‘Mr Galaxy’ puts dark matter on the map

It’s a funny place to find the start of a paradigm shift, the appendix of a paper. But nevertheless, there it is, in appendix A of a 1970 paper about spiral and flat galaxies: “There must be in these galaxies additional matter which is undetected… its mass must be at least as large as the mass of the detected galaxies, and its distribution must be quite different from the exponential distribution which holds for the optical galaxy.”

What Ken Freeman is describing in that paper is now known as dark matter: a mysterious and ubiquitous substance that makes up 97% of our galaxy, the Milky Way, and 93% of the universe.

The idea of dark matter had been kicking around for some time before Freeman actually nailed down what it might mean for galaxies and, in turn, the universe. In 1933, Swiss astrophysicist Fritz Zwicky proposed the concept of dark matter to explain the motions of galaxies in a nearby cluster called the Coma cluster; the galaxies were moving much faster than expected if all the mass of the galaxy was just in its stars and gas.

But, for nearly 40 years following Zwicky’s proposal, astrophysicists didn’t take it seriously; it wasn’t used in papers and nor did it form part of any analysis. “It just came too early. People had no idea what to do with this [idea],” says Freeman.

“It happens quite a lot – that something really new turns up and there is no framework to absorb it. People just don’t know what to do with it, so they don’t do anything with it. This is pretty common. It just reinforces the idea that if you want to discover something, you need to choose the right time, as well as making the discovery,” says Freeman.

Freeman never set out to solve the mystery. He just wanted to work out mathematically how galaxies were supposed to rotate. He compared his results to the available data, and that’s when he realised astrophysics had a big problem.

“If [galaxies are] just made out of stars and gas, and those are distributed the way that we see, then the question is, ‘Well, what rotation would you expect?’. And you can work this out, but nobody had done it properly. So I did it properly. And I compared it to some of the data that was around at the time, but that data wasn’t really very good. But it was pretty clearly a problem: the galaxies weren’t rotating the way that would be expected if all the mass was in the gas and stars,” Freeman says.

What that meant was that each individual galaxy had to be surrounded by a dark matter halo; that dark matter wasn’t scattered randomly throughout the universe or found in the empty voids between galaxies, as some astrophysicists surmised at the time. Through the 1970s, the dark matter revolution gained momentum as radioastronomers mapped out galaxies in more and more detail. The key breakthrough came after they figured out how to map out the gas in the galaxies, by using several radioantennas to simulate the effect of one big antenna. Because the gas extends much further out than the stars, the effects of dark matter become more apparent.

The “clincher”, says Freeman, came in 1978 with the work of a PhD student using a large radio telescope, Westerbork, in Holland. “That machine produced really good data. There was really no question about it. Everybody at that point accepted the fact that we really did have a problem.”

“[Freeman] says to this day that the 70s were the most exciting phase, for him, in astronomy. He says it was a very exciting time. There were big things happening,” says Joss Bland-Hawthorn, a colleague of Freeman’s and a professor of physics at the University of Sydney. “They were discovering that galaxies were really interesting objects with very interesting properties.”

“It was pretty exciting!” says Freeman. “By 1978, the whole picture of what was in the universe was changing quite fast because of this dark matter issue.”

“Due to Freeman, astrophysicists have a much better idea of how galaxies hold together, and how they live out their lives,” says nobel laureate Brian Schmidt from the Australian National University. “In the 1970s, he deduced that galaxies had dark matter… this was a key step, for the astrophysics community to believe that dark matter existed.”

FREEMAN’S 1970 paper is now one of the most cited single-author papers in the field of astrophysics. But the irony is, the vast majority of those citations are not for figuring out the importance of dark matter to galaxies.

The other key finding in the paper concerned the disk of the galaxy, the part that extends out from the bright bulge at the centre and is spectacled with stars. In the Milky Way, for example, the Sun, Earth and the rest of our solar system are found in the disk. Freeman found that the brightness of almost all galaxies’ disks is the same.

“It’s a disk that just dies away – shaped like a discus that track and field athletes throw. We call that the Freeman Disk,” says Bland-Hawthorn. And the brightness of the surface of the disk is always the same, which tells astrophysicists important information about how the discs of galaxies were assembled in the early universe. This came to be known as the Freeman Law.

Most people, after having a physical law and part of a galaxy named after them and kick starting the dark matter revolution, might sit back and enjoy the tributes as they pour in. But Freeman doesn’t seem to take much notice of such things. “Yeh, it’s alright,” he laughs, when I ask about having laws named after him. I can almost hear him shrug down the phone line.

“He’s a very humble person,” says Bland-Hawthorn, by way of explanation. “I haven’t heard Ken talk himself up in the 30 odd years I’ve known him.”

There would be a lot to talk up if he ever wanted to start. He is a Fellow of the Australian Academy of Science and Royal Society of London. He won the Pawsey Medal of the Australian Academy of Science in 1972, the Dannie Heineman prize of the American Institute of Physics and the American Astronomical Society in 1999 and the Prime Minister’s Prize for Science in 2012, which came with $300,000 prize money.

“Ken Freeman has been the leading person in figuring out how galaxies work. He’s the one person in the world that you could call Mr Galaxy,” says Schmidt.

OUR GALAXY, THE Milky Way, is a special place. It special because it’s ours and it’s the only galaxy you or I will ever live in. But it’s also special because it is the galaxy that we know the most about, and always will know the most about.

“We can see a hundred billion stars around us. In any other galaxy you see fewer stars because they are further away, and things look dimmer,” says Bland-Hawthorn. “Ken’s genius was to think about how we could interpret what we see in our own galaxy, the Milky Way, and how we could learn about all the other galaxies in the universe, and going back through time to the very early universe.”

In the 1980s, Bland-Hawthorn and Freeman met at a conference at Princeton University. They found in each other a like-minded soul, and instantly began long, in-depth discussions about astrophysics and galaxies over dinners. The seeds were sown in those discussions, and from them the new field of galactic archaeology has emerged. Using the light from a star, the pair figured out that you could deduce the chemical constituents of the star – of particular interest are the heavier elements such as iron and strontium – and that could then be used to work out which stars were born together, at the same time and in the same region of the galaxy.

In 2000, the pair were invited to write a review for the millennium issue of the U.S. journal Science on galactic archaeology, which got an “amazing reception” says Bland-Hawthorn. Then, in 2002, Freeman was asked to write a much longer article for the Annual Reviews of Astronomy and Astrophysics. “I’d been asked to write an article on the ‘new galaxy’. Now, I had no idea what they meant,” says Freeman.

In the resultant paper, Bland-Hawthorn and Freeman explored the galactic archaeology idea further. “We said things in the article like ‘We’re going to have to look at million of stars and do detailed chemistry and we should build an instrument to do this’,” says Bland-Hawthorn. “The referees of that article thought we were crazy. They said ‘No one is going to care about millions of stars. No one’s going to care about millions of stars and their chemistry, and no one is going to bother building an instrument for this’,” Bland-Hawthron recalls. He remembers one reviewer who sent 19 handwritten pages of comments, critising the paper.

“Today, 10 years on, most of what we said is accepted. Most people take it for granted now,” says Bland-Hawthorn.

Indeed, the instrument, HERMES, is in its final stages of construction at the Australian Astronomical Observatory in Siding Spring, New South Wales. HERMES can gather the light from 350 stars simultaneously, which then get automatically analysed for chemical elements.

“We’ll get that instrument sometime early next year, and then we’ll have about six months of shaking it down, making sure everything is working, and then we’ll start this project,” says Freeman.

During the course of the project, Freeman thinks they collect information about a million stars, which could tell us how the stars in our galaxy formed and how it evolved to be the galaxy we see today.

“HERMES will be one of Ken Freeman’s great legacies to astronomy,” says Bland-Hawthorn.