dark-matter-galaxy

Dark matter at the heart of the Galaxy

A new study is providing evidence for the presence of dark matter in the innermost part of the Milky Way, including in our own cosmic neighbourhood and the Earth’s location. The study demonstrates that large amounts of dark matter exist around us, and also between us and the Galactic centre. The result constitutes a fundamental step forward in the quest for the nature of dark matter.

dark-matter-galaxy
The image displays the rotation curve tracers used in our paper over a spectacular photo of the disc of the Milky Way as seen from the Southern Hemisphere (credit to the photo below). The tracers are colour-coded in blue or red according to their relative motion with respect to the Sun. The spherically symmetric blue halo illustrates the dark matter distribution inferred from our analysis.
Background image credit: Brunier http://apod.nasa.gov/apod/ap080104.html

In the modern cosmological paradigm dark matter pervades the whole Universe and is the main component of galaxies. One of the emblematic cornerstones of this paradigm dates back from the 1970s, when Vera Rubin, Kent Ford and others measured the speed at which the gas revolves around the centre of spiral galaxies, thus deriving the so-called ‘rotation curve’ way beyond the extension of the luminous disc. This provided a way to trace the total gravitational potential and effectively ‘weigh’ the galaxies out to their periphery. The end of the story is well-known: the observed flat rotation curves indicated the presence of large amounts of dark matter.

This is (relatively!) straightforward for many external spiral galaxies, located along particularly convenient lines of sight and with particularly convenient inclination angles. But in the case of our own Galaxy, the mighty Milky Way, a spiral itself, there is not much we can do. We are sitting inside the stellar disc, about 8 kpc off-centre, and from this position it is very hard to measure the rotation of gas and stars with the needed precision. Therefore, it has been historically challenging to uncover the existence of dark matter in the Milky Way. Actually, in the outskirts of the Galaxy it was possible to ascertain the presence of dark matter with reasonable degree of confidence, but the same was never done in the innermost regions, where baryons (i.e. gas and stars) contribute the most to the total mass budget.

That is where our new work Evidence for dark matter in the inner Milky Way comes in. First, we set up a comprehensive compilation of published measurements of the motion of gas and stars in the Milky Way. This defines with unprecedented precision the rotation curve of our Galaxy, which tracks the total gravitational potential. Second, we constructed a wide array of data-based models for the visible components, namely stellar bulge, stellar disc and gas. This takes into account the current uncertainty in baryonic modelling, especially towards the inner part of the Galaxy, and allows us to estimate the baryonic contribution to the total gravitational potential. The discrepancy between these two components is striking and statistically significant already inside the solar circle, calling for the need of significant amounts of dark matter between us and the Galactic centre. In short, we obtained a direct observational proof of the presence of dark matter in the innermost part the Milky Way.

So what? Well, this result is a confirmation of long-standing theoretical expectations. In fact, simulations of galaxy formation do suggest the presence of dark matter in these inner regions of the Galaxy. However, the actual amount has been at the centre of a lasting debate in the community, mostly because even if dark matter is expected to be there it is not expected to be the major component. That is, baryons dominate the gravitational potential in the inner regions of the Galaxy, and so extracting the subdominant dark matter component is very challenging. Our findings pave the way for observational determinations of the quantity of dark matter in these regions with higher precision than ever before. This is of crucial importance for the worldwide experimental efforts in direct and indirect searches for dark matter particles.

– Miguel Pato, OKC member, miguel.pato@fysik.su.se and Fabio Iocco, former OKC member

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