A primordial merger of galactic building blocks

The prevailing cosmological model is labeled by its two principal components: Λ, which represents dark energy, and CDM, which stands for cold dark matter. Because of its coldness, primordial dark matter readily clumped together under its own gravity whenever a random fluctuation raised the local density. From those clumps and the baryonic matter they entrained grew the first galactic building blocks: primordial dwarf galaxies.

Those galactic building blocks merged to form larger galaxies in a process that continues to play out. But some of the dwarf galaxies, having failed to find partners, have remained single, small, and faint. Indeed, it wasn’t until 2007, when the 10-meter Keck 2 was brought to bear on promising candidates, that the first examples were discovered in orbit around our own galaxy.

A typical ultrafaint dwarf galaxy (UFD) has a luminosity equivalent to just 10⁵ Suns, contains 1000 times as much dark matter as baryonic matter, and has a brightness profile that falls off exponentially from the center. But last year, MIT’s Anirudh Chiti and his collaborators discovered that 7 of 19 red giant stars in a UFD called Tucana II are located far beyond the dwarf’s central core. The brightness profile of Tucana II falls off not like a typical UFD’s but like an elliptical galaxy’s.

Yuta Tarumi and Naoki Yoshida of the University of Tokyo and Anna Frebel of MIT have recently published an explanation for the unusually extended halo of Tucana II. Using numerical simulations, they charted the evolution of six UFD progenitors. Four went on to form dwarf galaxies whose profiles resembled those of a typical UFD observed today. But two of the simulated progenitors merged at a time 510 million years after the Big Bang.

After the initial collision, the smaller of the two progenitors passed through the larger and shed a trail of stars in its wake. Then, the progenitors’ mutual gravitational attraction pulled them back together, and they passed through one another a second time. After a further 2500 million years, a single, stable dwarf galaxy remained. Its brightness profile was similar to that of an elliptical galaxy—and Tucana II’s.

The simulations also revealed that the merger triggered a second burst of star formation. The resulting stars were richer in iron and other elements than those of the progenitors’ first generation.

UFDs are of intense interest. Besides being the building blocks from which galaxies and larger structures are assembled, they harbor the first generation of stars. Being 99.9% dark matter, they also offer one the best prospects for finding the first evidence that dark-matter particles interact with each other through forces other than the gravitational force. That clue could be decisive in identifying what dark matter is made of.

The observations of Tucana II by Chiti and others and the simulations by Tarumi, Yoshida, and Frebel do not make UFDs less interesting. Rather, they will help astronomers evaluate which UFDs are truly pristine. Those UFDs will be the best candidates from which to draw inferences about some of the most important unanswered questions in astronomy and physics. (Y. Tarumi, N. Yoshida, A. Frebel, Astrophys. J. Lett. 914, L10, 2021 .)