Faint Galaxy w/ Zero Dark Matter Finding Draws Heated Criticism

When astronomers announced that the faint galaxy NGC 1052-DF2 had no dark matter, many astronomers felt they were being premature. In fact, the assumptions may have been bad science.

Published on 31st May, 2018

Hello Space Fans and welcome to another edition of Space Fan News. In this episode, astronomers respond to the claim made by the authors of the Nature paper stating the galaxy known as NGC1052-DF2 - a galaxy claimed to have no dark matter - by offering alternative explanations and expressing frustration that such extraordinary claims were made based on such flimsy evidence. What did they say? Stay right here… So back on March 19th I uploaded a video telling you guys about the discovery of a galaxy called NGC1052-DF2 or Dragonfly 44, a galaxy that the team of astronomers who studied it said had no dark matter. But the thing is, this would have been the only galaxy found to not have any dark matter in it whatsoever. What’s more, this result was used to put a nail in the coffin for the MOND theory of galaxy rotation. So to take a quick step back for a second, astronomers have found evidence that every galaxy has dark matter in it. They can’t see it directly but they can see the effect the dark matter has on the rotation rate of the galaxy. A galaxy that has no dark matter would rotate like the image on the left while one with no dark matter in it should rotate like the galaxy on the right. When you add up all the stuff you can see in a galaxy, you get something called its luminous mass and includes all the dust, cool gas, hot gas and stars that are all shining or radiating at a wide variety of wavelengths. For example, dust can be seen in radio, cool and hot gasses in the infrared, and stars can be seen in infrared, through visible and up past ultraviolet. So we can see most of the normal matter in a galaxy and this is usually called the luminous mass of the galaxy. There is another kind of galactic mass, called a dynamical mass and is inferred by looking at the motions of things like globular clusters within the galaxy to get a handle on things we can’t see like black holes and dark matter. If the dynamical mass is way different from the luminous mass, then astronomers assume there are lots of black holes and dark matter in that galaxy. And most galaxies are like this. But this galaxy wasn’t. It was a very dim, diffuse galaxy whose luminous and dynamical masses appeared to be close. To get the dynamical mass, the authors measured the velocities of 10 globular clusters that they assumed were gravitationally bound to the ultra diffuse galaxy. If this result is confirmed, then it would be the the first ever case of such a galaxy, out of hundreds of billions of galaxies. But in a response published on the astronomy preprint site Astro-ph earlier this month, and whose link I put in the description box below if you want to follow along, some astronomers have a real problem with the methodology and the conclusions reached in this paper that went out in one of the most prestigious scientific journals in the world, Nature. They respond: This galaxy is very dim, and there is no way to measure for sure how stable the orbits of the globular cluster are, nor can we even tell if they are gravitationally bound to the galaxy itself. If the globular clusters aren’t really part of the galaxy, then they can’t have anything to contribute with respect to the dynamical mass. There is also no evidence that the galaxy is the same at all angles (it may be a disk and we’re looking at it face on) nor is there evidence that all the stars and clusters in the galaxy are gravitationally stable. Objects in the galaxy may still be interacting in a chaotic way. If it were stable, which is known as virialization, then we might be able to use these clusters as the authors attempted. But we don’t even know that. What we do know reasonably well is that this galaxy has a luminous mass of about 10^8 solar masses, that’s a hundred million solar masses, and as far as we know, galaxies this small do not contain significant globular cluster systems. Based on the paper, the authors claim that this galaxy contains 1000 times more globular clusters than would be expected in a galaxy this size. Say what? If this is true, then the luminosity function (this is a way of finding what’s inside a galaxy based on its luminosity distribution), would be different from all other galaxies in the universe. Something these guys find very suspicious. This galaxy would truly be different from any other known galaxy and would not follow a luminosity function which has been proven to be universal. So it is with a huge amount of incredulity that astronomers discover that this analysis was published at all. These guys are saying the findings in this paper don’t even come close to the 5 sigma deviation standard to warrant a paper of this magnitude to be published, much less in Nature. Reading from the final paragraph of the response: Having said all the above, we cannot avoid to ask ourselves how could such a result have been published at all, wondering how a similar paper would have been received if the conclusion were the other way around. This points to the responsibility of journals that at present adopt standards orders of magnitude lower to publish results favouring the dark matter hypothesis compared to the ones required to papers claiming the opposite. Our sad conclusion is that science cannot progress this way. Well that’s it for this episode Space Fans, I want to humbly thank all of the Deep Astronomy Patreon Patrons whose support this month really saved our behinds at DA headquarters. In part, because of your support, I was able to recover from a laptop crash that I’m not sure I could have done without your help. You guys really are making a difference. Thanks to all of you for watching and as always, Keep Looking Up! Here is the link to the response on astro-ph: https://arxiv.org/abs/1805.04817