Looking for the edge of the Galaxy

How big is the Milky Way? Given our location, buried deep within the spiral disk that defines our galaxy, this has been a difficult question to answer. Over the last century, astronomers have surveyed the sky and charted the distances to stars, revealing the disk of the Milky Way to be about 160,000 light-years across, showing warps and flairs of galactic collisions, and circling the immense bulge of stars hosts the black hole at its heart.   

The edge of the Milky Way’s stellar disk is not well-defined, but the density of stars falls rapidly after about 80,000 light-years from the Galactic centre. Astronomers can point to this and be confident that the vast majority of stars lie within this radius. But have they truly established the edge of the Galaxy? 

By the late twentieth century, astronomers realised there was a lot more to a galaxy than the starlight we see. By tracing the motions of gas and stars, astronomers, including Vera Rubin in the US and Australia’s Ken Freeman, realised that there was also unseen mass, a lot of unseen mass, more than ten times that is visible. We now know that this “dark matter” dominates the mass of the Milky Way, and every other galaxy we have observed.

Everything we can see in our Universe, through the interaction of electromagnetic radiation is considered baryonic, or ‘normal’, matter. This matter absorbs, reflects, emits and interacts with electromagnetic spectrum (such as radio waves, visible light waves, x-rays). But matter can also be detected through its gravitational influence - and the more matter (mass) an object has, the higher its gravitational influence. 

In 1933, Fritz Zwicky (a colourful character) was observing the Coma galaxy cluster and measured the total light the cluster produced by its galactic members. He was also able to measure the motion and velocities of individual galaxies within these clusters - which is when he stumbled across something interesting. 

There was too much gravity, which in turn was driving higher velocities of galaxies, that could be accounted for - based on the amount of light the cluster was outputting. There was an unseen, dark matter that was generating more gravity. 

For decades there was not much interest in the idea until, by the 1970s, new technology and new observations started to hint that something strange was going on. Astronomers including Ken Freeman in Australia and Vera Rubin in the US deduced that there was indeed an additional gravitational signature surrounding galaxies that could not be accounted for with the visible light measured. The idea of Dark Matter was reignited. 

In the late 1990s, astrophysicists were studying the expansion of the Universe by observing distant supernovae explosions using the Hubble Space Telescope - and found that counter to what was expected - the Universe was expanding at an accelerated rate. They termed this force, Dark Energy and were able to calculate how much of the total energy density of the Universe it contained. 

It turned out that this force accounted for the majority component of our Universe - roughly 68%. Once scientists knew this value, they could then place a value against the total amount of dark matter in the Universe, and it turns out it’s about 27%. This means - everything else we see and have ever observed with our variety of telescopes, all the “normal” (baryonic) matter - only makes up 5% of the entire Universe.

Dark matter is spread more extensively than stars, and astronomers wondered how much further it went, but being invisible, detecting any edge in dark matter is far more difficult than for stars. Now Alis Deason, a Royal Society Researcher at the University of Durham, thinks she has the answer.

Deason and her international team started by looking at synthetic universes, supercomputer simulations of the evolution of matter in an expanding cosmos. These simulations are extremely detailed, allowing astronomers to trace the flow of gas into stars, and the impact of supernovae and black holes on the growth of galaxies. And, of course, they can accurately map out the ebb and flow of dark matter and its impact on the stars we can see.

Deason found that the distribution of dark matter influenced the motions of dwarf galaxies, small systems with only a few billion stars, as they orbited a larger galaxy. Importantly, the orbital properties of the dwarfs also revealed the location of the edge of dark matter, where its gravitational influence dwindled rapidly.

Looking at the dwarf galaxies around the Milky Way, Deason concluded that the edge of the Milky Way’s dark matter lies almost one million light-years from the centre of the Galaxy. This is an astonishingly large distance, much larger than the extent of the stellar disk. But it is also surprising as the Milky Way is not alone in this patch of the universe.

The Andromeda Galaxy is about two million light-years from the Milky Way, and observations suggest that its mass is comparable to our own. Given that Andromeda is heading towards the Milky Way at about 120km/s, in a few billion years the two galaxies will collide, destroying their spiral structure, and merging into a featureless elliptical galaxy. But given that the extensive dark matter haloes of these two giant galaxies are probably already in contact, the true collision has already begun.

The true nature of dark matter remains a mystery, a mystery that is the focus of astronomers and physicists around the globe. Most think that it is a fundamental particle, something that we have not nailed down in our theories or observed at the Large Hadron Collider, so the hunt continues. Some tantalizing clues have come from an experiment in Italy, in a laboratory hidden in the Gran Sasso road tunnel under the Alps. Here sits DAMA/LIBRA, a series of sodium iodide crystals encased in sensitive photodetectors, waiting to detect a tiny flash of light if a dark matter particle crashed into an atom in the crystal. The experiment is sensitive to background radiation from the rocks in the tunnel and cosmic rays from space and it’s essential that physicists can accurately calibrate this in the search for dark matter.      

DAMA is not the only “direct detection” experiment search for dark matter, but it is the one that claimed an intriguing detection. As well as the background, DAMA sees an excess in light flashes which they interpret as being interactions with dark matter particles. But this signal is not constant but smoothly varies over the period of a year. 

What is the source of this variation? Physicists suggest that it might be due to the Earth’s orbit about the Sun, which is strongly tilted with regards to the Sun’s motion around the Galaxy. For a portion of the Earth’s orbit, its motion adds to that of the Sun, whilst the other portion of the orbit subtracts from the Solar motion. This means that the Earth motion changes relative to the underlying dark matter of the Galaxy and this modulates the rate of dark matter detections in DAMA.

Of course, there are many other things that vary on the timescale of a year, such as the seasons and even the quantity of traffic flowing through the Gran Sasso tunnel, and physicists are continually checking whether all these external effects could be producing the observed signal.

One obvious test would be to examine the signal of an instrument like DAMA in the Southern Hemisphere as if the observed signal is due to dark matter, it should match that seen in the North, but if it is due to something boring like weather or people heading on holiday, then the signal should be shifted by six months. But, as yet, such a functioning dark matter detector does not exist. Yet!

Early next year, in a gold mine in the town of Stawell in Victoria, SABRE, short for Sodium-iodide with Active Background Rejection, will be turned on, with an identical detector opening Italy. As more sensitive versions of DAMA, these will search for the annual variation already claimed, and in a few years, we will know whether the claimed variations are terrestrial and boring, or extra-terrestrial and represent the first true detection of dark matter. 

Very soon, we might actually shine a light on the dark side that dominates the Milky Way.