Tuesday, September 13, 2011

Herschel paints new story of galaxy evolution

Galaxy forming stars
A galaxy accretes mass from rapid, narrow streams of cold gas. These filaments provide the galaxy with continuous flows of raw material to feed its star-forming at a rather leisurely pace.

This theoretical scenario for galaxy formation is based on the numerical simulations presented by Dekel et al., 2009 (Nature, 457, 451D). However, the actual process of stream accretion onto a galaxy has never been directly observed and it remains speculative. Credits: ESA–AOES Medialab. HI-RES JPEG (Size: 1476 kb)

Herschel's view of GOODS-North
GOODS-North is a patch of sky in the northern hemisphere that covers an area of about a third the size of the Full Moon. This images was taken by Herschel at the following infrared wavelengths: 100μm (blue), 160μm (green) and 250μm (red). North is up and East is left. Credits: ESA/GOODS-Herschel consortium/David Elbaz . HI-RES JPEG (Size: 585 kb)

In the nearby, present-day Universe, such high birth rates are very rare and always seem to be triggered by galaxies colliding with each other. So, astronomers had assumed that this was true throughout history.

Herschel now shows that this is not the case by looking at galaxies that are very far away and thus seen as they were billions of years ago.

David Elbaz, CEA Saclay, France, and collaborators have analysed the Herschel data and find that galaxy collisions played only a minor role in triggering star births in the past, even though some young galaxies were creating stars at furious rates.

By comparing the amount of infrared light released at different wavelengths by these galaxies, the team has shown that the star birth rate depends on the quantity of gas they contain, not whether they are colliding.

Gas is the raw building material for stars and this work reveals a simple link: the more gas a galaxy contains, the more stars are born.

Herschel's view of GOODS-South
GOODS-South is a patch of sky in the southern hemisphere that covers an area of about a third the size of the Full Moon. This images was taken by Herschel and NASA's Spitzer space telescope at the following infrared wavelengths:24 μm (blue), 100 μm (green) and 160 μm (red). North is up and East is left. Credits: ESA/GOODS-Herschel consortium/NASA/JPL-Caltech/David Elbaz .HI-RES JPEG (Size: 1036 kb)

"It's only in those galaxies that do not already have a lot of gas that collisions are needed to provide the gas and trigger high rates of star formation", says Dr Elbaz.

This applies to today's galaxies because, after forming stars for more than 10 billion years, they have used up most of their gaseous raw material.

The research paints a much more stately picture of star births than before, with most galaxies sitting in space, growing slowly and naturally from the gas they attract from their surroundings.

"Herschel was conceived to study the history of star formation across cosmic time", says Göran Pilbratt, ESA Herschel Project Scientist.

"These new observations now change our perception of the history of the Universe."

Further information

Notes for Editors

'GOODS–Herschel: an infrared main sequence for star-forming galaxies' by D. Elbaz et al. is published in Astronomy & Astrophysics, 533, A119. It is available online at:

For further information, please contact:

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int

David Elbaz
CEA Saclay
Tel: +33 1 69 08 54 39
Email: delbaz@cea.fr

Göran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt@rssd.esa.int