The Absolute Cosmos: Solved! Dark Matter mystery surrounding ultra diffuse galaxy DF4

Image credit: Montes et al., ApJ, 2020

In 2019, scientists announced the discovery of a small, faint, ultra diffuse galaxy (UDF). To their surprise, they found galaxy shockingly lacking dark matter. The galaxy is called NGC 1052-DF4 or DF4 and it lies 44 million light years away from us.

The discovery challenged our current galaxy formation models, as dark matter is vital for galaxies to form in the first place. DF4 represents the proof that galaxies could survive without sufficient dark matter.

But now according to a new study, scientists discovered that DF4’s missing dark matter can be explained by the gravitational effects of nearby massive galaxy.

As per scientists DF4 is actually lacking in dark matter because its already been removed by hungry its neighbour. This hungry neighbour is called galaxy NGC1035 and its gravitational pull is removing DF4’s stars and dark matter.

As of now, scientists can not detect dark matter directly, its existence can only be inferred by the gravitational effects it seems to have on visible matter.

Most of the galaxies have much more gravity than their detectable matter is suppose to create. Some invisible mass in our universe is creating this extra pull.

As per our current understanding of galaxy formation, without this invisible mass , there won’t be sufficient gravity for matter to collapse down to form infant galaxies.

While studying DF4, scientists found that velocities of star clusters inside it showed way too less dark matter than there should have been.

This ultra diffuse galaxy is very hard to see, so scientists used some of the world’s most powerful observatories (IAC80 Telescope, the Gran Telescopio Canarias, and the Hubble Space Telescope) to observe it.

Observations revealed that stars of DF4 were getting pulled out due to interaction with much larger spiral neighbour, galaxy NGC 1035. This process is called tidal disruption, where a larger body gravitationally disrupts the smaller one.

According to scientists, previous papers showed that DF4 has a symmetrical shape indicating that no outside force was acting on it. But the recent observations show that DF4 is indeed getting effected by its nearby spiral neighbour.

This is just the starting point of interaction. DF4’s inner part is well within its shape. But in the outer, fainter parts tidal tails can be seen- stars that already got separated from DF4.

And since the dark matter envelopes galaxies in a large halo, this tidal disruption would remove most of the DF4’s dark matter before actually affecting its stars.

When the dark matter content of a galaxy falls below ten to fifteen percent of its total mass, the galaxy’s stars start getting stripped. This was found consistent with the scientists’ observation data.

Ultra diffuse galaxies (UDFs) tend to have high amount of dark matter content, about ninety nine percent of galaxy’s total mass. Scientists estimated that DF4’s dark matter content is just one percent of its total mass.

In time NGC1052-DF4 will be dismantled by its large spiral neighbor NGC1035 with some of their stars drifting free in deep dark space.

For more information: The Galaxy “Missing Dark Matter” NGC 1052-DF4 is Undergoing Tidal Disruption

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