Mystery of Ultra Diffuse Galaxy with NO Dark Matter Solved

Remember that ultra diffuse galaxy I told you about in SFNs 226 and 232? You know, the one with NO dark matter? Well turns out there is dark matter there.

Published on 8th Jun, 2019

Looking to get started in astronomy? Try OPT: (aff link) Hello space fans and welcome to another edition of Space Fan News. In this episode, remember that ultra diffuse galaxy I told you about in SFNs 226 and 232? You know, the one with NO dark matter? Well turns out there is dark matter there after all. This was always a controversial finding, back in SFN 226 I reported that the group who found Dragonfly 44, an ultra diffuse galaxy with very little dark matter in it, had also taken a closer look at another galaxy, NGC1052-DF2 and wrote a paper, in Nature of all places, announcing that according to their observations, this galaxy rotated just like a galaxy would if it had no dark matter at all in it. Basically they were comparing the galaxies luminous mass (the dust in radio, the hot and cool gases in the infrared, and the stars in infrared through visible) with the dynamical mass which is inferred by looking at the motions of easy to see things like globular clusters within the galaxy to get a handle on things we can’t see like black holes and dark matter. If those two masses are close or the same, there isn’t much dark matter there, and according to the Dragonfly team, they were close, so close that there couldn’t be any dark matter there. In most galaxies, those two masses are way off, the luminous mass is much less than the dynamical mass because dark matter is affecting the motions of the star clusters. So when this announcement came out it got immediate blowback and in SFN 232 I outlined the problems astronomers had with it. To remind you, they claimed that the galaxy was too 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. They also had a problem with the number of globular clusters they claimed to have measured in a galaxy this small. The original authors claimed 1000 times more than should be there in a galaxy this size. Anyway watch SFN’s 226 and 232 for a more complete accounting of the drama, there’s links in the description and up here. OK so now fast-forward to today. Astronomers did a new analysis on this galaxy and they found that, hang on a minute, this thing is a lot closer to us that anyone thought. The previous estimates and measurement of dark matter in NGC1052-DF2 were reliant on the distance to the galaxy - which they thought was 64 million light years away. So by looking at the galaxies using five different methods some of which included photometry with Hubble and the ground-based Gemini Telescopes they recalculated the distance to NGC1052-DF2. Now astronomers estimate that this ultra diffuse galaxy is 42 million light years away and I know what you’re thinking, “Why don’t astronomers get their heads out of their collective you-know-whats and just put 42 everywhere they need a number?” We’ve known for a long time now that 42 is all we need to know, we can stop doing science now, we know the answer. Anyway, this means that because the galaxy is closer than we thought, the mass measurements taken before - you know, the luminous and dynamical masses - are smaller if the galaxy is this far away. Specifically, the amount of luminous mass from the stars is reduced by about one quarter. Stay with me here, if the amount of luminous mass is lower the proportion of normal matter is smaller and the ratio of luminous to dynamical is smaller, which means this galaxy is in fact acting like all the others because the difference between the two types of mass in this galaxy is larger - and we have a galaxy with dark matter in it. For some reason, I can’t get the image of an astronomer sticking their tongue out at the first team out of my head here. But let’s keep going, the lack of dark matter was previously inferred by the slow movement of star clusters but with the galaxy being closer, it turns out they are moving at a pretty normal rate, again just as if they were in a galaxy that has dark matter in it. [tongue] Astronomers of this report say that with this revised distance, the galaxy appears to be a rather ordinary low surface brightness galaxy with plenty of room for dark matter. So I guess this means it’s not ultra diffuse, I’m not sure about that part though. Finally, according to the article I read on this from Science Alert, there is another galaxy NGC1052-DF4 which also appears to have no dark matter and also appears to be 63 million light years away, they are looking at that galaxy too to see if it is closer than we think. Results so far suggest that it is, so dark matter wins to live another day. Well that’s it for this episode space fans, thank you Deep Astronomy Patreon Patrons for all of your support for what we do here, and to OPT telescopes, if you need a scope, they are a great company with good customer service that I stand behind 100% Thanks to all of you for watching and as always, Keep Looking Up!