Dwarf galaxies and globular clusters

The Milky Way as reconstructed from Gaia DR2 data, highlighting the location of nearly 90 satellites of the Milky Way: globular clusters (in blue) and dwarf galaxies (in red) with annotations. The arcs shown along with each satellite show the trajectories of the objects, and more specifically the path these objects take in the next 10 million years for the globular clusters (in blue) and in the next 100 million years for the dwarf galaxies (in red). Image created by Maarten Breddels.

The background image shows the Milky Way reconstructed from Gaia DR2 data, the highlights in different colours show the location of nearly 90 satellites of the Milky Way: globular clusters (in blue) and dwarf galaxies (in red), the better known ones being the Large and Small Magellanic Clouds in the bottom right part. The curves show the trajectories of the objects. These allow us to see how the globular clusters will move in the next 10 million years, and for the dwarfs in the next 100 million years. For many of these objects, such motions had never been measured before.   

Gaia is a unique mission because it covers all areas of astrophysics, from planets, stars, galaxies and cosmology. Gaia is currently making a 3D map of the Galaxy and our immediate neighbourhood in the universe. But not just that, it is actually measuring how objects move through space. This is extremely challenging as the apparent motions are very tiny because the objects are very far, and so one can barely see them move. But Gaia can, like no other project ever before.

Since the Gaia data becomes public at the same time for the whole world, and there is no dataset to compare to (because nothing measures up to Gaia due to its vastness and precision), a big question is how to verify the Gaia data. The DPAC Consortium selected a few science topics where the data could be used for superficial analysis and to check if it made sense and to identify problems if any. The figure above shows the results of the efforts to measure the proper motions of satellites around the Milky Way. Our team focused on 12 small satellite galaxies (10 of the smallest galaxies orbiting around the Milky Way and the Large and Small Magellanic Clouds) and 75 globular clusters. To facilitate their use by the astronomical community, we release our measurements in various tables as well as for example the characteristic parameters of their orbits.

Measuring proper motions for these objects is interesting for a lot of different reasons. It is through motions that we can understand the forces of nature. Such measurements can therefore be used to model the dark matter distribution around the Milky Way, to establish how the satellite system of the Milky was assembled, to determine the evolution of the satellites themselves…

Because the quality of the data is so superb, some of the analysis were extremely straightforward. As simple as plotting the data in the direction to the object. An example of this are the Large and Small Magellanic Clouds. We can “see" for the first time how coherent the rotation pattern is for the Large Cloud (and even the imprint of the bar!), and interestingly, for the first time using stars, very clearly also in the Small Cloud. We have also detected rotation signatures in at least 5 globular clusters (this gives us clues to their formation, which is an open puzzle).

One of the questions that we were most curious about is whether the dwarf galaxies move on a single plane around the Milky Way. This has been a point of debate for more than a decade, because it seems to be at odds with what cosmology (and in particular cold dark matter models predict), and some researchers stated it was one of the biggest challenges to the model. Now we are able to answer this question.  As the image shows, the dwarf galaxies of the Milky Way are all on highly inclined orbits with respect to the Galactic disk. However, their orbits are not on a single plane although some of them appear to have fairly coherent motions: about six objects move in the same sense as the figure shows, namely the Magellanic Clouds, Ursa Minor and Draco (which themselves appear to be a group), Carina and Fornax. This is probably an indication of filamentary infall which is very characteristic of the cosmological model (the cosmic web), and makes the link between the Milky Way and its environment and cosmology in a way that would be difficult to see for any other galaxy in the Universe.

Using Leo I, the most distant dwarf galaxy satellite (at 840,000 light years away), and assuming it is bound to the Milky Way, we were also able to derive a lower limit for the mass of our Galaxy of 10¹² solar masses. This is a very reasonable value (given previous measurements), and it indicates about 20 times more mass than what we can see in stars and gas, so is evidence of dark matter. We will be able not just to refine this determination of the mass using the data set provided here (with the clusters motions constraining the mass distribution at small radii and the dwarf galaxies at large distances) but also with other methods based on Gaia DR2 data. Astronomers will also try figure out how this mass is distributed which is more critical for knowing what dark matter is made of, or for establishing if we need to change the law of gravity.

One of the things we have learned through the analyses carried out by our team is that the dataset is so rich and of such excellent quality, that there is a lot to explore for every one of us in the astronomical community. For example, just by inspecting this image we made for outreach purposes, we have also been able to identify a pair of globular clusters, NGC5053 and NGC5024, that are traveling together through space. Therefore, even though for many of us the opening of the archive will feel at the beginning as a rat race, this actually shows that people quickly will realise that the Gaia “pie” is almost infinitely large and can satisfy many astronomers' hungry minds!

More information, including visuals can be found here.

Credits: ESA/Gaia/DPAC, Amina Helmi, Maarten Breddels and co-authors of the paper "The kinematics of globular clusters and dwarf galaxies around the Milky Way"

Published: 25 April 2018