Astronomers unravel 20-year dark matter mystery with new computer models

The Fornax dwarf galaxy is one of our Milky Way’s neighbouring dwarf galaxies. 

The Milky Way is, like all large galaxies, thought to have formed from smaller galaxies in the early days of the Universe.

These small galaxies should also contain many very old stars, just as the Milky Way does, and a team of astronomers has now shown that this is indeed the case.

This image was composed from data from the Digitized Sky Survey 2. 

Credit: ESO /Digitized Sky Survey 2

Astronomers at the University of Texas at Austin believe they have discovered the answer to a 20-year debate over how the mysterious cosmic "dark matter" is distributed in small galaxies.

John Jardel 

Graduate student John Jardel and his advisor Karl Gebhardt found that the distribution, on average, follows a simple law of decreasing density from the galaxy's center, although the exact distribution often varies from galaxy to galaxy.

The findings are published today in The Astrophysical Journal Letters.

Dark matter is matter that gives off no light, but that astronomers detect by seeing its gravitational tug on other objects (like stars).

Theories abound on what dark matter might be made of—unseen particles, dead stars, and more—but nobody knows for sure.

Though mysterious, understanding the nature of dark matter is important, because it makes up most of the matter in the universe.

The only way to understand how the cosmos evolved to its present state is to decode dark matter's role.

For that reason, astronomers study the distribution of dark matter within galaxies and on even larger scales.

The Lonestar supercomputer is a resource of the Texas Advanced Computing Center (TACC) at The University of Texas at Austin. 

It is a Dell Linux cluster with 5,840 processing cores, and a peak performance of 62 teraflops (62 trillion floating-point operations per second). 

Since its launch in 2006, Lonestar has provided more than 85 million computing hours to approximately 1,100 researchers across the nation. Credit: TACC/UT-Austin

Dwarf galaxies, in particular, make great laboratories to study dark matter, Jardel says, because they contain up to 1,000 times more dark matter than normal matter.

Normal galaxies like the Milky Way, on the other hand, contain only 10 times more dark matter than normal matter.

For the past 20 years, observational astronomers and theorists have debated how dark matter is distributed in galaxies.

Observational astronomers, through their studies of telescope data, have argued that galaxies have a fairly uniform distribution of dark matter throughout.

Theorists, backed by computer simulations from the 1990s, have argued that dark matter density decreases steadily from a galaxy's core to its hinterlands. The disagreement is known as the "core/cusp debate."

Jardel's work set out to study the question using both data from telescopes and newly developed computer modeling. The project started out "not assuming core or cusp theory is right," he says, "but just asking 'what is it?.' These new models allowed us to take this approach."

Jardel used telescope observations of several of the satellite galaxies orbiting the Milky Way, including the Carina, Draco, Fornax, Sculptor, and Sextans dwarf galaxies.

The work involved running many supercomputer models for each galaxy to determine the distribution of dark matter within it, using the university's Texas Advanced Computing Center (TACC).

More information: dx.doi.org/10.1088/2041-8205/775/1/L30