Tracing back the origin of Globular Clusters in the Milky Way

Title: No globular cluster progenitors in Milky Way satellite galaxies

Authors: Pierre Boldrini, Jo Bovy

First Author’s Institution: 1. Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France 2. Université de Lorraine, CNRS, Inria, LORIA, F-54000 Nancy, France

Status: Accepted to MNRAS

I’ve described globular clusters (GCs) as glittering jewels and ancient relics (see this astrobite). If you think you don’t know much about them, you are not alone! Astronomers also debate about how they formed and where: they could have formed in dark matter clumps (like a mini galaxy), or in gravitationally bound gas clouds (like a more dense version of normal star formation).

They are very old and sturdy; because the stars in them are so closely packed, they can survive galaxy mergers and still be observed today. There’s evidence to indicate some GCs in the Milky Way (MW) were not born here. Given that many dwarf galaxies have fallen into the MW, a question naturally arises: did the accreted satellite galaxies bring GCs into our Galaxy?

<<< Rewind 

Thanks to the Gaia mission, we have accurate positions and velocities of 170 GCs in the MW. The velocities tell you how tightly GCs are bound to the Milky Way. GCs that have high velocities and orbits reaching far beyond the MW center are more likely to have been deposited by a satellite. The authors of today’s paper rewind their orbits and check whether any GCs could have been part of the known satellite galaxies.

If you want to know where GCs came from, simply rewind their orbits and see where they were in the past. Of course, in practice it is never that simple. Starting from the position of the GC, you calculate the gravitational force it feels from the MW, which then changes the velocity of the GC and takes it to a different position. To rewind orbits carefully, you need to account for the past evolution of the MW: the authors model how the MW grew in mass as the satellite galaxies lost mass. You also need to include the gravitational effects from other MW satellites like the LMC.

Once the orbits have been calculated, the authors define two criteria for association. For a GC to be associated with a satellite galaxy, it has to be close in physical distance and within the galaxy’s escape velocity. The authors checked this method on the Sagittarius dwarf galaxy with known GCs and recovered the association. It works! Then what’s the result for the rest of the MW GCs?

Results

Table 1: From left to right, the columns give for each progenitor candidate: likely associated GCs; the probability of having been bound to the progenitor candidate assuming maximum and minimum mass loss for the MW satellite galaxies. Only GCs with a non-zero probability for one of the dwarf masses are listed. None of these GCs show any clear association with the eight classical MW dwarfs. Reproduced from Table 4 in the paper.

When the authors apply the method to check all known MW GCs, they do not find any GCs that could be associated with the known satellite galaxies. Is that a disappointment? Not necessarily!

Even though the authors did not find any association to the 11 known galaxies, many GCs are still believed to be accreted based on their composition. Then why do their orbits not trace back to any progenitor? The authors assumed GCs always had only stars and no dark matter, but if GCs formed with some dark matter content, then the modeling would lead to a wrong orbit. Another possibility is that the progenitor satellite galaxy could have dissolved and is not observable today as one of the 11. Also, our assumption about the MW mass history could be wrong and affected the reconstructed orbits. In any case, the origin of the accreted GCs are still a mystery.

Astrobite edited by Ciara Johnson

Featured image credit: NASA 

About Zili Shen

Hi! I am a Ph.D. student in Astronomy at Yale University. My research focuses on ultra-diffuse galaxies and their globular cluster populations. Since I came to Yale, I have worked on two "dark-matter-free" galaxies NGC1052-DF2 and DF4. I have been coping with the pandemic and working from home by making sourdough bread and baking various cookies and cakes, reading books ranging from philosophy to virology, going on daily hikes or runs, and watching too many TV shows.

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