Researcher presents evidence of dark matter in the Milky Way

A study published in Nature Physics compares the gravitational profile of the galaxy's central portion to the profile it would have if it consisted entirely of luminous matter.

By José Tadeu Arantes

Agência FAPESP – Robust proof of the existence of dark matter in the region between the solar system and the center of the Milky Way has been obtained by Fabio Iocco, a researcher affiliated with São Paulo State University’s Theoretical Physics Institute (IFT-UNESP) and the ICTP South American Institute for Fundamental Research (ICTP-SAIFR) in Brazil. A paper reporting the study, “Evidence for dark matter in the inner Milky Way” by Iocco et al., was published online by Nature Physics on February 9.

“We obtained this evidence by measuring the rotation of our galaxy with great precision,” Iocco told Agência FAPESP. “We then used the rotation data to calculate the galaxy’s gravitational attraction, and from the gravitational attraction, we arrived at its mass. Its calculated mass is greater than that of its luminous matter (stars and gas) alone. The difference points to the existence of another material component in the region, and this has to be dark matter.”

Iocco is Italian and is working in Brazil supported by a scholarship and research grant from FAPESP’s Young Investigators Grants program.

The hypothesis that the universe contains an unknown form of matter, called dark matter because it has never been detected via direct astronomical observation, was formulated in the 1970s, when scientists were first able to produce very precise calculations of the rotation of gas around the center of a spiral galaxy.

This rotation is difficult to measure in the Milky Way because all human observers are located inside the galaxy, roughly halfway between its periphery and its center.

“Measuring gas and star rotation with the necessary precision has been a major challenge for all these years because of our position in the galaxy,” Iocco said. “It’s particularly difficult in the region between the Sun and the center of the Milky Way, where there’s a high concentration of stars and gas, thus contributing more to the amount of mass.”

“We had to compile two enormous datasets. We needed indicators of total gravitational potential and rotation curves, including variations in the velocities of stars, gas and masers [sources of electromagnetic emission]. For this purpose, we compiled all the data recorded in the literature since the 1960s. We also needed the distribution of visible matter. In this case, because of the lack of consensus in the literature regarding the morphological structure of the galaxy, we collected data on all the existing models instead of running the risk of choosing the wrong model.”

According to Iocco, neither these datasets nor the distribution of visible matter are compatible with the calculated galactic rotation curve. “This is one of the reasons why we’re so sure of having proved the existence of dark matter in the region, including the Solar System,” he said.

There has been a great deal of speculation about the composition of dark matter. Iocco preferred not to express an opinion.

“We didn’t set out to answer that question in the paper,” he said. “Indeed, we didn’t assume the existence of any kind of dark matter. The evidence for it emerged from our calculations. There must be some kind of matter, something that exerts gravitational attraction, and it must be dark, non-baryonic, and non-compact. The findings of our forthcoming studies using the same data should provide more precise answers to questions about the distribution of this dark matter, which could help to determine the nature of this material component, be it through direct or indirect detection.”

Fabio Iocco was born in Naples and graduated from the University of Naples Federico II, where he also earned a PhD. While working on his doctorate, he spent three years at Stanford University, California, as a visiting researcher. Once back in Europe, he worked at the Arcetri Observatory in Florence, Italy, and later in Paris, Stockholm and Madrid, before coming to São Paulo.

The paper “Evidence for dark matter in the inner Milky Way” (doi: 10.1038/nphys3237), by Iocco et al., can be read on the Nature Physics website at www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3237.html.