Scientists reverse-engineer our Galaxy’s primordial dark matter halo for the first time, and it is nothing like cold dark matter halos.
Visualization of the primordial dark matter halo (left) inferred through reverse engineering from the rotation curve of the galaxy in the compressed halo (right).
The gravitational forces that keep stars and gas orbiting within galaxies are much stronger than can be accounted for by the attraction of the observed stars and gas. The usual explanation for this fact is that galaxies are enveloped in a “halo” of cold dark matter that makes up most of the mass of galaxies. This mysterious substance is generally postulated to be composed of otherwise unknown elementary particles, in contrast to the familiar normal or “baryonic” matter such as gas, stars and planets. Since dark matter particles have not yet been detected, astrophysicists try to constrain their nature by measuring the distribution of dark matter in and around galaxies. Cosmological simulations that neglect baryonic matter make clear predictions for the density profiles of primordial dark matter halos, but only for the situation before a galaxy formed inside them.
“Primordial halos are often confused with current halos derived directly from observed dynamics, such as rotation curves,” said Dr. Pengfei Li, assistant professor at Nanjing University in China and the leading author of this study, “but they differ fundamentally because the density profile of a primordial halo is changed significantly as the ordinary matter condenses to make stars within it.”
As gas collects in a dark matter halo and forms a galaxy, the gravitational pull at its center becomes stronger, causing the halo to contract. The research team developed an innovative approach to reverse-engineer the properties of primordial dark matter halos from observations of how rapidly galaxies rotate today. Using this technique, and the rotation curve measured with the latest Gaia data, they derived the primordial dark matter halo of our Milky Way for the first time.
The inferred primordial halo is quite different from that expected. Its inner part has a surprisingly large “core” of near-constant density, whereas cold dark matter simulations predict “cuspy” halos which have a steep density peak in their centers.
“It is now clear that the observed high density of current halos in massive galaxies is caused by baryonic compression,” Pengfei Li explained. “Once we reverse halo contraction the primordial halos are actually cored.”
Cored dark matter halos were observed in dwarf galaxies, in which baryonic compression is negligible. This discrepancy with predictions is generally attributed to energetic feedback from star formation, such as supernova explosions, which can drive out gas from small galaxies and thereby transform a cusp to a core. But for massive galaxies like our Milky Way, it is unclear how to form such a large core in the cold dark matter paradigm because such feedback is overwhelmed by the compression effect.
The authors also found that the outer region of the primordial halo shows a much steeper decline than expected. No known mechanisms explain this phenomenon. The steep decline is caused by the quickly declining observed rotation curve from Gaia, which remains debated.
“For a long time, scientists focused on the inner structure of dark matter halos, but the challenge in the outer region may be as serious as in the inner region if the declining rotation curve is confirmed by Gaia’s next data release”, commented Dr. François Hammer, co-author of the study.
This research, published in Astronomy & Astrophysics Letters, is led by the School of Astronomy & Space Science in Nanjing University, a top astronomy institute in China, and in collaboration with Case Western Reserve University (USA), Leibniz-Institute for Astrophysics Potsdam (Germany), Paris Observatory (France), and the University of Arizona (USA).
Link to the article: https://www.aanda.org/10.1051/0004-6361/202557021