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Telescopes and space missions

Telescopes and space missions

Dark matter seen in the Milky Way’s core

10 Feb 2015
In the dark: the Milky Way on a clear night

 

An international team of astronomers has found the best evidence yet that the inner core of the Milky Way contains significant quantities of dark matter. The result confirms the long-standing belief that the centre of the Milky Way is rich in dark matter, just like its outer regions. While the researchers have deliberately avoided using any specific models of dark matter in their analysis, they are confident that further studies of the galactic core could help identify which models are most viable.

Scientists first inferred dark matter’s existence from the fact that galaxies such as the Milky Way rotate faster than would be expected if they were held together by just the gravitational forces between visible matter such as gas, dust and stars. While it is apparent that the gravitational attraction of invisible dark matter is holding galaxies together, it has proved very difficult to measure the distribution of dark matter in the core of the Milky Way. This is because the complicated distribution and dynamics of conventional matter in the core makes it very tricky for astronomers to work out exactly where the dark matter should be.

In the new research, Fabio Iocco of the ICTP South American Institute for Fundamental Physics in São Paolo and colleagues in Sweden and the Netherlands have combined data from several recent observations of the Milky Way and compared it with theoretical predictions of how fast the core should be rotating.

Tricky measurements

The team looked at 2780 measurements of the motions of interstellar gas, stars and interstellar masers. These provide information about the rotation rate of our galaxy at distances between 3–20 kpc from its centre. To put this in perspective, the Sun is about 8 kpc from the centre and the vast bulk of the Milky Way lies within an 18 kpc radius. The team combined these data to arrive at the angular velocities of the galaxy at a number of different radii. The researchers then compared these figures with the angular velocities that would be expected if the galaxy contained no dark matter. This is tricky, explains Iocco, because we are inside the galaxy and moving with it, and this perspective makes it difficult to determine both the distance and the circular motion of other objects. “There is not full agreement in the literature on the exact distribution of stars in the Milky Way,” he notes.

Most researchers studying the galaxy choose “their favourite model of the morphological distribution of visible matter”, says Iocco. In this study, however, the team considered every accepted possibility in the literature, calculating the rotation curve – the rotation rate of the galaxy as a function of radius – that would be predicted by this distribution if there were no dark matter present. “None of these fit the observed rotation curve,” says Iocco, which implies that “none of the possible distributions of visible mass fit the total mass inferred in the galaxy – there is some missing mass even in the worst case”.

The team calculated the difference between the observed and theoretical rotation curves at a large number of different radii between 3–20 kpc. Differences are seen at all radii and although the statistical significance is relatively small at 3 kpc, it rises to above 5σ beyond 6–7 kpc. This figure of 5σ is considered to signify a discovery in particle physics.

Does Newtonian dynamics hold true?

This result means that there are significant quantities of dark matter well inside the 8 kpc radius of the Milky Way, provided that Newtonian dynamics holds true. This last qualification is crucial, because a minority of astrophysicists argue that the discrepancies between predicted and observed rotation curves are better explained by modifying Newtonian dynamics at large distances, rather than the presence of invisible matter (see “Gravity’s dark side”). The researchers believe, however, that by examining the galactic dynamics on comparatively small scales, their results will shed some light on this debate. Indeed, the team plans to address this issue in the future.

Jorge Peñarrubia of the Royal Observatory of Edinburgh believes that the research is an important step towards quantifying the amount of dark matter in the Milky Way. “The number of data they’ve compiled is certainly going to be crucial in trying to determine the amount of dark matter in our galaxy,” he says. However, he adds that “The next step will be to try to construct a dynamical model that can explain the motions that other people have measured. That’s going to be the difficult part.” Dan Hooper of Fermilab in Chicago agrees. “This study shows – conclusively in my view – for the first time that there is about the same amount of dark matter that we had predicted there should be in the innermost parts of the Milky Way,” he says. “It’s a pretty big step forward and one that I hope will continue as we get more information from things like the Gaia telescope, which will be able to measure even more stars with more precision.”

The research is described in Nature Physics.

  • In the following video Luke Davies of the University of Bristol explains how the presence of dark matter is inferred from the rotation of galaxies.

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