The light galaxy at the heart of a dark dispute
Dark matter is considered the glue that holds galaxies together – but a few years ago, a team of astronomers claimed to have found a galaxy that didn’t have any. Other scientists later argued it was a calculation error, but now the original team has used Hubble to make more robust observations and confirm their findings.
This new study is the latest episode in a long-running saga that really illustrates the scientific method at work. In 2018, a team of astronomers from Yale announced the discovery of DF2, a dwarf galaxy that appeared to have no, or at least very little, dark matter. This raised new questions about how the galaxy could have formed at all without that crucial glue holding it together.
The following year, an independent team checked the Yale team's work and conducted their own analysis, concluding that DF2 had a normal amount of dark matter. And now, the original team has studied the galaxy in more detail, and is saying they were right the first time – DF2 is dark-matter-deficient. So what’s really going on here?
The discrepancy comes down to distance. All the measurements of a galaxy’s dark matter content are dependent on how far away the galaxy is from Earth, and this is the main point of difference that the two teams are arguing about.
The original study said that DF2 was about 64 million light-years away, based on surface brightness fluctuation, which measures the variance in the light distribution of the stars. From this they were able to infer the galaxy’s mass, and found that the visible stars accounted for almost all of it. There was no “room” left for dark matter.
But in the second study, astronomers from the Instituto de Astrofísica de Canarias (IAC) in Spain revised that distance to just 42 million light-years. They used five different methods to arrive at that figure, finding that the galaxy was likely much closer but contained fewer stars. When the galaxy was re-examined with this in mind, its stars were only found to account for about a quarter of its mass, indicating a normal matter-to-dark matter ratio.
And now, the original Yale team has gone back to the drawing board, collecting more data to try to measure DF2’s distance and dark matter content more accurately. The astronomers used 40 orbits’ worth of data from Hubble to take images of the galaxy with longer exposures to provide a deeper view.
Specifically, the team focused on red giant stars in the outskirts of DF2. These aging stars have an intrinsic brightness, allowing astronomers to determine how far away they are by measuring their apparent brightness. This technique is called tip of the red-giant branch (TRGB), and it’s considered one of the most accurate methods.
The Yale researchers say that the TRGB technique not only validates their original finding that DF2 has very little dark matter, but the revised distance puts it at about 72 million light-years away – even further than their previous estimate.
By their calculations, DF2 is almost the same size as the Milky Way, but is far more diffuse, containing only 0.5 percent of the stars and at most just 0.25 percent of the dark matter.
However, there’s another unusual wrinkle to the story. The TRGB method was one of those used by the IAC team to reach their conclusion that the galaxy is much closer. How the two teams reached wildly different results remains to be seen, and potentially leaves the door open for further counterarguments.
If DF2 does turn out to be light on dark matter, it will have some major implications for cosmology. For one, it could be the exception that proves the rule – if dark matter was merely a mathematical misunderstanding, as has been suggested, then the same error should apply evenly to every galaxy observed. Finding one that happens to have little dark matter adds evidence that it is a physical substance that can be present in various amounts.
It does, however, raise other questions. The gravitational pull of dark matter is thought to be pivotal in the formation of galaxies, so how did DF2 form without it? One recent study proposes an explanation: DF2 and DF4 – a neighboring dwarf galaxy that’s also quite light – may have had their dark matter stripped away due to tidal forces from a much larger galaxy nearby.
The researchers say that future studies should focus on finding other galaxies that lack a dark heart, which could help unlock the mysteries.
The research was published in the Astrophysical Journal Letters. The team describes the work in the video below.