Dwarf galaxies pose new questions about dark matter and the early universe that models are struggling to answer

Abstract

Late one night in October 2015, Andrew Wetzel was fretting. For 15 days, his cosmological models had been swirling virtual dark matter around cybernetic gas and dust and slowly generating a synthetic galaxy approximately the size of our own Milky Way, and Wetzel was about to receive the results. “I finally got the plot up to compare our simulation with the Milky Way,” recalls Wetzel, an assistant professor of physics at the University of California, Davis. “I went to bed very happy that night.” Cosmological simulations such as this one have gotten better at reproducing dwarf galaxies, but questions remain. This simulation shows a Milky Way-like galaxy at the center surrounded by a realistic population of satellite dwarf galaxies and a stellar halo. Image credit: Andrew Wetzel (University of California, Davis, CA) and Phil Hopkins (California Institute of Technology, Pasadena, CA). That’s because, for the first time, a simulation had accurately reproduced a realistic population of dwarf galaxies near the Milky Way. These small companions orbit larger galaxies like planets around a star, and for a decade and a half, they had presented astronomers with a big problem. Simulations suggested that thousands of companion galaxies should surround the Milky Way, but telescopes had seen only a handful. Researchers seemed to be in the dark about some important aspect of the cosmos. With the latest simulations from Wetzel and his team, the mystery of the missing satellites might appear to be explained (1). But more questions are emerging, with newly discovered galaxies more diverse than anyone predicted, and simulations, as yet, unable to recreate their richness. There may even turn out to be too many dwarfs for simulations to contend with. Future supercomputer models and next-generation telescopes will address these conundrums, potentially illuminating a critical stage in the growth of the …