The center of our galaxy contains a supermassive black hole called Sagittarius A star, or Sag A*, which has a mass four million times the mass of our Sun. The black hole's powerful gravitational pull should compress the dark matter in its vicinity into a dense "spike" that would emit intense gamma rays.
A new study of this black-hole enhancement of dark matter annihilation has been made by Brian Fields, Stuart Shapiro and Jessie Shelton from the Department of Physics and the Department of Astronomy at Illinois.
These scientists show for the first time that the predicted strength of a dark matter gamma-ray signal from a canonical black hole spike of Sag A* actually far exceeds what has been observed through the Fermi Gamma-ray Space Telescope. While the observed gamma rays may still originate from dark matter annihilations in the galactic halo near the center, the spike would add more than is observed.
Written by Siv Schwink
(L-R) Professor of Astronomy and Physics and Chair of the Department of Astronomy Brian Fields, Professor of Physics and Astronomy Stuart Shapiro, and Assistant Professor of Physics Jessie Shelton. Background image, a Fermi Gamma-Ray Space Telescope image of the excess gamma rays at the center of the Milky Way Galaxy, NASADark matter is one of the greatest mysteries facing scientists today. It's estimated that nearly 85 percent of the universe is made up of dark matter, but the only concrete evidence for its existence has been indirect, through gravitation. In the last few years, mounting evidence has suggested that high-energy gamma rays imaged at the center of our Milky Way Galaxy could include a signal from dark matter. New theoretical findings out of the University of Illinois at Urbana-Champaign address this suggestion.
NASA's Fermi Gamma-ray Space Telescope imaged high-energy gamma rays coming from the Galactic Center. Detailed analysis of these images has revealed that the gamma-ray signal is brighter than can be accounted for by known gamma-ray sources. Debate among astronomers and physicists proceeds over whether dark matter annihilations might account for the excess gamma rays imaged at the center of our Milky Way Galaxy.
The center of our galaxy contains a supermassive black hole called Sagittarius A star, or Sag A*, which has a mass four million times the mass of our Sun. The black hole’s powerful gravitational pull should compress the dark matter in its vicinity into a dense "spike" that would emit intense gamma rays.
A new study of this black-hole enhancement of dark matter annihilation has been made by Brian Fields, Stuart Shapiro, and Jessie Shelton from the Department of Physics and the Department of Astronomy at Illinois.
GC gamma-ray flux. Points are the Fermi source 2fGL J1745.6-2858 near Sgr A*. Curves show the DM annihilation signal from the spike and halo within 1°. Top: the canonical adiabatic spike plus halo (γc=1.26, γsp=2.36). Middle: same as top, but for γsp=1.8. Bottom: same as top, but for γsp=1.5. The dashed black line shows the inner 1° of the halo alone. Shaded bands vary the halo index between 1.2 ≤ γc ≤ 1.3.
These scientists show for the first time that the predicted strength of a dark matter gamma-ray signal from a canonical black hole spike of Sag A* actually far exceeds what has been observed through the Fermi Gamma-ray Space Telescope. While the observed gamma rays may still originate from dark matter annihilations in the galactic halo near the center, the spike would add more than is observed.
In it, the team outlines plausible alternative galactic histories that would predict different spike signals. They propose alternative explanations for this weaker-than-expected gamma-ray signal, even if it does originate from dark matter: for example, the black hole's growth and dark matter spike might be disturbed during the evolution of the Milky Way.
Shapiro remarks, “This line of inquiry has huge implications for our understanding of the history of the Galactic Center and the particle physics of dark matter annihilations. The search is on at Illinois to identify further clues to solve this puzzle in our cosmic backyard and to help unlock the mysteries of dark matter.”