Herschel weighs the key ingredients for making galaxies

The Lockman Hole, as observed by Herschel
A region of the sky called the Lockman Hole, showing thousands of distant galaxies seen as they were just a few billion years after the Big Bang. Image credit: ESA / Herschel / SPIRE / HerMES

Astronomers have used Herschel 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. 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.

Simulations of distribution of dark matter and dark matter clumps
Left: simulations of the distribution of dark matter in the Universe around 3 billion years after the Big Bang. Right: clumps of dark matter (red), with those larger than 300 million times the mass of the Sunhighlighted in yellow. Image credit: Virgo Consortium / A. Amblard / ESA

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."