Around the Milky Way there are many river-like structures. Being made up of stars, they are called stellar streams. How did these stellar streams form?
A recent study led by Prof. Gang Zhao and Dr. Jiang Chang from National Astronomical Observatories of Chinese Academy of Sciences (NAOC) has reproduced the formation process of the newly discovered Cetus stream in a computer, using a series of high-resolution dynamics numerical simulations. The results have been published in The Astrophysical Journal.
"Stellar streams are the remnants of dwarf satellite galaxies that swallowed by the Milky Way, but have not been fully digested," said Dr. Jiang Chang, the first author of the research paper. "The accretion process is not that the Milky Way swallowed the dwarf galaxy in one bite, but it peeled the dwarf galaxy layer by layer from outside to inside by tidal stripping, just like peeling an onion. The stripped stars distributed in their original orbits, and they formed a river-like structure, that is, a stellar stream."
The Milky Way galaxy grows by constantly devouring dwarf satellite galaxies. Scientists call this process the galaxy merger. Through the study of the merging history of the Milky Way, we can know how the Milky Way formed and evolved.
In their previous study, the researchers discovered the Cetus stream based on the observational data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also known as Guoshoujing Telescope) Survey and the Sloan Digital Sky Survey. Then, in this work, the formation history of this stellar stream was reconstructed in the supercomputer through numerical simulations, and a simple portrait of the Cetus Stream progenitor before being swallowed by the Milky Way was provided.
"Our work shows how the Milky Way slowly peeled apart and swallowed a dwarf galaxy with a mass of about 20 million times of the sun over a period of 5 billion years," said Dr. Jiang Chang. "This swallowing process is very important for the growth of the Milky Way. Through the study of stellar streams, we can understand how the Milky Way accreted dwarf galaxies and grew into nowadays such great mass," added by Prof. Gang Zhao, the co-corresponding author of this work.
Satellite galaxies are usually not stripped by tide force quickly and all form stellar streams at once. Instead, there always remains a core structure composed of relatively dense stars. Some previous research works put forward the hypothesis that the globular star cluster NGC 5824 is a core structure associated with the Cetus Stream. But in this work, the researchers 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. Jiang 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."
The distribution of stellar streams is usually throughout the sky. While LAMOST helped to discover the Cetus stream in the northern sky, the researchers also found the candidate counterpart of the Cetus stream in the southern sky, by comparing the results of their numerical simulations with the stellar streams found by the Dark Energy Survey (DES) through international cooperation. That is, the Palca stream. This points out the direction for future studies.
"There are a large number of merging relics in the Milky Way similar to the Cetus stream," said Prof. Gang Zhao. "They compose a treasure house for studying the structure and formation history of the Milky Way. It can also help us to better understand how galaxies in the universe have formed and evolved."
Figure: The simulated process of Cetus Stream progenitor merging with the Milky Way. (Credit: Jiang Chang).
The paper can be accessed at https://iopscience.iop.org/article/10.3847/1538-4357/abc338.