The way spiral galaxies rotate suggests that there should be more matter than we can actually see, which astronomers think may be dark matter. Almost five decades later, we still haven’t worked out what dark matter is, though. Now the rotation of mini-spiral galaxies might give some new clues about dark matter.
In a new study, published in the Monthly Notices of the Royal Astronomical Society, researchers from SISSA (The International School for Advanced Studies) in Italy have looked at 36 dwarf spiral galaxies on a wide range of luminosities.
These mini-galaxies tell us that there are more interactions between dark matter and ordinary matter than we have thought so far. Dark matter is believed to be composed of weak interactive massive particles (WIMPs), which have mass and emit gravity but don’t interact with other forces.
"Our observations, however, disagree with this notion," said lead author Ekaterina Karukes in a statement. "If, for a given mass, the luminous matter in a galaxy is closely compacted, so is the dark matter. Similarly, if the former is more widespread than in other galaxies, so is the latter."
The concentration of stars maps the concentration of dark matter very well, which has never been seen before on such a scale. The researchers think that it can be explained away by blaming other types of effects.
“These 36 items are the tip of the iceberg of a phenomenon that we will probably find everywhere and that will help us discover what we cannot yet see," co-author Professor Paolo Salucci said.
Their observations only fit some of the different models of dark matter and none of them fit in the Standard Model of particle physics, which describes how fundamental particles interact, so there’s plenty of room for speculation.
It will be interesting to see if this idea of a more interacting dark matter could explain other extreme galaxies, like Dragonfly 44. Unlike these galaxies, Dragonfly 44 is made of 99.99 percent dark matter, quite the opposite set up to these mini-spirals.