In materials issued with the new research, Helmi said that the chemical signature of many wrong-way stars was demonstrably different than “native” Milky Way stars.
“And they are a fairly homogeneous group, which indicates they share a common origin,” she said. “The youngest stars from Gaia-Enceladus are actually younger than the native Milky Way stars in what is now the thick disk region. This means that the progenitor of this thick disk was already present when the fusion happened, and Gaia-Enceladus, because of its large size, shook it and puffed it up.”
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Size is a tricky concept when working on the galactic scale, but Helmi’s team estimate that Gaia-Enceladus, the dearly departed galaxy, was slightly more massive than the Small Magellanic Cloud (SMG), a dwarf galaxy with a total solar mass about one percent of the Milky Way. (By the way, the Milky Way is slowly eating the SMG, too.)
An interesting final note: When Helmi finally pieced together all the data from the Gaia info dump, she discovered that the numbers looked awfully familiar. They reminded Helmi of simulations performed by a former Ph.D. student more than ten years ago. The student’s simulations of the merging of a large disc-shaped galaxy with the young Milky Way produced results that were totally in line with the Gaia data.
“It was amazing to look at the new Gaia data,” Helmi said, “and realize that I had seen it before.”