Some 163,000 lightyear of the Milky Way a much smaller, much older galaxy: Tucana II, so named after the tropical bird-like constellation in which it resides. Sitting at the edge of our galaxy’s gravity, Tucana II offers researchers the opportunity to understand the composition of the earliest galactic structures in the universe.
Now it has a team of astronomers found evidence of an extensive halo of dark matter around the galaxy. Their research was published today in the journal Nature Astronomy.
“We know [dark matter] is there because to keep galaxies bound, there has to be more matter than what we see visible from starlight, ”said Anirudh Chiti, an astronomer at the Massachusetts Institute of Technology, during a telephone conversation. “That led to the hypothesis that dark matter is an ingredient that holds galaxies together; without it, galaxies we know, or at least the things on their edges, would fly apart. “
A dark matter halo is a region of gravity bound matter in space. (The dark matter halo of the Milky Way extends much further the pinwheel that makes up the visible stuff of our galaxy). The team found that Tucana II’s gravitational limits are between three and five times as heavy as previously thought that even some of the oldest galaxies will have dark matter halos.
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Tucana II happens to be the most chemically primitive galaxy we know today, meaning some of its stars have very low metal content (the heavier elements of the universe were produced later in time). The team realized that Tucana II had the halo of dark matter when observations of stars in that part of the sky revealed that the stars were moving together.
“If you only look at the area of the sky where the Milky Way exists, you don’t really see a clustering or over-density of stars,” Chiti, Who is lead author of the recent article, said. “It is only when you look at their speeds and realize that it is a group of stars moving at the same speed that you realize that a galaxy exists there.”
As study co-author Anna Frebel, also an astronomer at MIT, put it in a university press release, the swirl of Tucana II’s movement resembles “ bath water running down the drain. ” It is undeniable that some of the galaxy’s peripheral stars are older than stars closer to the galactic center. The team hypothesizes that Tucana II may be the result of a previous galactic fusion, a cosmic collision in which one primitive galaxy was consumed by another, resulting in stars of different origins in the same galaxy.
Whether that theory of the origin of Tucana II is true or not, a similar clash is certainly in the future. Being in the gravitational region of the much more massive Milky Way, the relatively small galaxy will eventually be swallowed up by ourselves.
Although astronomers know how to spot dark matter halos, they still don’t know exactly what dark matter is. In addition to finding halos around galaxies, researchers are also looking for the identity of dark matter in mysterious signals from neutron stars and in the form of small, theoretical black holes.