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NASA/ESA Hubble Space Telescope
In 2018 an international team of researchers using the NASA/ESA Hubble Space Telescope and several other observatories uncovered, for the first time, a galaxy in our cosmic neighbourhood that is missing most of its dark matter. This discovery of the galaxy NGC 1052-DF2 was a surprise to astronomers, as it was understood that Dark matter (DM) is a key constituent in current models of galaxy formation and evolution. In fact, without the presence of DM, the primordial gas would lack enough gravity pull to start collapsing and forming new galaxies. A year later, another galaxy that misses dark matter was discovered, NGC 1052-DF4, which further triggered intense debates among astronomers about the nature of these objects.
Now, new Hubble data  have been used to explain the reason behind the missing dark matter in NGC 1052-DF4, which resides 45 million light-years away. Mireia Montes of the University of New South Wales in Australia led an international team of astronomers to study the galaxy using deep optical imaging. They discovered that the missing dark matter can be explained by the effects of tidal disruption. The gravity forces of the neighbouring massive galaxy NGC 1035 are tearing NGC 1052-DF4 apart. During this process, the dark matter is removed, while the stars feel the effects of the interaction with another galaxy at a later stage...
Globular clusters are thought to form in the episodes of intense star formation that shaped galaxies. Their compact sizes and luminosity make them easily observable and they are therefore good tracers of the properties of their host galaxy. In this way, by studying and characterising the spatial distribution of the clusters in NGC 1052-DF4, astronomers can develop insight into the present state of the galaxy itself. The alignment of these clusters suggests they are being “stripped” from their host galaxy, and this supports the conclusion that tidal disruption is occurring.
By studying the galaxy’s light, the astronomers also found evidence of tidal tails, which are formed of material moving away from NGC1052-DF4 — this further supports the conclusion that this is a disruption event. Additional analysis concluded that the central parts of the galaxy remain untouched and only ∼ 7% of the stellar mass of the galaxy is hosted in these tidal tails. This means that dark matter, which is less concentrated than stars, was previously and preferentially stripped from the galaxy, and now the outer stellar component is starting to be stripped as well...
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Very interesting. If I understand correctly, the optical information provides evidence for tidal disruption in the galaxies. But there is no direct evidence that the DM was actually stripped, it is a hypothesis that since tidal forces are present, DM could have been stripped before. Shouldn't this extra DM affect NGC 1035 behavior (gravity should have pulled it towards NGC 1035)? Do we have any evidence of DM being stripped before? Since all evidence for DM is only indirect, to prove that it has been displaced from place A (periphery of the galaxy) to place B (closer to a larger mass neighbor) you should show gravitational effects on B.
Secondly, the NASA article states that "without the presence of DM, the primordial gas would lack enough gravity pull to start collapsing and forming new galaxies". I was not aware that DM was necessary for galaxy formation modeling, I mean DM was invoked to explain abnormally high speeds in galactic periphery due to its gravitational effects, not galactic formation processes.
Trying to clarify my ideas...