<div class="pull-right"><img src="https://steemitimages.com/DQmPXD17k2pwGYf3HuHuw5tmAM6KZLZn6J21GfjUK8pyHeT/image.png"><br><center><sub><b>Galaxy NGC 1052-DF2</b><br>NASA, ESA, and P. van Dokkum <a href="https://commons.wikimedia.org/wiki/File:NGC_1052-DF2_a_ghostly_galaxy_lacking_dark_matter.jpg">link</a>
&nbsp;&nbsp;CC BY 4.0 license</sub></center></div>

#### Intro

For the first time, a galaxy has been discovered that is apparently devoid of dark matter. The galaxy with the designation NGC 1052-DF2 lies about 60 million light-years from Earth in the constellation Cetus.

The recent discovery of this galaxy may have some interesting implications for the field of dark matter, particularly for its competing hypotheses such as MOND and the more sophisticated variants of MOND.

#### Galactic Rotation Curves

A galaxy that is said to exist without dark matter simply means that the measured galactic rotation curve of the stars will be what you would expect it to be if dark matter did not exist. In other words, you take a photo of a galaxy and then you make good estimates of how the mass is distributed in that galaxy based on the stars that you see.

<div class="pull-right"><img src="https://steemitimages.com/DQmdcKtnPXLWdeaR9mn4dtFF12ggfmNKoXTAnKbveY9Y7m4/image.png"><br><center><sub><b>Galactic Rotation Curves, Predicted vs Observed</b><br>PhilHibbs <a href="https://en.wikipedia.org/wiki/Dark_matter#/media/File:GalacticRotation2.svg">link</a>
CC BY-SA 3.0 license</sub></center></div>

Using some basic physics you can then work out what the velocities of these stars needs to be for them to stay in orbit. For most galaxies this calculation turns out to be very wrong hence the need to introduce the dark matter concept to fix the problem.

The galactic rotation curve image provided explains the situation graphically. It shows the rotation curve of your typical spiral galaxy in which the dotted blue curve is the predicted orbital velocity if dark matter did not exist and the solid red curve is the what astronomers measure (until now). Dark matter needs to be introduced to help explain the 'flatness' of the measured velocity curve as it goes out out to a large distances from the centre.

#### The Threat To MOND

If the MOND hypothesis and its ilk were true then the flat curve phenomenon should always work in every galaxy (see my earlier post on MOND [here](https://steemit.com/steemstem/@procrastilearner/dark-matter-versus-mond)). There should be no anomalies such as seen in NGC 1052-DF2 because gravity needs to work the same everywhere no matter what the descriptive theory may be (whether it is general relativity plus dark matter or MOND without dark matter).

So it seems then that this anomalous galaxy either has had its dark matter stripped from it sometime in its history or it formed without the assistance of dark matter. A third option is that it is a spiral arm or other remnant flung off of another much larger galaxy during a galaxy-galaxy collision.

#### Closing Words
It is still early days in this particular story. More measurements of this galaxy will be needed to confirm the claims and no doubt they will be performed over the next few years.

Whatever the final answer turns out to be this promises to be an interesting astronomy story to follow.

*Thank you for reading my post.*

#### Post Sources

[1] [Galaxy NGC 1052-DF2 Wikipedia article.](https://en.wikipedia.org/wiki/NGC_1052-DF2)
[2] [Dark Matter Goes Missing In An Oddball Galaxy.](https://www.nasa.gov/feature/goddard/2018/dark-matter-goes-missing-in-oddball-galaxy)
[3] [Researchers find a galaxy without dark matter.](http://earthsky.org/space/ngc-1052-df2-galaxy-without-dark-matter)
[4] [Dark Matter Wikipedia article.](https://en.wikipedia.org/wiki/Dark_matter)