There are literally dozen of dwarf galaxies around the Milky Way that continue to slowly be absorbed into ours. These galaxies are of great interest to astronomers as they can teach us a lot about cosmic evolution, such as how smaller galaxies merged into larger structures over time. Since they are considered relics of the very first galaxies in the universe, they also resemble “galactic fossils”.
Recently, a team of astrophysicists from the Massachusetts Institute of Technology (MIT) observed one of the oldest of these galaxies (Tucana II) and noticed something unexpected. At the edge of the galaxy, they observed stars in a configuration that suggests that Tucana II has an expanded halo of dark matter. These results suggest that the oldest galaxies in the universe had more dark matter than previously thought.
The research was led by physics student Anirudh Chiti from MIT’s Kavli Institute for Astrophysics and Space Exploration and Anna Frebel, Associate Professor of Physics for the Silverman Family Career Development at MIT. They were joined by several colleagues from Kavli as well as the observatories of the Carnegie Institution of Washington, the ANU Research School of Astronomy and Astrophysics, and UC Berkeley.
Part of the virtual universe, a billion light years in diameter, shows how dark matter is distributed in space, with dark matter surrounding the yellow clumps that are connected by dark filaments. Photo credits: Joachim Stadel, UZH
In summary, dark matter refers to the invisible mass that astronomers began to theorize about in the 1960s. It makes up 85% of the matter in the universe and about a quarter of its total mass-energy density. While all attempts to find a candidate particle for dark matter have (so far) been unsuccessful, scientists can observe its influence on large-scale structures (such as galaxies and galaxy clusters).
A perfect example of this is dark matter halos, which refers to a local mass concentration that penetrates, surrounds and holds together galaxies, groups and clusters of galaxies. The presence of these halos is determined by observing the rotation curves of galaxies and the movements of galaxies in groups and clusters that astronomers have found to be inconsistent with the amount of matter they can see (also known as “luminous matter”) .
Tucana II is an ultra-weak dwarf galaxy located approximately 163,000 light years from Earth, towards the Tucana constellation. Due to the age of its stars (all old and very faint red stars) and its low metallicity, Tucana II is one of the most primitive dwarf galaxies known. Previously, astronomers had identified stars around their core with such low metal content that the galaxy was considered the oldest known ultra-weak dwarf galaxy.
For their study, Chiti, Frebel and their team observed Tuscana II to determine whether this ancient galaxy could contain even older stars – whose investigation could provide insights into the formation of the universe’s first galaxies. It is estimated that these formed around 13 billion years ago, just 800 million years after the Big Bang. To test this, they received data from the SkyMapper Telescope, an optical ground-based telescope in Australia.
The proximity of the extremely faint dwarf galaxy Tucana II as imaged with the SkyMapper telescope. Credits: Anirudh Chiti, MIT
They then used an image filter to identify particularly faint, metal-poor stars and combined their observations with an algorithm (developed by Chiti) to identify them. In addition to the previously identified stars near the core, they observed nine new ones on the edge of Tucana II. They also found that they were in a configuration that suggested they were being caught by the galaxy’s gravitational pull.
This was surprising as they were far from the core, suggesting that Tucana II has an expanded halo of dark matter that is three to five times as massive as previously thought. “Tucana II has a lot more mass than we thought to bind these so distant stars,” said Chiti. “This means that other relic first galaxies are likely to have such extended halos as well.”
Chiti and Frebel followed these results using data previously obtained from the Magellan telescopes at the Las Campanas Observatory in Chile. These observations indicated that the nine new stars were even more metal-poor (older) than those in the core. These results are the first evidence that ultra-weak dwarf galaxies have extensive halos and could have significant implications for cosmological theories. As Frebel explained:
“This probably also means that the earliest galaxies formed in much larger halos of dark matter than previously thought. We thought that the first galaxies were the smallest, faintest galaxies. But they were perhaps many times larger than we thought and yet not that small. “
Illustration of what the night sky could look like billions of years from now as the Andromeda Galaxy slowly approaches merging with the Milky Way. Photo credit: NASA / ESA / Z. Levay and R. van der Marel, STScI / T. Hallas; and A. Mellinger
In addition, the imbalance between ancient stars near the core and even older stars on the outskirts could indicate that Tucana II may have been the result of one of the first mergers in the universe. This process of “galactic cannibalism” is taking place all over the universe today and will take place in about 3.75 billion years between the Milky Way and the neighboring Andromeda galaxy.
However, until now it has been unclear whether or not early galaxies merged in a similar fashion. In this context, Frebel claims that what they observed could be another:
“We may see the first signature galactic cannibalism. A galaxy may have eaten one of its slightly smaller, more primitive neighbors, who then buried all the stars on the outskirts. Tucana II will eventually be eaten by the Milky Way, no mercy. And it turns out that this ancient galaxy has its own cannibalistic history. There are probably many more systems, perhaps all of which have those stars blinking on the outskirts. “
In the near future, the team plans to use the same approach to observe other ultra-weak dwarf galaxies around the Milky Way. If they happen to find many other cases of very old stars orbiting near the edges of dwarf galaxies, it suggests that dark matter played a particularly important role in the fusion of ancient galaxies and their subsequent evolution.
The study describing its results, “Chemical Abundances of New Member Stars in the Tucana II Dwarf Galaxy,” recently appeared in the Astrophysical Journal. The research was made possible in part thanks to support from NASA and the National Science Foundation (NSF).
Further reading: MIT
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