Absence of dark matter shows gravity of situation

ASTRONOMERS have found yet another galaxy that contains essentially no dark matter, and it has them scratching their heads once again.

The nature of dark matter has been a puzzle for astronomers for a long time. It interacts with normal matter only through its gravitational influence, but it is through this very effect that dark matter seems to have contributed to the formation of galaxies.

Astronomers think that in the early universe, there would not have been sufficient “clumpiness” of normal matter to have triggered the formation of galaxies. The enigmatic dark matter, however, would indeed have formed concentrations, and it is those that caused the normal matter to come together in clumps, which became the building blocks of galaxies.

Therefore, galaxies and dark matter should go together.

However, a few years ago, two galaxies, NGC 1052-DF2 and NGC 1052-DF4, were found to contain essentially no dark matter at all. I wrote about this surprise in 2018 and 2019. Once the second one was found, it seemed clear that it heralded the discovery of still more of them.

An analysis of the DF4 galaxy, at least, has shown that its interaction with the nearby galaxy NGC 1035 has stripped its dark matter that previously existed. In this case, at least, this was a relief to astronomers.

However, DF2’s lack of dark matter is still a mystery, and it would not relate to the reason behind the lack in DF4, because they are about 6.5 million light years apart.

Since the observations of DF2 and DF4, astronomers have studied several other galaxies and found them to be at least quite deficient in dark matter. Re-investigation of one of them, called AGC 114905, using the Very Large Array (VLA), a group of radio telescopes near Socorro in New Mexico, has revealed that it shows no evidence of any dark matter. So that galaxy, especially, is currently standing out in astronomers’ minds as a reason to rethink their ideas of galaxy formation.

The research was conducted by a team of astronomers led by Pavel E. Mancera Pina, of the Kapteyn Astronomical Institute in the Netherlands.

You may wonder how astronomers know that dark matter is present at all, given that we can’t see it. However, it is not necessary to see material in order to know it is there.

A topical example is the discover of extrasolar planets — planets orbiting other stars. In the vast majority of cases, we haven’t seen them. They are detected by various methods, mainly either because they can temporarily block a small part of the star’s light as they move in front of it, or because their gravity causes a “wobble” in the position of the parent star.

It is gravity, too, that gives away the presence of dark matter. Its existence was discovered by Fritz Zwicky (1898-1974), who in the 1930s observed that the motions of galaxies in a cluster of them called the Coma Cluster was being affected by an unidentified gravitational force.

For individual galaxies, the high speed at which the stars or other material in their outlying parts revolve around the galaxy’s centre reveals the presence of dark matter.

Similarly, if those outer regions are moving relatively slowly, as one would normally expect based only on the ordinary matter in the galaxy, it is an indication of the absence of dark matter.

The observations were made using radio telescopes because the researchers wanted to measure the speed of cool hydrogen, rather than the stars themselves. This hydrogen is detectable using radio observations, made at wavelengths much longer than those of visible light.

It is estimated that there are several hundred billion galaxies in the universe. However, apart from our own Milky Way galaxy, which takes its name from the familiar milky band of light surrounding us, there are only three that are visible to the unaided eye.

A pair that we see in our southern sky are two of the closest to ours: the Magellanic Clouds, named after the explorer Ferdinand Magellan.

They look like dim clouds, but are actually made up of stars that are too faint to see individually without a good telescope.