#df4

The First Galaxy Without Dark Matter Is About To Be Torn Apart

“Although it’s only affecting roughly 7% of the stellar mass right now, this tidal interaction with a large, massive neighbor is sufficient to solve this dark matter puzzle. The reason its stars are so old is because it was created long ago; the reason it has practically no dark matter is because the dark matter is being actively stripped out of it right now; the reason it still survives today is because it’s undergoing active disruption, and will likely be destroyed in short order, at least on cosmic timescales.

The whole point is this: you can’t have a long-lived galaxy without dark matter. You can lose your dark matter through a tidal interaction, which creates a stellar aggregation known as a tidal dwarf galaxy, but these are transient: short-lived, and easy to tear apart. The mystery of DF4 is that it looks like an ultra-diffuse galaxy, not a tidally disrupted galaxy, because it was an ultra-diffuse galaxy up until very recently. The tidal disruption affected the dark matter first, and only now — now that it’s almost entirely gone — are the stars starting to be disrupted, too. With this new discovery, the puzzle may be completely solved, teaching us why DF4 has no dark matter, after all.”

If a galaxy loses its dark matter halo, it should be very easy to destroy. Without that extra gravity to hold it together, it’ll simply fall apart when it gets near another source of mass. But somehow, galaxy NGC 1052-DF4, known as “DF4″ for short, has last formed stars some ~7 billion years ago, but has no dark matter to speak of.

Ask Ethan: What's The Real Story Behind This Dark Matter-Free Galaxy?

“I read a study that said the mystery of a galaxy with no dark matter has been solved. But I thought that this anomalous galaxy was previously touted as evidence FOR dark matter? What's really going on here, Ethan?”

Imagine you looked at the Universe, and saw a galaxy unlike any other. Whereas every other galaxy we’ve ever looked at exhibited a large discrepancy between the amount of matter that’s present in stars and the total amount of gravitational mass we’d infer, this new galaxy appears to have no dark matter at all. What would you do? If you’re being a responsible scientist, you’d try to knock down this galaxy by any scrupulous means possible. You’d wonder if you had mis-estimated one of its properties. You’d try to re-confirm the measurements with different instruments and techniques. And you’d wonder if there weren’t an alternative explanation for what we were seeing.