Dark Matter Galaxy: 99.99% Mystery

The nature of dark matter remains a deep mystery for astrophysicists; yet they see its manifestation every time they observe galaxies. Called CDG-2, one of them breaks records since it seems to be made up of 99.99% of this substance which is certainly invisible, but whose gravitational effects are measurable. Only a few rare star clusters are visible there, and it is precisely thanks to these that it was identified.

A new class of galaxies dominated almost entirely by dark matter?

It is often by studying the most astonishing cases that discoveries are made. Here, a team of astronomers was working on a very special class of galaxies, because they are very faint: the UDGs (for Ultra Diffuse Galaxies), having noticed that they had five to seven times more dense clusters of stars than typical galaxies. Hence this question: “Could UDGs in fact be evidence of a new class of galaxies dominated almost entirely by dark matter, of which they would be the least ‘dark’ members? ?”

These scientists then combined data from the European Euclid telescope (specialized in the search for dark matter, in fact), the venerable Hubble and Subaru, a Japanese observatory based in Hawaii, to confirm the surprising detection of four globular clusters linked to no known luminous galaxy.

This “black galaxy”, nicknamed CDG-2 (for Candidate Dark Galaxy 2), is the first spotted solely thanks to a few large, very bright groups of stars. They alone produce up to a third of its total light. In comparison, in our Milky Way, these same groups of stars represent only 0.1% of the luminosity. This is proof that CDG-2 does not play in the same league: in visible light, it “weighs” 6 million times the mass of the Sun, but 57 billion “real” masses, or 99.99% dark matter.

Some of the most extreme properties of any known galaxy

The authors of the publication claim that their results “demonstrate that CDG-2 has some of the most extreme properties of any known galaxy” Indeed, if we summarize its properties, here is what it gives:

• CDG-2 shines like 6 million suns, which is ridiculously dim for a galaxy.
• Its total mass (ordinary matter + dark matter) is however approximately 57 billion solar masses.
• Its globular star clusters represent about 1/6th of its total luminosity.
• CDG-2 is so diffuse that astronomers cannot locate its center.

New class of galaxies or statistical strangeness?

In 2022, the lead author of the study, Dayi Li, had already found a CDG-1 galaxy with similar properties in the same Perseus cluster. However, by his own admission, its nature still remains uncertain. With this second candidate, whose statistical reality is much more robust, the question really arises. Can entire galaxies form from pure dark matter halos, when their gas would have been siphoned off by the presence of more ordinary galaxy clusters?

In any case, it is an incredible feat. Uncovering tiny star clusters 300 million light years away seemed unimaginable not so long ago. The team of astrophysicists would now like to analyze this CDG-2 galaxy spectroscopically using the James-Webb space telescope to confirm their hypotheses.

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