Title: The Cocytos Stream: A Disrupted Globular Cluster from Our Last Major Merger?

Authors: C. Aganze, V. Chandra, R. H. Wechsler, T. S. Li, S. E. Koposov, L. B. Silva, A. Carrillo, A. H. Riley, M. Valluri, O. Y. Gnedin, M. Heiger, C. Rockosi, R. Carlberg, A. Byström, N. Kizhuprakkat, et al.

First Author’s Institution: Kavli Institute for Particle Astrophysics & Cosmology, Stanford University, Stanford, CA, USA

Status: To be submitted to ApJ, available on arXiv

Our galaxy is not a static thing. It is changing and evolving constantly, but on timescales of billions of years. Just as we cannot observe human evolution in real-time, the same is true for galactic evolution. So, for both sciences we turn to archeology to get answers. While paleoanthropologists dig up the remains of humans’ evolutionary ancestors to determine where we came from, astronomers turn their telescopes to observe the remains of the galaxies which the Milky Way (MW) has devoured in order to grow into what we see today. The authors of today’s paper have dug through a tremendous amount of MW stellar data to determine the origins of the Cocytos stellar stream and how it relates to our galaxy.

Step 1) Find a Stream

Stellar streams (or simply, streams) are an association of stars orbiting together in a galaxy in a long, thin line. These stars originally formed in a dwarf galaxy or a globular star cluster (GC), but were stretched out as their original host was shredded apart by tidal forces as it fell into the more massive MW. These streams offer insights into the galactic merger history of our galaxy, since the stars hold onto properties of their original hosts, such as orbital paths, momenta, and chemical compositions. These streams can be difficult to identify, however, since there are so many stars in the MW, and identifying smaller groups moving together, especially far out in the stellar halo where streams are found, is difficult.

However, the authors of this paper had the power of large surveys to help them in their search. They used DESI and Gaia spectroscopic and astrometric data, which covers major, uninterrupted swaths of the sky, allowing them to get data on the movements of over 10 million stars. They fed this data into a cluster finding algorithm which returned several major streams already known and studied, and also one stream we know much less about.

The Cocytos Stream was discovered in 2009, but was only observed with photometric data, meaning they had the positions of the stars but not their motions. Here the authors conduct a much more thorough analysis of the stream to figure out where it most likely came from. They were able to identify 65 stellar members of the stream, each with information about their velocities, energies, angular momenta, and metallicities. Even though Cocytos overlaps with a few other known streams in the sky, they are able to distinguish which stars belong to which stream thanks to their analysis of their orbits and velocities.

Step 2) Find Its Source

To figure out what exactly Cocytos was prior to its infall, the authors compare various properties of the member stars to those of stars which belong to dwarf galaxies and GCs. Although its size and magnitude suggest it was originally a dwarf galaxy, its velocity dispersion and metallicity info end up suggesting a GC origin instead. This is because tidal forces can make GCs become disrupted and much more extended, but there’s not really a mechanism that would make the stars in a dwarf galaxy all end up having high metallicities and relatively similar velocities, something that is common for GCs. 

So, Cocytos likely came from a GC, but where did that GC come from? There are a few major galaxy mergers that the MW has undergone in its history that we already know about, most notably the Sequoia merger and the Gaia-Sausage Enceladus merger (GSE) … yes it’s really called that. The authors decided to compare the properties of the Cocytos stars to stars associated with those two mergers to learn more.

There is plenty of overlap between the Cocytos stars and the GSE stars when plotting their energies, momenta, metallicities, and radial actions, which is the difference between radial kinetic and potential energy integrated over time (see Figure 2).  However, there is not as much overlap with the Sequoia stars, implying that GSE is the most likely original host for this stream. Interestingly, they also find that a further away stream, 300-S, and a nearby GC, Pyxis likely both came from the same origin as Cocytos, while the T24 stream, which is located right next to Cocytos, very likely has a different origin.

Step 3) See Where The Current Takes You

The authors lay out the next steps that need to be taken to really understand this stream. First, we would like to have an idea of the age of this stream and compare it to what we would expect if its origin really is from a GSE merger GC. All of the member stars they have identified are giants, meaning they can’t date the age of the stream using the main sequence turn-off because there is not enough diversity in the members they are observing. Using a telescope that would be capable of reliably detecting smaller, and therefore dimmer, stars in the stream would solve this problem.

In addition, the authors highlight the need for detailed chemical abundance measurements to confirm their hypothesis. While metallicity is a good proxy for chemical abundance, it is broad, so having more information about the specific composition of this stream is necessary. 

Conclusions

The authors of today’s paper were able to conduct a detailed analysis of the properties of the Cocytos stream, which was previously much less well-studied. They were able to come up with a hypothesis about its origin and connect other streams and GCs to it as well. Most excitingly, this study highlights the capabilities of large surveys like DESI and Gaia (as well as Rubin in the near future!) to be able to identify these faint, disrupted structures and learn more about the history of our galaxy.

Astrobite edited by: Joe Williams

Featured image credit: edited from “Gaia observes the Milky Way ESA”, CC BY 4.0, via Wikimedia Commons