A small, mysterious galaxy 44 million light-years away is lastly yielding up its secrets and techniques. Revealed final yr to have a shockingly low quantity of dark matter, the galaxy NGC 1052-DF4 posed a vital problem to our fashions of galaxy formation.
Those fashions but stay one other day. According to new analysis, NGC 1052-DF4 is certainly missing in darkish matter – however solely as a result of one other close by galaxy nicked it.
“The 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 brief) was announced last year, and it was instantly a dilly of an astronomical pickle. It was the second galaxy of its sort – a faint, ultra-diffuse galaxy, or UDF – to be discovered significantly dark-matter depleted. The first was NGC 1052-DF2 (DF2), and DF4 represented a affirmation that galaxies with inadequate darkish matter might exist.
The drawback was that, in response to our present fashions, darkish matter is critical for galaxies to type within the first place.
We do not know what darkish matter is, and we will not detect it straight, however we do know that the majority galaxies have far more gravity than their regular, detectable matter might create. There’s some hidden mass within the Universe creating that additional pull, and with out it, in response to our understanding of galaxy formation, there would not be sufficient gravity for matter to break down all the way down to type child galaxies.
The matter appeared to attract near a decision when a workforce of astrophysicists discovered that DF2 was actually a lot closer to us than first thought. That meant it has a lot much less mass than preliminary calculations steered, and the proportion of regular matter was a lot decrease. Once calculations based mostly on the revised distance have been accomplished, DF2 had a fairly regular quantity of darkish matter.
Then they centered their consideration on DF4. It, too, seemed to be much closer… however one thing was nonetheless not fairly proper. The velocities of clusters of stars contained in the galaxy have been nonetheless suggesting that there was a lot much less darkish matter than there ought to have been.
The faint galaxy may be very laborious to see, so Montes and her colleagues booked time on a few of the world’s strongest telescopes to see if they may work out why.
Using the IAC80 Telescope, the Gran Telescopio Canarias, and the Hubble Space Telescope, they detected stars being pulled out of DF4, according to an interplay with the a lot bigger spiral galaxy NGC 1035. This course of, whereby a bigger physique gravitationally “disrupts” a smaller one, is named tidal disruption.
“Initial papers showed that the galaxy has a very ‘relaxed’ symmetrical shape, suggesting that no outside forces were perturbing it,” Montes said.
“But our deep images show that this galaxy is in fact being affected by its neighbour galaxy – it’s just caught in the beginning of the interaction. The inner part of the galaxy keeps its shape, but the outer, fainter parts are where you see these ‘tidal tails’: stars that have been already separated from the galaxy.”
The tidal tails discovered streaming out of DF4. (Montes et al., ApJ, 2020)
Since darkish matter surrounds galaxies in a giant halo, this tidal stripping would take away many of the smaller galaxy’s darkish matter earlier than affecting the celebrities, Montes famous. Stars solely begin to be stripped as soon as the darkish matter content material falls under 10 to 15 % of the galaxy’s whole mass.
That’s according to the workforce’s observations. UDFs are likely to have a excessive proportion of darkish matter – round 99 % of the whole mass of the galaxy. In the case of DF4, the workforce estimated that darkish matter makes up only one % of the whole mass.
Because darkish matter is principally the gravitational glue that holds galaxies collectively, this additionally signifies that DF4’s time on this Universe is proscribed.
“With time,” said astrophysicist Ignacio Trujillo of the Instituto de Astrofísica de Canarias, “NGC1052-DF4 will be cannibalised by the large system around NGC1035, with at least some of their stars floating free in deep space.”
But at the very least we do not have to return to the drafting board on galaxy formation.
The analysis has been printed in The Astrophysical Journal.
