassive black holes with masses m1 and m2, with m1 < m2. The authors investigate stellar orbits around the smaller black hole (m1). Credit: Mockler et al. 2023
And Now Three’s a Crowd
To explore the effects of a binary supermassive black holes on the
rate of tidal disruption events, the authors perform dynamical
simulations of the three-body problem we just set up above. They focus
in particular on the effects of the eccentric Kozai–Lidov (EKL) mechanism,
which is a dynamical effect in a three-body system that allows the
eccentricity and inclination of the outer binary (i.e., the star and the
lower-mass black hole) to oscillate. EKL oscillations can lead to
extreme eccentricities, which is a great way to make tidal disruption
events happen! To explore the effects of EKL on the system, the authors
test different combinations of binary masses and stellar density
profiles. There’s a large range of possible parameters in this problem,
so they limit their tests to those in which the timescale for the EKL
mechanism is the shortest dynamical timescale (which leads to EKL being
the dominant mechanism driving the system’s evolution).
The simulations revealed that there should be a burst of tidal disruption events lasting 1–100 million years, depending on the exact simulation parameters. During this time period, the tidal disruption event rates greatly exceed that expected from two-body relaxation, which is what sets the rates of these events in single supermassive black hole systems. However, if the stars near the black hole are not replenished after this period, either from star formation near the galactic nucleus or some dynamical effects, then the rates of EKL-driven tidal disruption events drop to less than those of two-body relaxation. This is highlighted in Figure 2, which shows the EKL-driven tidal disruption event rate as a function of time in these dynamical simulations. So, our best hope for catching tidal disruption events around the smaller black hole in a binary pair is relatively quickly after it enters the binary.
Figure 2: Rate of tidal disruption events occurring around the smaller supermassive black hole as a function of time in the simulations. The shaded blue regions represent different masses of the smaller supermassive black hole, each of which is 10 times less massive than the larger supermassive black hole. The shaded grey region shows the observed rate of optically selected tidal disruption events, and the grey hashed region denotes the rate of tidal disruption events in “post-starburst” (PSB) galaxies (galaxies seen about a few millions of years after a recent burst of star formation, which is often driven by a merger). Finally, the dashed and dotted lines show the rates of tidal disruption events from two-body relaxation (i.e., ordinary tidal disruption events around a single supermassive black hole). The simulations show a burst of tidal disruption events relative to the two-body relaxation rate for the first 1–100 million years. Adapted from Mockler et al. 2023
The simulations revealed that there should be a burst of tidal disruption events lasting 1–100 million years, depending on the exact simulation parameters. During this time period, the tidal disruption event rates greatly exceed that expected from two-body relaxation, which is what sets the rates of these events in single supermassive black hole systems. However, if the stars near the black hole are not replenished after this period, either from star formation near the galactic nucleus or some dynamical effects, then the rates of EKL-driven tidal disruption events drop to less than those of two-body relaxation. This is highlighted in Figure 2, which shows the EKL-driven tidal disruption event rate as a function of time in these dynamical simulations. So, our best hope for catching tidal disruption events around the smaller black hole in a binary pair is relatively quickly after it enters the binary.
Figure 2: Rate of tidal disruption events occurring around the smaller supermassive black hole as a function of time in the simulations. The shaded blue regions represent different masses of the smaller supermassive black hole, each of which is 10 times less massive than the larger supermassive black hole. The shaded grey region shows the observed rate of optically selected tidal disruption events, and the grey hashed region denotes the rate of tidal disruption events in “post-starburst” (PSB) galaxies (galaxies seen about a few millions of years after a recent burst of star formation, which is often driven by a merger). Finally, the dashed and dotted lines show the rates of tidal disruption events from two-body relaxation (i.e., ordinary tidal disruption events around a single supermassive black hole). The simulations show a burst of tidal disruption events relative to the two-body relaxation rate for the first 1–100 million years. Adapted from Mockler et al. 2023
Finding Supermassive Black Hole Binaries with Tidal Disruption Events
To end, the authors leave us with a potential way to search for
supermassive black hole binaries using these tidal disruption events.
This method relies upon the fact that the two black holes in the binary
will dominate two different observable properties. On one hand, the
gravitational potential of the galactic nucleus where these two black
holes reside will be dominated by the larger of the two black holes,
meaning that host galaxy properties that scale with the galaxy’s central
black hole mass will be set by this larger black hole. On the other
hand, the light curve from a given tidal disruption event is set by the
mass of the black hole that the star is accreting onto, which in this
case is the smaller black hole. This means that if we see a tidal
disruption event that seems to be coming from a small black hole, but
it’s actually happening in a galaxy that’s far too big to host such a
black hole, then there’s strong evidence that this could be a
supermassive black hole binary system! And so, while three may be a
crowd, this unlucky star will actually shed some light on its black hole
companions as it leaves the party.
Original astrobite edited by Mark Dodici.
About the author, Megan Masterson:
I’m a 3rd-year PhD student at MIT studying transient accretion events around supermassive black holes, including tidal disruption events and changing-look active galactic nuclei. I primarily use X-ray observations to observe the inner accretion flow of these transients, but I am also interested in multi-wavelength follow-up to get the full picture of these fascinating systems. In my free time, I enjoy hiking and watching soccer.
Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
