Weighing the KEY ingredient for making galaxies

Astronomers have used Europe’s Herschel Space Observatory to reveal just how much dark matter it takes to give rise to a galaxy bursting with stars. The findings are a key step in understanding how dark matter – an invisible substance that pervades our Universe – contributed to the birth of massive galaxies in the early Universe.

Dr David Parker, Director of Space Science and Exploration at the UK Space Agency, which provides the UK funding for Herschel, said, “Once again, the Herschel team have pushed the boundaries and brought us another step closer to understanding the complex creation and evolution of our Universe. As always, we’re immensely proud of the outstanding work of our UK scientists who are playing key roles in this world-leading space project. Herschel is a jewel in the UK's space programme.

Galaxies like our own Milky Way formed billions of years ago from clouds of gas collapsing under gravity. The way in which the gas collapses depends on the amount of dark matter in the neighbourhood. "If you start with too little dark matter, then a developing galaxy would peter out. But if you have the just the right amount of dark matter, then a galaxy bursting with stars will pop out.” said astronomer Asantha Cooray of UC Irvine in Calif., who lead this new research appearing in the Feb. 24 issue of the journal Nature.

The right amount of dark matter turns out to be a mass equivalent to around 300 billion Suns. For reference, the dark matter surrounding our Milky Way galaxy weighs in at the equivalent of roughly 1 trillion Suns.

Herschel – the world’s largest space telescope - launched into space in May 2009. The mission's large, 3.5m telescope detects far-infrared light from a host of objects, ranging from asteroids and planets in our own solar system to faraway galaxies. This research was part of the HerMES project, which uses Herschel to look at these distant galaxies and is led by Seb Oliver, of University of Sussex, and Jamie Bock, of NASA’s Jet Propulsion Laboratory. Herschel is a flagship mission of the UK Space Agency, which funds the UK's involvement in the UK-led SPIRE instrument.

The team used Herschel to measure infrared light from massive, star-forming galaxies in the distant Universe, using images of two regions of the sky in the constellation of Ursa Major. One of these regions is an area of the sky called the “Lockman Hole” and is shown opposite. The GOODS-North field is shown to scale as an inset, and is around the same size as the Full Moon as seen in the sky. These regions are almost completely devoid of objects in our Galaxy, making them ideal for studying the distant Universe.

The huge distance means that light has taken 10-11 billion years to cross the Universe, so the galaxies are seen when the Universe was only around 3 billion years old.
Astronomers think that these and other galaxies formed inside halos, or clumps, of dark matter, and were forming stars hundreds of times more rapidly than galaxies in today’s Universe. The rapid star formation produced a lot of interstellar dust, which is what is glowing at the far-infrared wavelengths observed by Herschel.
"This measurement is important because we are homing in on the very basic ingredients in galaxy formation," said Alexandre Amblard of UC Irvine, lead author of the Nature paper. "In this case, the ingredient, dark matter, happens to be an exotic substance that we still have much to learn about."
In this new study, Hershel was able to uncover more about how this galaxy-making process works by acquiring maps of the infrared light that comes from collections of very distant, massive star-forming galaxies. The most distant galaxies are so far away that Herschel cannot see them individually, but rather sees the pattern as their light blurs together. This pattern of light, called the cosmic infrared background, is like a web that spreads across the sky. Because Herschel can survey large areas of the sky very quickly with high resolution, it was able to create the first detailed maps of the cosmic infrared background.

"It turns out that it's much more effective to look at these patterns rather than the individual galaxies," said Jamie Bock. "This is like looking at a picture in a magazine from a reading distance -- you don’t notice the individual tiny dots but you see the big picture. Herschel gives us the big picture of these distant galaxies, showing the influence of dark matter."

Normal matter – the stuff that makes up people, planets, stars and galaxies – is far outweighed by dark matter. There is around 5 times as much dark matter as normal matter in the Universe, and while it can’t be seen by any telescope, it does give a gravitational tug on the matter we can see.

Giant clumps of dark matter act like gravitational wells to collect gas and dust needed for making galaxies. The gas and dust become much denser as they fall into the well, allowing new stars to form. When there are eventually enough stars, a galaxy is born.

But this only happens if there is enough dark matter to keep the gas and dust clumped together – if there is enough dark matter, several galaxies will form in the same clump. The maps showed that the galaxies are more clustered into groups than previously believed, which means that they tend to lie in larger clumps of dark matter – in those clumps which contain more matter than around 300 billion Suns.

"I find it amazing that with these results we are able to understand the link between mass and star formation” said Seb Oliver. “Particularly when the mass can never be seen, the star-formation is shrouded in dust and invisible to normal telescopes, and only 15% of the dust emission can be resolved into individual galaxies – heroic work!"

Below is a simulation of a region of the Universe, showing the way the dark matter (blue) was distributed around 3 billion years after the Big Bang. It forms a cosmic web, with clumps, or halos, where it has collapsed due to gravity. The clumps come in a range of sizes, and the amount of dark matter in each one affects whether it can form galaxies. The clumps which are large enough to form galaxies, like our own Milky Way, are shown in red. The region of space is around 1/3 of the size of the area imaged in the Lockman Hole field.