The Milky Way is a dynamic museum of ancient merging relics, river-like streams of stars stripped from dwarf satellite galaxies that flow through the galaxy revealing its history and structure that allow astronomers to better understand how galaxies in the universe have formed and evolved.
Remnants of Dwarf Satellite Galaxies
“Stellar streams are the remnants of dwarf satellite galaxies that are swallowed by the Milky Way, but have not been fully digested,” said Zhao Gang from National Astronomical Observatories of Chinese Academy of Sciences (NAOC). “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 Cetus Stream
New research led by first author Gang and Chang Jiang, also with NAOC, simulated the formation process of the newly discovered Cetus stream through a series of high-resolution dynamics numerical simulations, and provided a simple portrait of the Cetus Stream progenitor before being devoured by the Milky Way.. How these stellar streams formed remains unclear. In a previous study, the researchers discovered the Cetus stream based on the observational data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Survey and the Sloan Digital Sky Survey.
“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 Zhao Gang, the co-corresponding author of the study.
The simulated process of Cetus Stream progenitor merging with the Milky Way. (CHANG Jiang)
The Globular Star-Cluster Hypothesis
In satellite galaxies, there always remains a core structure composed of relatively dense stars. Some researchers put forward the hypothesis that the globular star cluster NGC 5824 in the constellation Lupus is a core structure associated with the Cetus Stream. But in this work, the researchers overturned this hypothesis through detailed numerical simulations. Globular star clusters are extraordinary, densely packed objects, holding a million stars in a ball only about 100 light-years across on average, dating back almost to the birth of the Milky Way.
“The globular cluster NGC 5824 is not the remnant core structure corresponding to the Cetus stream, because the dynamic feature is not correct,” 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.”
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–the Palca stream.
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