Galaxies without dark matter may have formed after a big collision

In 2018 and 2019, a team led by Dutch astronomer Peter van Dokkum (Yale University) announced the discovery of two diffuse galaxies – called DF2 and DF4 – that were discovered to contain little or no dark matter. This announcement sparked a heated debate as astronomers believe that dark matter is a crucial component in the formation of galaxies.

The discussions certainly are not over yet. But what has been exposed a bit so far is how similar DF2 and DF4 are. They are about the same size, clear, and have the same shape. Also, both contain a strange set of very bright globular clusters. In addition, it is in close proximity to the bright elliptical galaxy NGC 1052.

in the last temper natureThe publication, van Dokkum and his collaborators suggest that DF2 and DF4 not only resemble each other like two drops of water, but also arose during the same event. This thought is not surprising: last year Korean researchers have already shown by computer simulations that exotic “ducks” such as DF2 and DF4 can indeed form during a collision between galaxies.


Based on this, van Dokkum and his team reconstructed where and when their paths might intersect, using the current velocities and positions of the two galaxies. In doing so, they came up with a scenario in which a small satellite galaxy of NGC 1052 collided with a transient galaxy about 8 billion years ago.

In this collision, the dark matter would have escaped from the galaxies, and the natural matter gas in the galaxies had slowed down. This gas then strained together in a series of clumps that collapsed under its own gravity to form new galaxies – except for the missing dark matter.

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Under this scenario, more dark matter-free galaxies could exist between DF2 and DF4. And at the ends of the hypothetical rope, there could be galaxies containing an abundant amount of dark matter.

To verify, the team searched the catalog of galaxies around NGC 1052 and found nine more galaxies in addition to DF2 and DF4. Two of them, RCP 32 and DF7, could be remnants of the original collision.

Further research will show whether the series of intriguing galaxies in NGC 1052 actually have a common origin. Van Dokkum and his colleagues plan to make follow-up observations with the European Very Large Telescope in Chile and the Hubble Space Telescope. This will allow them to measure the speeds and distances of galaxies, as well as determine whether they all contain bright globular clusters, such as DF2 and DF4. The team also hopes to have a chance to determine the masses of RCP 32 and DF7 using the new Webb Space Telescope.

Image: Hubble telescope image of the “transparent” galaxy NGC 1052-DF2, which appears to contain almost no dark matter. (NASA, ESA, STScI, Zile Shen (Yale), Peter van Dokkum (Yale), Shani Daniele (IAS) / Alyssa Pagan (STScI))

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