Mysterious Signal comes from very Old Stars at the centre of the Milky Way

Go to the profile of Oscar Macias
Mar 12, 2018
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The paper published in Nature Astronomy can be found here:

The gravity from ordinary matter – stars, interstellar gas and dust - alone is not sufficient to keep Galaxies from falling apart. An abundance of roughly five times as much Dark Matter as ordinary matter is needed to hold Galaxies together.  It is now well established that the Galaxy which we inhabit, the Milky Way, is embedded in an enormous spherically shaped clump or “halo” of this strange substance.

Structure of the Milky Way Galaxy
Illustration of the structure of the Milky Way: The bright visible disk of stars and gas is embedded in a large, roughly spherical dark matter halo. Densities of both visible and dark matter rise towards the Galactic centre, where there is also an extended bulge of stars. [Image credit:L. Jaramillo & O. Macias/Virginia Tech.]

Unlike most ordinary matter, Dark Matter particles are invisible to optical telescopes, but may smash against each other and radiate gamma-ray photons a billion times more energetic than visible light. Such a glow of Dark Matter emission is expected to be brightest at the Centre of the Milky Way due to the very high concentration of Dark Matter particles in that region of the sky. 

The Fermi Gamma-Ray Space Telescope (launched in 2008 and still in operation) has allowed scientists to have the clearest ever view of the gamma-ray sky at the few giga-electron volt energy range. In one of the most interesting recent developments in our quest for Dark Matter, several independent studies of Fermi Satellite data uncovered a mysterious gamma-ray signal originating from the Galactic Centre (hereafter Fermi Galactic Centre excess) that was easily accommodated by some of the best theoretically motivated Dark Matter models.

The Fermi Gamma-Ray Space Telescope
The Fermi Gamma-Ray Space Telescope scans the full sky every 3 hours detecting photons about 1 billion times more energetic than visible light. [Images Credit: NASA/Aurore Simonnet, Sonoma State University. Photo-illustration: Sandbox Studio]

The centre of our Galaxy may be rich in Dark Matter, but it is also rich in stars. As shown in the artistic illustration below, the stars of the Milky Way are distributed in three main structures: a central bulge, a dominant disk and a diffuse stellar halo. N-body simulations show that the Milky Way bulge was formed through an entangled process of stellar orbit evolution - disk stars originally orbiting in the plane of the Galaxy slowly transition to bulge orbits via dynamical instabilities. The instantaneous spatial distribution of the bulge stars results in bulge shapes that can be distinctively non-spherical.  

The stellar structures of the Milky Way Galaxy.
Artist's illustration of the main stellar structures of the Milky Way Galaxy. The bulge stars are distinctively non-spherical. Our new study compares maps of the various stellar populations of the Galactic bulge against the spatial distribution of the Fermi Galactic centre excess data. [Credit: L. Jaramillo & O. Macias/Virginia Tech.]

Near infrared data from the Diffuse Infrared Background Experiment instrument on board the COBE satellite first established the non-spherical nature of the Milky Way bulge. This was later confirmed by stellar count maps of different surveys (e.g. 2MASS, OGLE-II and VVV survey). The image below displays new diffuse infrared measurements taken with NASA's Wide-field Infrared Survey Explorer (WISE), which reveals the non-spherical morphology of the bulge stars. 

Milky Way Galaxy as seen by the WISE infrared telescope
Image of the central regions of the Milky Way taken with the NASA infrared space telescope WISE. The spatial distribution of the bulge stars differs from a spherical shape. Our analysis uses stellar maps obtained with WISE and COBE/DIRBE data, among others. [Image credit: NASA/JPL-Caltech/D.]

Our new study, recently confirmed by an independent team, shows that there is a detailed match between the projected maps of various stellar populations of the Galactic bulge and the spatial morphology of the Fermi Galactic Centre excess signal. This finding essentially requires that an astrophysical source connected to these stars is responsible for the Galactic centre gamma-ray excess and rules out a Dark Matter explanation of the signal.

Go to the profile of Oscar Macias

Oscar Macias

Postdoctoral Researcher, Virginia Tech

My research interests lie at the interface of particle physics and astrophysics. A central theme of my research program is the possibility that Dark Matter particles - which are invisible to optical telescopes - may produce gamma-ray photons as the result of collisions with each other. A clear-cut detection of this signal in regions where Dark Matter is abundant and the background is well understood would have profound implications for our understanding of the Universe.

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