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Comment by wanda | Hacker News Reader

(I'm no astrophysicist, so forgive me if my comment is stupid or naive)

From the Wikipedia article [0] on NGC 1052:

    NGC 1052 shows also two small jets emerging from its 
    nucleus as well as a very extended disc of neutral 
    hydrogen, far larger than the galaxy itself, 
    all these features suggesting a gas-rich galaxy 
    collided and merged with it 1 billion years ago
    producing all the above features.

Is it possible that NGC 1052-DF2 is simply the remnant of the aforementioned gas-rich galaxy suspected of having collided with NGC 1052, and that the apparent dark matter deficiency is a result of that collision?

Or do we know what happens when galaxies collide well enough to conclude with reasonable certainty that this is not the case?

[0]: https://en.wikipedia.org/wiki/NGC_1052

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The point is that the galaxy shows that dark matter exists /independently/ (i.e. as its own particles) of normal matter. This is huge.

Up until now, there was a branch of physics that tried to explain dark matter as a phenomenon that is caused by normal matter. This discovery shows that such an approach is unlikely to work.

For all of this, it does not matter /why/ there is no (not a lot) dark matter in that galaxy. Although that is probably the next question physicists are after!

Ah, gotcha! That is huge indeed. I appreciate the clarification.

This galaxy (NGC 1052-DF2) is very sparse. The size of the Milky Way, but only 0.5% as many stars. So I think there is still some wiggle room for the modified gravity theories here.

Minor clarification in case someone else similarly misreads your post at first glance:

Dark matter is not normal matter, and does not exist as particles of normal matter. It's completely distinct, and we don't even know what it is made of. This finding gives good evidence that it's actually real, rather than either simply not existing ("gravity behaves strangely at long distances" hypotheses) or just being normal matter that isn't easily detectable, thus contributing to a galaxy's gravity but not what we can see of it.

This new discovery is of a galaxy whose mass does correspond to what we can see, meaning that we aren't missing obvious explanations for other galaxies, which rules out a lot of hypotheses that might otherwise explain the mass that we can't see for other galaxies.

I know you know this, but it's possible to misinterpret your comment.

That is true of some forms of dark matter (e.g. WIMPs). It is not true of others (e.g. MACHOs) which ARE classical matter, just not visible to telescopes.

This finding rules out most if not all of the MACHO candidates though. There's no way you could strip them away cleanly like this.

Dark matter is (probably) pretty damn normal, while we don't know precisely what particles make up the majority of it there's half a dozen particles predicted by the Standard Model (how much more normal than standard can you get?) that are dark.

Normal aka ordinary aka baryonic matter. Dark matter is decidedly not normal. The Standard Model encompasses everything that could theoretically exist, while normal matter (the kind of stuff you and I are made of) is a small subset of that.

This feels like a leap in logic to me. It seems that it is possible that a 'dark force' could be not operating as strongly in this galaxy as easily as the possibility that dark matter is not being present. Without a mechanism to explain why either one exists, I don't see how this observation could rule one or the other out.

Not sure if this analogy works?

Imagine one group postulated a new type of matter to explain the effects of EM radiation. Another group theorized there was a new force. Then there was observed some charge particles moving at constant velocity and the conclusion was the new matter theory had to be correct since we observed regular matter without the effects of EM radiation.

> The point is that the galaxy shows that dark matter exists /independently/ (i.e. as its own particles) of normal matter. This is huge.

Didn't the bullet cluster already show that?

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