【Press Release】The Smallest Galaxies in Our Universe Bring More About Dark Matter to Light

Press Releases

Graduate School of Science,Tohoku University
Institute for Cosmic Ray Research(ICRR),The University of Tokyo
Kavli Institute for the Physics and Mathematics of the Universe(Kavli IPMU),The University of Tokyo

Our universe is dominated by a mysterious matter known as dark matter. Its name comes from the fact that dark matter does not absorb, reflect or emit electromagnetic radiation, making it difficult to detect.  

Now, a team of researchers has investigated the strength of dark matter scattered across the smallest galaxies in the universe using stellar kinematics.

“We discovered that the strength of dark matter is quite small, suggesting that dark matter does not easily scatter together,” said professor Kohei Hayashi, lead author of the study.

Much is unknown about dark matter, but theoretical and experimental research, from particle physics to astronomy, are elucidating more about it little by little.

One prominent theory surrounding dark matter is the “self-interacting dark matter (SIDM) theory.” It purports that dark matter distributions in galactic centers become less dense because of the self-scattering of dark matter. 

However, supernova explosions, which occur toward the end of a massive star’s life, can also form less dense distributions. This makes it challenging to distinguish whether it is the supernova explosion or the nature of dark matter that causes a less dense distribution of dark matter.

To clarify this, Hayashi and his team focused on ultra-faint dwarf galaxies. Here a few stars exist, rendering the influences of supernova explosions negligible.

Their findings showed that dark matter is dense at the center of the galaxy, challenging the basic premise of SIDM. Images from the dwarf galaxy Segue 1 revealed high dark matter density at the center of the galaxy, and that scattering is limited. “Our study showed how useful stellar kinematics in ultra-faint dwarf galaxies are for testing existing theories on dark matter,” noted Hayashi. “Further observations using next-generation wide-field spectroscopic surveys with the Subaru Prime Focus Spectrograph, will maximize the chance of obtaining dark matter’s smoking gun.”

Fig.1: Dark matter distributions. The redder colored regions show high dark matter density regions. The left panel shows the case that dark matter distribution becomes high dense in the center of a galaxy, while right one show less dense dark matter distribution predicted by self-interacting dark matter theory. Segue1 prefers high dark matter density like that the left panel. Copyright: Kohei HAYASHI
Fig.2: The strength of dark matter scattering (y-axis) versus the averaged relative velocity between dark matter and itself (x-axis). The points with error bars come from other galaxies estimated by previous studies. The red shaded region shows the result from Segue 1 ultra-faint dwarf galaxy. Copyright: HAYASHI et al.

<Publication Detail>
Title: “Probing Dark Matter Self-interaction with Ultra-faint Dwarf Galaxies”
Authors: Kohei Hayashi, Masahiro Ibe, Shin Kobayashi, Yuhei Nakayama, Satoshi Shirai
Journal: Physical Review

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Tohoku University, Graduate School of Science, Tohoku University