This story takes a quick look at the ratio of visible matter to invisible dark matter in a nearby dwarf galaxy. 

Large, healthy galaxies like ours, with star formation and a complex structure, are thought to form through the merger of dwarf galaxies. We can even observe myriad small galaxies being gravitationally pulled into our own Milky Way and shredded as our galaxy consumes them. Researchers study today’s surviving dwarf galaxies to understand what went into making our galaxy. One of these dwarf galaxies is Tucana II, a dwarf galaxy faintly visible in the southern sky. This system has very primitive stars that are a thousand times less rich in heavy elements than our Sun and that recent observations revealed to be larger than previously thought.

In new work published in Nature Astronomy with first author Anirudh Chiti, astronomers were able to identify knots of super bland stars far from the galaxy’s core. The average star in Tucana II has a thousand times fewer heavy elements than our Sun, as we mentioned before, and these new stars have one-third of those heavy elements. Since heavy elements form over time inside stars, this implies that the stars in the outskirts formed from more primitive — more bland — material than the core.

In fact, this dwarf galaxy may actually be the result of two even smaller dwarf galaxies merging, with the two sets of stars coming from different systems.

The motions of these stars also allow us to measure the galaxy’s mass, and it appears this system has significantly more — three to five times more — dark matter than previously thought. According to co-author Anna Frebel: We have thought that the first galaxies were the tiniest, wimpiest galaxies. But they actually may have been several times larger than we thought, and not so tiny after all.

The two galaxies, now appearing as one, will eventually be consumed by the Milky Way. It’s a galaxy-eat-galaxy universe out there.

More Information

MIT press release

“An extended halo around an ancient dwarf galaxy,” Anirudh Chiti et al., 2021 February 1, Nature Astronomy