Astronomers have just solved a mystery of dark matter by saving our models of galactic formation

A small and mysterious galaxy 44 million light years away is finally revealing its secrets. Revealed last year to have an incredibly low amount of dark matter, the galaxy NGC 1052-DF4 presented a significant challenge to our galactic formation models.

Those models still live another day. According to new research, NGC 1052-DF4 is effectively devoid of dark matter, but only because another nearby galaxy has affected it.

“Dark matter isn’t there because it’s already been removed,” said astrophysicist Mireia Montes of the University of New South Wales in Australia and the Space Telescope Science Institute.

“We found that the gravitational pull from the nearby massive galaxy NGC1035 is removing its stars and dark matter.”

(Montes et al., ApJ, 2020)

The discovery of NGC 1052-DF4 (or DF4 for short) was announced last year, and it was immediately an astronomical pickle. It was the second galaxy of its type – a faint, ultra-diffuse galaxy, or UDF – to be found seriously depleted of dark matter. The first was NGC 1052-DF2 (DF2), and DF4 was a confirmation that galaxies with insufficient dark matter could exist.

The problem was that, according to our current models, dark matter is necessary for the formation of galaxies in the first place.

We don’t know what dark matter is and we can’t detect it directly, but we do know that most galaxies have far more gravity than their normal detectable matter could create. There is a hidden mass in the Universe that creates that extra attraction, and without it, according to our understanding of galaxy formation, there would not be enough gravity for matter to collapse to form small galaxies.

The matter seemed to be nearing a resolution when a team of astrophysicists discovered that DF2 was actually much closer to us than previously thought. This meant that it has much less mass than the initial calculations suggested, and the proportion of normal matter was much less. Once the calculations based on the modified distance were completed, DF2 had a fairly normal amount of dark matter.

So they focused their attention on DF4. That too seemed to be much closer … but something was still not quite right. The velocities of star clusters within the galaxy still suggested that there was far less dark matter than there should have been.

The faint galaxy is very difficult to see, so Montes and his colleagues booked time on some of the most powerful telescopes in the world to see if they could figure out why.

Using the IAC80 telescope, the Gran Telescopio Canarias and the Hubble Space Telescope, they detected stars that were extracted from DF4, consistent with an interaction with the much larger spiral galaxy NGC 1035. This process, whereby a gravitationally larger body “interrupts” a smaller one, it is known as a tide interruption.

“Initial documents showed that the galaxy has a very ‘relaxed’ symmetrical shape, suggesting that no external forces were disturbing it,” Montes said.

“But our deep images show that this galaxy is actually affected by the neighboring galaxy: it was only captured at the beginning of the interaction. The inner part of the galaxy retains its shape, but the outer and fainter parts are where you see them.” tide tails “: stars that have already been separated from the galaxy.”

Tide tails were found exiting DF4. (Montes et al., ApJ, 2020)

Because dark matter surrounds galaxies in a large halo, this tidal stripping would remove most of the smaller galaxy’s dark matter before affecting the stars, Montes noted. Stars begin to be stripped only when the dark matter content drops below 10-15% of the galaxy’s total mass.

This is consistent with the team’s observations. UDFs tend to have a high percentage of dark matter, about 99 percent of the galaxy’s total mass. In the case of DF4, the team estimated that dark matter makes up just one percent of the total mass.

Since dark matter is basically the gravitational glue that holds galaxies together, this also means that DF4’s time in this Universe is limited.

“Over time,” said astrophysicist Ignacio Trujillo of the Istituto de Astrofísica de Canarias, “NGC1052-DF4 will be cannibalized by the great system around NGC1035, with at least some of their stars floating free in deep space.”

But at least we don’t have to go back to the drawing board of galaxy formation.

The research was published in The Astrophysical Journal.