Scientists weigh out ingredients for the perfect galaxy

Herschel space telescope finds out how much dark matter it takes to make a galaxy - <em>News</em>

Friday 18 February 2011
Adapted from a news release issued by the UK Space agency and NASA

Astronomers working with Europe's Herschel Space Observatory have found out just how much dark matter it takes to make a new galaxy bursting with stars.

The discovery is a key step in understanding how dark matter - an invisible substance that pervades our universe - contributed to the creation of massive galaxies early in the life of the Universe, about 13.7 billion years ago.

According to the scientists' calculations, the recipe for a perfect galaxy requires just about 300 billion times the mass of our Sun - almost 6 followed by 41 zeros kilograms - of dark matter.

"If you start with too little dark matter, then a developing galaxy would peter out. If you have too much, then gas doesn't cool efficiently to form one large galaxy, and you end up with lots of smaller galaxies," said Asantha Cooray from University of California Irvine, principal investigator of the new research, which appears in Nature. "But if you have just the right amount of dark matter, then a galaxy bursting with stars will pop out."

Herschel – the world’s largest space telescope - launched into space in May 2009. The mission's large telescope detects far-infrared light from a host of objects, ranging from asteroids and planets in our own solar system to faraway galaxies.

Physicists from Imperial College London played a key role in conceiving, designing and developing Herschel's Spectral and Photometric Imaging Receiver (SPIRE) instrument and HerMES, the Herschel Multi-tiered Extragalactic Survey, over the last 20 years, and more recently have been instrumental in developing the software to convert masses of raw data from space into images and spectra that can be studied.

Dr David Clements is one of the Imperial physicists on the international team. He said: "This work shows just how powerful the Herschel Space Observatory is, allowing us to get new insights into the mysterious dark matter that is impossible for us to see directly. And this paper is based on only a small fraction of the data we're going to get from Herschel, so there's lots more exciting science to come!"

The calculated distribution of dark matter (Credits: The Virgo Consortium/Alexandre Amblard/ESA)

Early galaxies go through periods of forming stars that are much more vigorous than we find in our present-day Milky Way. About half of the light from these baby stars, on average, is hidden in veils of interstellar dust, which Herschel sees glowing in infrared light with its large 3.5-meter telescope.

The scientists used the telescope to measure infrared light from massive, star-forming galaxies located 10 to 11 billion light-years away. Astronomers think that these and other galaxies formed inside halos, or clumps, of dark matter.

These giant clumps of dark matter act like gravitational wells to collect the gas and dust needed for making new galaxies. When gas and dust falls into a well, it condenses and cools, allowing new stars to form. If enough stars form, then a new galaxy is born.

In the new study, Hershel was able to uncover more about how this galaxy-making process works, by mapping sources of infrared light that comes from collections of very distant, massive galaxies.

The maps showed that these galaxies are more clustered into groups than previously believed. The amount of this galaxy clustering depends on the amount of dark matter, so, after a series of complicated numerical simulations, the astronomers were able to determine exactly how much dark matter is needed to form a single star-forming galaxy.

Herschel is a flagship mission of the UK Space Agency, which funds the UK's involvement in the UK-led SPIRE instrument. 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."

For further information please contact:

Simon Levey
Research Media Officer
Imperial College London
email: s.levey@imperial.ac.uk
Tel: +44(0)20 7594 6702
Out of hours duty press officer: +44(0)7803 886 248