But scientists are still unable to explain the existence of galaxies.
In 2018, astronomers made a remarkable discovery. Encounter a Galaxy (NGC 1052-DF2) It appears to have almost no dark matter. The discovery was remarkable, because until then it was believed that dark matter dominates all galaxies. In fact, astronomers believed that all galaxies were born in massive dark matter halos.
Dark matter is an unproven substance that we can’t see. So the existence of this issue is inferred from what we can see. Very concretely, this involves looking at rapidly orbiting galaxies and galaxy clusters. These are spinning really fast and – judging by what you can see – you would expect them to disperse. But this does not happen. And that would be due to the invisible matter holding things together – by giving the galaxy extra mass.
The fact that NGC 1052-DF2 does not appear to contain any dark matter has spoiled everything we thought we knew about galaxies and their formation. No wonder the results were immediately called into question. In particular, the claim that the galaxy was 64 million light-years away has been questioned. A year later he appeared until Investigation Which concluded that the galaxy was much closer than previously thought and must still consist largely of dark matter.
But new distance measurements from the Hubble Space Telescope – the most accurate ever – now show that NGC1052-DF2 is slightly further away than previously thought. This reinforces the idea that the galaxy contains hardly any dark matter. “I think it makes sense for people to question our findings (from 2018, editor), because they were so unusual,” researcher Peter van Dokkum says. “It would be nice if there was a simple explanation for that, like measuring the wrong distance.” But this does not appear to be the case. Van Dokkum does it well. “I think it’s more fun and interesting if it’s actually an alien galaxy.
Distance and dark matter
Why does the distance between us and NGC 1052-DF2 determine whether or not a galaxy contains dark matter? It has to do with the mass of galaxies. As mentioned, galaxies are usually made up of visible matter (think stars) and invisible matter (dark matter). The total mass of the galaxy can be calculated by studying the motions of the stars. Their speeds are actually affected by gravity (which in turn results from the total mass of the galaxy). To calculate only the mass of visible matter, you need to determine the absolute brightness of the galaxy. Because a galaxy is brighter, the more stars there are in that galaxy and the more stellar mass it has. But to determine the absolute brightness, you need to know how far the galaxy is from us. Because the farther away the galaxy is, the less bright it is. Go back to NGC 1052-DF2 for a moment. In 2018, it was said to be about 64 million light-years away from us. The galaxy looks very faint from Earth, but that can largely be traced back to the great distance between us and the galaxy. If you could see the galaxy up close, it would be very bright and therefore many stars and a large stellar mass. So large, in fact, that the stellar mass is almost equal to the total mass of the galaxy, so there is hardly any dark matter. In 2019, researchers reported that the galaxy was much closer to Earth. In this scenario, the faint galaxy would not be very bright and heavy. The result is a smaller stellar mass. In the meantime, the total mass remains the same. Thus a greater proportion of the total mass will be attributed to dark matter.
But now researchers have re-examined the distance between us and NGC 1052-DF2. They studied red giants at the edges of the galaxy using the Hubble Space Telescope. Then they used the brightness of these stars to calculate their distance. “Studying the brightest red giant planets is a common method for calculating the distance to nearby galaxies,” said researcher Zile Shen. Based on the red giants, NGC 1052-DF2 is 72 million light-years away. It is roughly in line with 2018 results and supports the conclusion that the galaxy’s total mass is almost entirely attributable to visible matter.
The whole iceberg
“For nearly every galaxy we look at, we can’t see most of the mass because it’s dark matter,” van Dokkum says. “But in this case it is: What you see is what you get. The Hubble Space Telescope really shows us everything. That is it. It’s not the tip of the iceberg. It’s the whole iceberg.”
Meanwhile, it is still not clear how this galaxy formed in the almost complete absence of dark matter. “There is an old saying that extraordinary claims require extraordinary evidence, and the new distance measurements support our previous conclusion that DF2 lacks dark matter,” Shin said. “Now we need to put the remote discussion behind us and focus on how these galaxies form.”
Because it is becoming increasingly clear that NGC 1052-DF2 is not alone. For example, in 2018, researchers already reported that galaxy NGC 1052-DF4 also appears to contain virtually no dark matter. And last year, an independent research group discovered 19 other dwarf galaxies that appear to be poor in dark matter.
The search continues for galaxies that contain hardly any dark matter. Because researchers want to clarify how “normal” these still strange ducks are. And what other unique characteristics they might have. This can give more ideas about how it might look. Finally, by comparing these dark matter-deficient galaxies with dark-matter-rich galaxies, the researchers also hope to gain more clarity about what still mysterious dark matter is.
The Hubble data on which this study is based was collected some time ago. Hubble is currently inactive. The space telescope has been dealing with computer problems since Sunday. NASA is working hard to fix this.
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