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Title: The First Robust Evidence Showing a Dark Matter Density Spike Around the Supermassive Black Hole in OJ 287
Authors: Man Ho Chan and Chak Man Lee
First Author’s Institution: The Education University of Hong Kong
Status: Published in ApJL
The nature of dark matter is still a big mystery to astronomers and physicists. Most of the evidence for dark matter comes from studying its gravitational effects on visible matter. We now know that dark matter makes up about 85 percent of the total matter in the universe and that it forms dark matter halos in which galaxies are embedded.
It is predicted that the supermassive black hole residing in the center of a galaxy can affect the shape of the dark matter density near the center of the galaxy. The dark matter gets redistributed to form a “spike” in the density distribution, causing the rate of dark matter annihilation to rise and resulting in strong emission of gamma rays. However, we have not detected any strong gamma-ray emission near a supermassive black hole, including the one in our galaxy — Sagittarius A* (Sgr A*).
In today’s article, the authors propose an alternative method to confirm the existence of a dark matter density spike at the center of a galaxy. They use data from a supermassive black hole binary called OJ 287. A binary consists of two black holes in orbit around each other. The binary will lose energy through gravitational waves, causing the orbit to decay and the orbital period to decrease. The authors use orbital data of OJ 287 to show the effects of a dark matter spike on the orbital period of the binary, providing strong evidence for the existence of a dark matter density spike around a supermassive black hole.
How Is the Supermassive Black Hole Binary Losing Its Energy?
Assuming the energy loss in the binary OJ 287 is dominated by gravitational waves, the measured orbital period decay rate suggests the two supermassive black holes will merge in about 12,000 years. The calculated total energy loss rate, however, turns out to be lower than the energy loss rate calculated due to gravitational wave emission. This suggests that there is some other mechanism causing the binary to lose energy.
The authors propose that the dark matter density spike can cause a drag force on the primary supermassive black hole. This drag force, called dynamical friction, can account for the additional energy loss rate. To test this, the authors compute the energy loss rate due to dynamical friction for a dark matter density distribution with a “spike-index” parameter to account for the density spike.
How “Spiky” Is the Dark Matter?
After accounting for the energy loss due to dynamical friction, the authors constrain the spike-index parameter by matching the calculated total energy loss rate to the observed value. This yields a narrow range of spike-index values that agree with the predicted value from a theoretical supermassive black hole growth model (see Figure 1).
Figure 1: The energy loss rate (dE/dt) of the binary with only gravitational radiation accounted for (green; GW only). The red curve shows the total energy loss rate with both gravitational waves and dynamical friction (GW + DF). The shaded region is the constrained total rate from observations. The blue dotted line indicates the spike index as predicted by an adiabatic supermassive black hole growth model. The dynamical friction from a dark matter density spike can account for the large orbital decay period observed. [Chan & Lee 2024]
Future low-frequency gravitational wave observations like those from the Laser Interferometer Space Antenna (LISA) can further examine supermassive black holes similar to OJ 287 to verify this result and help us understand the interactions between dark matter and supermassive black holes better. Also, studying the dynamical orbits of stars around a supermassive black hole like Sgr A* in our own galaxy can be another way of verifying the existence of a dark matter density spike. Future accurate observations of stellar orbits around Sgr A* should help us constrain the dark matter density spike model and shed more light on the properties of dark matter.
Original astrobite edited by Megan Masterson.
About the author, Pranav Satheesh:
I am a second-year graduate student in physics at the University of Florida. My research focuses on studying supermassive binary and triple black hole dynamics using cosmological simulations. In my free time, I love drawing, watching movies, cooking, and playing board games with my friends.