Around the Milky Way, there are many river-like structures made up of stars. They are called stellar streams. How these stellar streams formed remains unclear.

Researchers led by Professor Zhao Gang and Dr. Chang Jiang from the National Astronomical Observatories (NAOC) of the Chinese Academy of Sciences reproduced the formation process of the newly discovered Cetus stream using a computer.

The study was recently published in The Astrophysical Journal.

“Stellar streams are the remnants of dwarf satellite galaxies that were swallowed by the Milky Way, but have not been fully digested,” said Dr. Chang, the study’s first author. “During the accretion process, the Milky Way does not swallow a dwarf galaxy in one bite, instead peeling the dwarf galaxy layer by layer from outside to inside through tidal stripping. It’s just like peeling an onion. The stripped stars distribute in their original orbits, and they form a river-like structure, that is, a stellar stream.”

The Milky Way galaxy grows by constantly devouring dwarf satellite galaxies, a process that is called galaxy merging. By studying the Milky Way’s history of mergers, we can learn how the Milky Way formed and evolved.

In their previous study, the researchers discovered the Cetus stream based on observational data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also known as the Guoshoujing Telescope) Survey and the Sloan Digital Sky Survey.

Now, they have used a supercomputer to reconstruct the formation history of this stellar stream, relying on a series of high-resolution dynamics numerical simulations. The simulation provided a simple portrait of the Cetus stream progenitor before it was swallowed by the Milky Way.

The simulated process of the Cetus stream progenitor merging with the Milky Way [IMAGE: CHANG JIANG]

“Our work shows how the Milky Way slowly peeled apart and swallowed a dwarf galaxy with a mass about 20 million times that of the sun over a period of 5 billion years,” said Professor Zhao, the co-corresponding author of the study.

In satellite galaxies, there always remains a core structure composed of relatively dense stars. Some researchers have previously put forward the hypothesis that the globular star cluster NGC 5824 is a core structure associated with the Cetus stream. But during the recent progress achieved, the scientists overturned this hypothesis through detailed numerical simulations.

“The globular cluster NGC 5824 is not the remnant core structure corresponding to the Cetus stream, because the dynamic feature is not correct,” Dr. Chang said. “But we found that there is a strong correlation between the two. NGC 5824 should be a globular cluster in the Cetus stream progenitor galaxy.”

Stellar streams are distributed throughout the sky. While LAMOST helped to discover the Cetus stream in the northern sky, the researchers also found a candidate counterpart of the Cetus stream in the southern sky — the Palca stream.

“There are a large number of merging relics in the Milky Way similar to the Cetus stream,” said Professor Zhao. “They provide a treasure trove of information for those studying the structure and formation history of the Milky Way and help us to better understand how galaxies in the universe have formed and evolved.” 

Source: National Astronomical Observatories,

Chinese Academy of Sciences