Astronomers have used ESA's Herschel Space Observatory to identify thousands of previously undetected starburst galaxies - among the most prolific stellar factories in the Universe. Follow-up observations with the W. M. Keck Observatory in Hawai'i have provided redshift measurements for almost 800 of these galaxies, which appear to span the past 12 billion years of cosmic history.

The study confirms that, although less numerous than 'ordinary' galaxies, these infrared-bright galaxies played an important role in star formation throughout the history of the Universe. This rich data set will also allow future investigations into the physical mechanisms that triggered such intense star formation.

To unfold the history of how stars formed in galaxies across 13 billion years of cosmic evolution, astronomers are keen to study the most powerful galaxies. Also known as starburst galaxies, these produce stars at a much higher rate than 'ordinary' galaxies and might hold the key to the main processes that trigger star formation. Although rare in the local Universe, these extremely efficient star factories were much more abundant at earlier cosmic epochs.

Establishing how much these galaxies contributed to the total history of star formation in the Universe is a crucial clue in the investigation of how galaxies formed and evolved.

Starburst galaxies contain large amounts of gas mixed with dust; the latter absorbs starlight and re-emits it at longer wavelengths. As a result, starburst galaxies shine most brightly in the far-infrared and sub-millimetre portions of the electromagnetic spectrum and elude surveys carried out at optical wavelengths. Many distant 'obscured' galaxies were discovered in the late 1990s using sub-millimetre observatories, so they are also referred to as sub-millimetre galaxies.

A new study, led by Caitlin Casey from the Institute for Astronomy at the University of Hawai'i, USA, has measured redshifts for a large number of starburst galaxies discovered in photometric surveys from ESA's Herschel Space Observatory. These infrared-bright galaxies represent a new population of sources, which overlaps only in part with the already known class of sub-millimetre galaxies.

Based on a survey conducted with Herschel and followed up with the W. M. Keck Observatory, this study provides a valuable census of starburst galaxies and their properties - a rich data set that will enable future investigations of the processes underlying these prolific stellar factories.

"Using Herschel's unique combination of spectral coverage and sensitivity, we could reveal thousands of previously hidden starburst galaxies, most of which remain undetectable not only with ground-based optical observations, but with ground-based sub-millimetre observations as well," says Casey.

To study this sample of galaxies in depth, Casey and her collaborators gathered high-resolution spectra using the W. M. Keck Observatory.

"These galaxies are rather faint at optical wavelengths, so we needed about a hundred hours of observation at one of the largest telescopes on Earth to collect spectra for this many sources," explains Casey.

The astronomers used multi-slit masks - a technique that allows them to gather spectra of a few dozen neighbouring galaxies simultaneously. The masks were cut to match the positions of the sources identified in each field with Herschel. The result is an unprecedented census of 767 starburst galaxies with known redshifts - an enormous improvement over the previously available spectroscopic survey of sub-millimetre galaxies, which comprised only 73 objects.

The redshift, determined from the position of the spectral lines, provides astronomers with a measure of how long the light from each galaxy has travelled across the Universe. In turn, this indicates when, in cosmic history, the light from each galaxy was emitted. For most galaxies in this new census, their light was emitted during the past ten billion years; a fraction of them - about five per cent of the survey - lie even farther away from us, as their light was emitted when the Universe was only one to three billion years old.

"The Herschel data tell us how fiercely and prolifically these galaxies are producing stars. Combining this information with the redshifts provided by the Keck data, we can uncover the contribution of these infrared-bright starburst galaxies to the total amount of stars produced across the history of the Universe," comments Seb Oliver from University of Sussex, UK. Together with Jamie Bock from Caltech, USA, Oliver coordinates the HerMES Key Programme, within which the data have been collected.

Whilst the starburst galaxies detected by Herschel may be relatively rare among the galaxy population in general, they appear to be major players in terms of their star formation rate.

In fact, these infrared-bright galaxies seem to have contributed as much as 'ordinary' galaxies - which are much more numerous - to the total build-up of stars in the Universe, in particular during the last ten billion years. At earlier epochs, the supply of star formation provided by these galaxies is also significant, amounting to at least ten to twenty per cent of the total.

"Our study confirms the importance of starburst galaxies in the cosmic history of star formation. Models that try to reproduce the formation and evolution of galaxies will have to take these results into account," notes Casey.

The data set compiled by Casey and her collaborators is a great advance in the investigation of the physical mechanisms that trigger these intense starbursts. There are two main scenarios suggested by current theory: either a violent merger between galaxies, or a more steady process of gas accretion from the intergalactic medium. Follow-up observations of the starburst galaxies detected by Herschel may help astronomers figure out the relative importance of each process at different cosmic epochs.

"This census of starburst galaxies is a huge step forward in the study of galaxy formation and evolution," comments Goran Pilbratt, Herschel Project Scientist at ESA. "With these data, we can now further explore the dominant mechanisms producing the bulk of star formation in the Universe."

The study relies on photometric data acquired with ESA's Herschel Space Observatory at sub-millimetre wavelengths, and on spectroscopic data acquired with the W. M. Keck Observatory on Mauna Kea, Hawai'i, USA. The data are from observations of the following fields: Lockman Hole North, GOODS-North, ELAIS-N1, extended UKIDDS Ultra Deep Field, COSMOS, and extended Chandra Deep Field South.

The Herschel data have been acquired with the Spectral and Photometric Imaging Receiver (SPIRE) instrument on board Herschel as part of the Herschel Multi-tiered Extragalactic Survey (HerMES), a Guaranteed Time Key Programme designed to probe galaxy evolution at high redshift. HerMES is the largest Herschel Key Programme in terms of observing time and aims at studying the evolution of galaxies in the distant Universe. The project is carried out by a large international collaboration led by Seb Oliver (University of Sussex, UK) and Jamie Bock (Caltech, USA).