Mar 29 2018

Galaxy Without Dark Matter

Published by under Astronomy
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One of the cool things about astronomy is that the universe is a huge place, and we can look in any direction and see lots of stuff. Current estimates are that there are about 2 trillion galaxies in the visible universe. Galaxies vary in size, but have on the order of hundreds of billions of stars.

What this means is that even if a phenomenon occurs only in one in every billion galaxies, there are still two thousands of them out there for us to find. Or even a one-in-a-million star will occur hundreds of thousands of times in our own galaxy. So the more we look, the rarer and rarer stuff we will find.

Recently astronomers happened upon one such rare occurrence – a galaxy that appears to be devoid of dark matter. That’s not supposed to happen, so if it is confirmed by longer study will have significant implications for our understanding of the universe.

Dark matter is material that has a gravitational effect that we can observe, but does not radiate and so is “dark”. The existence of dark matter was first proposed by Fritz Zwicky to explain the apparent rotation of observed galaxies. Based on Newton’s gravitational equations, we can calculate how fast a galaxy should rotate based on how much mass it contains. Another way to look at this, if we observe how fast a galaxy is rotating, we can calculate how much mass it should contain. If it contained less than the needed mass, the stars would be flung away by their observed velocity.

The problem was, when we calculated the mass of galaxies based on what we could observe, we found only about 20% of the matter necessary to explain its rotational velocity. It was as if galaxies contained 80% invisible or “dark” matter.

Dark matter has since been largely confirmed. The bullet cluster is one such confirmation – essentially these are two colliding galaxy clusters. Pressure from all the intersellar gas caused the galaxies to slow down as they approached each other. Dark matter, however, interacts far more weakly (except through gravity) and so the dark matter just continued on through the other galaxies. When astronomers mapped out the matter in the cluster and the gravity (through lensing effects) they found that they were not in the same place. The gravitational effects had moved on, past the ordinary matter.

There was a lot of hot debate about the bullet cluster, but the consensus has come around to this being good evidence for dark matter.

The new discovery is of a galaxy,  NGC1052-DF2, that is described as a “ghost” galaxy, because it has very few stars. More importantly when astronomers calculated the ratio of dark matter to regular matter, based on rotation and visible matter, they found that it was essentially zero. There does not appear to be any dark matter.

If confirmed, this is another nail in the coffin of the dark matter skeptics. The alternative theory is that gravity behaves differently on the galactic scale (so called, modified Newtonian dynamics) but here is a galaxy without the apparent dark matter effect, and it is difficult to argue that the laws of physics are just different in this galaxy.

The simplest explanation is that, for some reason, this galaxy does not have any dark matter. But this does introduce a new mystery – how did that happen?

There are at least two possible types of explanation. One is that the galaxy lost its dark matter somehow, perhaps with an interaction with another galaxy. Or, perhaps this galaxy formed without dark matter. But here’s the thing, our current theory of how galaxies forms requires the mass from dark matter. So astronomers don’t currently know how a galaxy could form without dark matter.

In order for stars to form gas clouds have to collapse in on themselves, until the gas is compressed enough to ignite nuclear fusion. That collapse is due to gravity, and requires the gravity from dark matter to happen. For this reasons galaxies form around concentrations of dark matter. Prof Pieter van Dokkum, lead author of the study, put it this way:

“It’s not just galaxies,” explains van Dokkum. “The entire fabric of the universe is really the scaffolding of dark matter and everything else is pasted on it.”

What this means is that astronomers will have to come up with alternate hypotheses about how this ghost galaxy could have formed. Something else is going on, allowing the gas clouds to collapse into stars. In a way, however, this makes sense because there is likely one phenomenon that explains both its paucity of stars and absence of dark matter. Whatever is causing the star formation in this galaxy results in far fewer stars than regular galaxies with dark matter.

Science stories like this are always fascinating, but it is also important to see the bigger picture. They show case how regular science works day-to-day. Scientists love mysteries, when things don’t make sense, or they find a phenomenon that appears anomalous. That is where the science is – such anomalies point to new discoveries.

This reality is very different from the anti-science cartoon that portrays working scientists as wanting to support the status quo. That’s just nonsense.

These stories also showcase the process of science – new evidence is met with a mixture of excitement and caution, different interpretations are offered, and scientists may disagree but they all agree that in the end more evidence will settle the debate. When that new evidence comes in, they actually change their mind.

The closing quote in the BBC write up of this news item reveals this process:

Dr Richard Massey, a physicist at Durham University, agrees: “I’m genuinely very impressed with the work, and I’d use the conclusions to say that we should stare at these objects a lot harder for a lot longer – but I wouldn’t conclude anything profound about dark matter quite yet,” he told BBC News.

You rarely if ever hear anything like that from vaccine deniers, alternative medicine proponents, UFO enthusiasts, or the like.


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