From Tohoku University (東北大学; Tōhoku daigaku)(JP): “The Smallest Galaxies in Our Universe Bring More About Dark Matter to Light”

From Tohoku University (東北大学; Tōhoku daigaku)(JP)


Kohei Hayashi
Astronomical Institute, Tohoku Univeersity

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

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.

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

“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.

A schematic of dark matter distributions where red indicates regions with higher dark matter density. The illustration on the left indicates that dark matter distribution becomes denser in the center of the galaxy, as this study found, whereas the illustration on the right shows a less dense distribution of dark matter according to SIDM. ©Kohei Hayashi.

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 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 limited scattering.

A graph indicating the strength of dark matter scattering (y-axis) versus the average relative velocity between dark matter and itself (x-axis). Error bars indicate galaxy estimates by previous studies, whereas the red regions shows results from the Segue 1 ultra-faint dwarf galaxy. © Hayashi et al.

“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.”

NAOJ Subaru Prime Focus Spectrograph.

NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA,4,207 m (13,802 ft) above sea level.

Science paper:
Probing Dark Matter Self-interaction with Ultra-faint Dwarf Galaxies
Physical Review D

Dark Matter Background
Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, some 30 years later, did most of the work on Dark Matter.

Fritz Zwicky from http://

Coma cluster via NASA/ESA Hubble.

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.
Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.
Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science).

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL).

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970.

See the full article here.


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Tohoku University (東北大学; Tōhoku daigaku), or Tohokudai (東北大, Tōhokudai), located in Sendai, Miyagi in the Tōhoku Region, Japan, is a Japanese national university. It was the third Imperial University in Japan, the top three Designated National University along with the University of Tokyo and Kyoto University and selected as a Top Type university of Top Global University Project by the Japanese government. In 2020, the Times Higher Education ranked Tohoku University the top university in Japan.

In 2016, Tohoku University had 10 faculties, 16 graduate schools and 6 research institutes, with a total enrollment of 17,885 students. The university’s three core values are “Research First (研究第一主義),” “Open-Doors (門戸開放),” and “Practice-Oriented Research and Education (実学尊重).”