Spiky dark matter around supermassive black holes

Title: The first robust evidence showing a dark matter density spike around the supermassive black hole in OJ 287

Authors: Man Ho Chan, Chak Man Lee

First Author’s Institution: Department of Science and Environmental Studies, The Education University of Hong Kong, Hong Kong, China

Status: Available on arXiv

The nature of dark matter is still a big mystery to astronomers and physicists. Most of the evidence of 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 haloes in which galaxies are embedded. 

It is predicted that the supermassive black hole (SMBH) 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 re-distributed to form a ‘spike’ in the density distribution where dark matter annihilation and could result in strong emission of gamma rays. However, we have not detected any strong gamma-ray emission near an SMBH, including the one in our galaxy- Sgr A*.

In today’s paper, 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 OJ287. A binary consists of two black holes in orbit around each other, emitting gravitational waves (GW). The binary will lose energy through gravitational waves, the orbit decays, and the orbital period decreases. The authors use the use orbital data of  OJ 287, to show the effects of dark matter spike on the orbital period of the binary, providing strong evidence of the existence of a dark matter density spike around an SMBH.

How is the SMBH binary losing its energy?

Assuming the energy loss in the binary OJ 287 is dominated by GWs, the measured orbital period decay rate suggests the two SMBHs 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 GW 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 SMBH. 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 ‘spike-y’ 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 that agrees with the predicted value from a theoretical SMBH growth model (see Figure 1).

Figure 1: The energy loss rate of the binary with only gravitational radiation accounted for (green). The red curve shows the total energy loss rate with both gravitational waves and dynamical friction. The shaded region is the constrained total rate from observations. The blue dotted line indicates the spike index as predicted by an adiabatic SMBH growth model. The dynamical friction from a dark matter density spike can account for the large orbital decay period observed. Source: Figure 1 in the paper

It is possible that this discovery could be a coincidence, as the spike index derived from the SMBH growth model assumed may not apply to a binary like OJ 287. Numerical simulations suggest that an SMBHB can scatter dark matter particles, decreasing dark matter density and reducing the spike-index. Therefore the actual spike-index could be lower, but there are other mechanisms that replenish the inner regions with dark matter. The complex interactions between dark matter and binary SMBHs are not well understood, but this result could provide an important clue to understand them better.

Future low-frequency GW observations like LISA can further examine SMBHs similar to OJ 287 to verify this result and help us better understand the interactions between dark matter and SMBHs better. Also, studying the dynamical orbits of stars around a SMBH 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.

Astrobite edited by Megan Masterson

Featured image credit: Pranav Satheesh

About 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.

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