Unraveling the Cocytos Stream: A Stellar Fossil from the Milky Way’s Past

Christian Aganze and his team present an in-depth study of the Cocytos stream — a long, faint structure of stars believed to be a remnant from a past event in the Milky Way’s history. Stellar streams like Cocytos are important because they are formed when small star clusters or dwarf galaxies are torn apart by the gravitational pull of the Milky Way. The stars that make up these streams carry information about the original system they came from, making them valuable tools for studying how our galaxy was built over time.

Rediscovering Cocytos with Modern Surveys

The Cocytos stream was first discovered in 2009 using images from the Sloan Digital Sky Survey, but at the time, astronomers lacked enough data to understand what it really was. This paper reports its re-identification in the third year of the Dark Energy Spectroscopic Instrument (DESI) Milky Way Survey. By analyzing the motions and chemical properties of giant stars in the outer regions of the galaxy, the team was able to pinpoint a group of stars all moving together — evidence of a stream. Follow-up observations using the Magellan telescope confirmed that many of these stars share similar distances, velocities, and chemical compositions.

A Globular Cluster Origin?

One of the main questions the authors address is where Cocytos came from. By analyzing the stream's metallicity (its content of elements heavier than hydrogen and helium), they found it to be relatively metal-rich at [Fe/H] ≈ -1.3. This is more typical of globular clusters — dense groups of stars that formed early in a galaxy's life — than dwarf galaxies. The stream also has a fairly large velocity dispersion (about 8 km/s) and is quite thick, but these measurements still align more closely with a globular cluster origin, especially when considering its brightness and total mass.

Mapping the Motion: The Orbit of Cocytos

To understand the stream’s journey through the galaxy, the authors calculated its orbit using a combination of DESI and Magellan data. They found that Cocytos follows a stretched, eccentric path that takes it from a minimum distance of 23 kiloparsecs to as far as 100 kiloparsecs from the center of the Milky Way. This type of orbit is typical for stars and clusters that came in with the Gaia–Enceladus merger, a major event when the Milky Way absorbed another galaxy billions of years ago. Cocytos’s path is also similar to that of the Pyxis globular cluster, suggesting a potential connection between the two.

How Cocytos Compares to Other Streams

The researchers compared Cocytos’s properties — including its brightness, metallicity, and width — to other known stellar streams and clusters. They found that Cocytos is one of the brightest and most metal-rich globular cluster streams discovered so far. While it is thicker and more kinematically heated (meaning the stars move with a broader range of speeds) than most globular cluster streams, this could be explained by it having fallen into the Milky Way as part of a larger, now-disrupted system, which may have heated it up over time.

Ties to the Virgo Overdensity and the Milky Way’s Violent Past

Cocytos lies in a region of the sky called the Virgo Overdensity (VOD), an area thought to be filled with debris from a massive merger. The VOD and related structures, like the Hercules–Aquila Cloud and the Eridanus–Phoenix overdensities, are believed to have formed from the same or similar events as the Gaia–Enceladus merger. The Cocytos stream shares chemical and orbital properties with these features, further suggesting it was once part of a larger satellite galaxy that merged with the Milky Way.

Conclusion: A Stellar Clue to a Cosmic Collision

In sum, Cocytos appears to be a disrupted globular cluster that likely entered the Milky Way during the Gaia–Enceladus event. It stands out for its high metallicity, relatively large mass, and thick structure. While its origin is still being studied, the evidence strongly points to a connection with the dramatic history of galactic mergers that helped shape the Milky Way. This discovery highlights the power of modern spectroscopic surveys like DESI to reveal the hidden structure of our galaxy and piece together its evolutionary past.

Source: Aganze