The Smallest Galaxies in Our Universe Bring More About Dark Matter to Light

February 24, 2021
Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU)

Fig.1：Dark matter distributions. Redder regions show higher dark matter density regions. The panel to the left shows dark matter distribution increases in density at the center of a galaxy, while that to the right shows lesser density dark matter distribution as predicted by the theory of self-interacting dark matter. Dwarf galaxy Segue 1 prefers high dark matter density as shown in the left hand panel. (Credit: Kohei HAYASHI)

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 research team including the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Project Assistant Professor Satoshi Shirai 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 Kohei Hayashi, a former Kavli IPMU project researcher, and currently a project assistant professor at the Tohoku University and lead author of the study.

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 the Segue 1 ultra-faint dwarf galaxy. (Credit: HAYASHI et al.)

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 states 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.”

For more details, please refer to the press release by Tohoku University.
http://www.tohoku.ac.jp/en/press/smallest_galaxies_dark_matter.html

Paper details:
Journal: Physical Review D
Title: Probing Dark Matter Self-interaction with Ultra-faint Dwarf Galaxies
Authors: Kohei Hayashi (1), Masahiro Ibe (2, 3), Shin Kobayashi (2), Yuhei Nakayama (2), Satoshi Shirai (3)

Author affiliation:
1. Astronomical Institute, Tohoku University, Sendai, Miyagi 980-8578, Japan
2. Institute for Cosmic Ray Research, The University of Tokyo, Kashiwa, Chiba 277-8582, Japan
3. Kavli Institute for the Physics and Mathematics of the Universe (KAVLI IPMU, WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan

DOI: 10.1103/PhysRevD.103.023017 (Posted on January 20, 2021)
Abstract of the paper: (Physical Review D)
Pre-print: (arXiv.org page)

Research Contact:
Kohei Hayashi
Astronomical Institute, Tohoku University
Email: k.hayasi _at_ astr.tohoku.ac.jp
Website (Astronomical Institute, Tohoku University): https://www.astr.tohoku.ac.jp/en/index.html
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