5 dwarf galaxies in perfect alignment pose a challenge to our best model of the cosmos

A group of dwarf galaxies unusually aligned in a near-straight line, with most of them rotating in the same direction, has been discovered by astronomers. These unusual features pose challenge to predictions made by the prevailing cold dark matter (CDM) theory, which suggests dark matter — the enigmatic substance that affords the gravitational framework for galaxies and other large-scale structures — causes chaotic groupings of galaxies. In other words, as per this theory, galaxies aren’t likely to have a preferred alignment. So, experts think, the observed galaxies with unusually cohesive dynamics may be better explained by a different form of dark matter, termed warm dark matter (WDM).

The newfound galaxy group comprises of at least five members in an isolated pocket of the cosmos about 117 million light-years from Earth. Together, these galaxies span 502,408 light-years and seem to be aligned in a nearly vertical line from north to south, a configuration “not frequently seen in CDM simulations,” says study lead author Sanjaya Paudel (Research Professor at Yonsei University).

Moreover, data from DESI (the Dark Energy Spectroscopic Instrument), which maps millions of galaxies in deep space to construct massive 3D maps of the cosmos, disclosed that three of the five galaxies share the same direction of rotation — with their southern parts moving toward us and their northern parts moving away from us.

The most reasonable explanation for this coherent motion is that the galaxies formed from the same gas clump — a scenario that contradicts CDM paradigm, which, as mentioned above, typically predicts chaotic groupings with no preferred alignment, especially for dwarf galaxies like those in this particular study. “Why do they have the same rotation?” said Sanjaya Paudel. “They must have a connection with each other — that would be a very strong question that cannot be explained by the ΛCDM model.”

Forthcoming observations could very soon confirm whether the galaxies’ rare, near-straight line alignment is a chance projection or a real physical arrangement, the experts note in their paper. Essentially, the distances to the individual galaxies are not known, thwarting the team from constructing a 3D picture of the galaxies’ positions in space.

The scenario that presently offers a better explanation for these observed phenomena is that of warm dark matter, a hypothetical form of dark matter whose particles vary from cold dark matter when it comes to speed, mass, and, most significantly, how they clump together in the early cosmos.

WDM particles, for instance sterile neutrinos, do not clump as proficiently as CDM particles, which would favour the formation of galaxies from the same cloud of gas. Consequently, these galaxies would be more closely synchronized in both position and motion, resulting in the coherent dynamics observed in this study, said Paudel.