Despite the ubiquity of dark matter in every galaxy out there, it has been difficult to pin down how much influence the mysterious substance has in our own galaxy, particularly in the inner regions of it. This has contributed to some (possibly minor) issues for the cold dark matter model.

Over the years, many observations have provided data on the motions of objects in our galaxy. In a new study, researchers have put some of that data together; their analysis provides new evidence for the existence and abundance of dark matter in the inner Milky Way. It represents a fundamental step forward in the quest to understand dark matter.

Previous studies of the inner galaxy’s dark matter fall into two categories: theoretical models and measurements of local stars near the Sun. The problem with the modeling approach, the study’s authors argue, is that it necessarily relies on assumptions about the distribution of dark matter. And the problem with local measurements is that the measurements are compatible with the density of dark matter being zero “unless one makes strong assumptions,” as the authors put it. The new study benefits from not depending on such assumptions.

Rotation curves

One of the first pieces of evidence for dark matter came in the form of what's called the rotation curve of the Milky Way. Now a newer, up-to-date version is cementing our knowledge of the inner Milky Way’s dark matter.

A rotation curve is a simple graph that shows the velocities of objects in a system versus distance from the center of the system. In most rotation curves, the curve starts off high on the left side of the graph and then goes down as you follow it to the right, meaning that the further from the center of the system one looks, the slower the orbiting objects are moving. Our Solar System is one example of this: Mercury, the closest planet to the Sun, is moving the fastest, Venus is a little slower, and the speed continues to decrease as we move through the gas giants.

But the Milky Way’s rotation curve isn’t like that. Mysteriously, outside the bulge at the galaxy’s center, the curve mostly flattens out rather than sloping downward. Speeds throughout the galaxy’s spiral arms remain roughly the same as one looks further from the center. That observation led to the conclusion that extra mass is distributed throughout and surrounding the galaxy, mass that can’t be seen—dark matter.

Rotation curves are built by obtaining the velocities of lots of objects. The new study has compiled a newer, improved rotation curve, one which includes a lot more data from recent measurements. “This represents an exhaustive survey of the literature,” the authors write in their paper. “In total we have compiled 2,780 measurements, of which 2,174, 506, and 100 are from gas kinematics, star kinematics, and masers, respectively."

The new study takes this rotation curve data and compares it to the rotation curve expected from the baryonic matter alone (the normal matter, stars and planets and gas and such). This provides a better estimate of the total contribution of baryonic matter to the rotation curve, making it clearer just how much of an impact dark matter has on the motions of inner Milky Way objects. If there were no dark matter, that would have become clear in this study.

Thinking of everything

To be sure that the data really supports their conclusion, the authors considered every possible arrangement of baryonic matter, based the uncertainties in the various measurements. All of these possible arrangements were taken into account in their model—there’s simply no arrangement that could explain the measurements without dark matter. The evidence for dark matter gets stronger at greater distances from the galaxy’s center, but it's already strong enough well within the Sun’s orbit of the galaxy to confirm the presence of an unseen, diffuse material in the inner Milky Way.

The results persist even if the stars are moving in strange, unexpected patterns contrary to the researchers’ assumptions or if there are systematic variations in motion caused by spiral arms or a few other possible variations in their model change things. In all these cases, the changes to the results were minor and didn’t affect the authors’ conclusions.

“The observed rotation cannot be explained unless large amounts of dark matter exist around us, and between us and the Galactic center” explains Miguel Pato of the Department of Physics, Stockholm University, and one of the paper’s authors.

Implications

These robust results will play into attempts to understand the structure and evolution of the Milky Way in a cosmological context. Understanding how dark matter is distributed in galaxies is important to our study of the larger Universe as well, since the material is the main source of gravity holding galaxies together and forming larger-scale structures.

The work will also be important for the investigation into what dark matter is. Both direct and indirect searches for the identity of the particle—if it is a particle at all—will benefit from a more detailed understanding of its distribution in our galaxy.

“Our method will allow for upcoming astronomical observations to measure the distribution of dark matter in our Galaxy with unprecedented precision,” says Pato. “This will permit [us] to refine our understanding of the structure and evolution of our Galaxy, and it will trigger more robust predictions for the many experiments worldwide that search for dark matter particles. The study therefore constitutes a fundamental step forward in the quest for the nature of dark matter.”

Nature Physics, 2015. DOI: DOI: 10.1038/NPHYS3237 (About DOIs)