Astronomers have discovered the most distant stellar stream! This wispy trail of stars encircles a dim galaxy more than 100 million light-years awaya record for stream detection. It’s the first stellar stream ever observed beyond our Milky Way, and it could provide astronomers with their best way yet to chart the dark matter structure that surrounds all galaxies.
The crux of the discovery is an ultra-diffuse galaxy called UGC 9050-Dw1. This combination of features is a strange one, as these loosely-woven and sparse pop he of stars have a ghostly quality themselves as they float against the void of space. But this sparsity actually proved to be an extra bonus, as the component stars could be seen easily in the Hubble archive images as a thin arc in the background.
The arc was then identified as the debris of a globular cluster, a dense sphere of very old stars that was apparently tor- tured and flayed by the galaxy over billions of years. What is unique about this discovery is not the stream alone. Stellar streams are like heavenly breadcrumbs. If you zoom in on each of the stars along the ribbon, they all lie along similar lines, as if everything were looking toward the same final destination. This trajectory is solely determined by the galaxy’s overall gravitation; the mass of the celestial bodies in the galaxy and the invisible mass of dark matter that surrounds them.
With the knowledge of the stream’s positional points, it is possible to determine the entire mass of the galaxy: these findings indicated that UGC 9050-Dw1 was embedded in a massive, dark matter halo, in line with what theorists had expected of these ultra-diffuse objects. Since dark matter does not give off or bounce around light, astronomers have no direct way of observing it; they depend on the effects gravity has on normal matter to detect its presence. Before this investigation, stellar stream dark-matter analyses have only been performed on the one galaxy we have three-dimensional data for to work with, Earth.
With the stream mapped using Hubble, the team moved on to computer models and simulations. They ran through thousands of potential variations, by changing the original mass of the broken-up cluster, and the structure of the dark matter halo, until their virtual streams matched Hubble image.
When their models were most accurate, they discovered that the visible stars by themselves could not account for the stream’s trajectory. This is just an exhilarating future direction to take. Future survey and telescope projects will uncover many more stellar streams around distant galaxies. Each one would be an independent probe of dark matter on scales that has been rare to measure so far. Knowing the distribution of dark matter in both ellipticals and spirals is key in constraining theorybe it heavy particle, light particle, or even more bizarre.