Herschel shows galaxies had cool beginnings

The GOODS-North region of sky
One of the regions of sky studied by Herschel, containing hundreds of distant galaxies. Click for more information. Image credit: ESA/Herschel/SPIRE/HerMES

Observations from Herschel have shown that galaxies in the early Universe were cooler than those we see around us today. This indicates that early galaxies were more bloated, containing more dust, distributed over larger regions.

Around a thousand galaxies were studied, chosen because they are very distant and forming stars at very high rates. Because they are so distant, the galaxies are seen as they were when the Universe was much younger, with those studied here spanning a range of cosmic times between 1 and 10 billion years ago – a significant portion of its fourteen billion year history.

Most of these galaxies are seen when the Universe was about half its current age, a period during which galaxies tended to be much more “active” than those we see around us today, with some forming stars hundreds or even thousands of times faster than our Milky Way. Although rare, such “starburst galaxies” have produced as much energy over the course of cosmic history as all the other galaxies combined. This makes them crucial for studying the history of star formation in the Universe.

Stars form from massive clouds of gas and dust, and these early galaxies have large quantities of both. Most of the dust in these galaxies is cold by human standards, at temperatures of around −240 Celsius, and so can only be seen at far-infrared wavelengths. Using Herschel’s cameras, astronomers are able to study the properties of the dust and deduce the average conditions within each galaxy.

“These galaxies are all but invisible to optical telescopes, but Herschel sees the far-infrared glow from their dust,” says Dr Myrto Symeonidis, who led and carried out the research at UCL and is currently based at the University of Sussex. “With so many galaxies in the Herschel images, we can start to look at how galaxies have changed over the history of the Universe.”

Messier 82 as seen by Spitzer
The galaxy Messier 82, the closest example of a "starburst galaxy", as seen by NASA's Spitzer satellite. Click here for Herschel/s view of this galaxy. Image credit: NASA/JPL-Caltech

The newly formed stars in these galaxies heat up the dust, so galaxies which form stars more rapidly are expected to have higher dust temperatures overall. But the surprise for astronomers was that the galaxies in the younger Universe appeared cooler than those seen today. “There are two explanations for this surprising result,” explains Dr Symeonidis. “Either the earlier starburst galaxies were much larger than we thought, or they contain greater quantities of dust than predicted. In either case, they are different from those we see around us today.”

These results used images and data from Herschel’s two cameras, PACS and SPIRE. By studying seemingly blank regions of sky for long periods of time, these revolutionary instruments picked out the light from thousands of galaxies in the early Universe.

“Previous assumptions have generally assumed that these early starburst galaxies were similar to those in the local Universe,” commented Prof Seb Oliver, University of Sussex, who leads “HerMES”, one of the Herschel surveys used in this study. “What we’ve shown here is that this wasn’t always the case, we need to look more carefully at the conditions in these early galaxies”.

“This work brought together some of the best data collected over Herschel’s four year lifetime, showcasing the immense advances we can make in understanding how stars form and under what conditions”, points out Dr Dieter Lutz, from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who leads the “PEP” survey.

“Herschel’s cameras have provided unrivalled views of the early Universe, allowing us to study the distant past in unprecedented detail,” says Prof Matt Griffin, of Cardiff University, the lead scientist of the international team which designed, built and operated the SPIRE instrument on board Herschel. “We’ve only scratched the surface of the immense Herschel data archive, and there is undoubtedly much, much more to come”, he concludes.