Herschel sheds new light on star formation in early Universe
Do galaxies form their stars through violent and tumultuous merging events or rather via more steady and gentle processes? Scrutinising thousands of galaxies across the past 11 billion years of cosmic history with the Herschel Space Observatory of the European Space Agency (ESA), astronomers have, for the first time, been able to answer this long-standing question. Launched in May 2009, the Herschel Space Observatory is a sensitive far-infrared and submillimetre telescope capable of collecting and discerning the light from very faint galaxies emanating from the era when the first galaxies were formed to the present day. It started taking data in July of the same year, and the new images show distant galaxies building stars. Stars begin to form inside clouds of gas and dust, within thick dust cocoons that protect them until the moment they 'hatch'. In the pre-stellar core, as these cocoons are called, gravity squeezes gas and dust into the centre of the clump. This heats the gas, and the heat radiates into space before the embryonic star completes its collapse and finishes its formation. Today galaxies form stars at a rather leisurely pace - or at least this is what astronomers inferred from observing galaxies in the present-day Universe. Occasionally, interactions between massive galaxies may trigger intense bursts of star formation and cause the number of stars to increase much more swiftly. But what about in the past? Did galaxies experience periods of enhanced star formation, or have they always been producing stars at a steady and gentle rate? With its large telescope and spectral coverage that extends to very long wavelengths, the Herschel Space Observatory allowed astronomers to peer deeply into the Universe's past and gather new clues to tackle these questions. Dr David Elbaz from the Centre of the Commissariat à l'énergie atomique et aux énergies alternatives (CEA) in Saclay, France and his collaborators analysed the Herschel data and found that galaxy collisions have played only a minor role in triggering star births in the past - even though merging events were more common in the early history of the Universe than in more recent epochs. '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', explains Dr Elbaz. 'Galaxies that form stars very efficiently are mostly just a scaled up version of 'normal' galaxies like our Milky Way.' The study was based on the deepest Herschel images of the Great Observatories Origins Deep Survey (GOODS) fields, two carefully selected patches of the northern and southern sky. As they do not contain any bright stars and are not contaminated by strong emissions coming from the Milky Way, they are ideal for studying galaxy evolution. The astronomers looked at the relation between the total infrared output of the galaxies, which quantifies their star formation activity, and a measure of their infrared emission at wavelengths around 8 µm. The latter, due to carbon-rich dust grains populating the interstellar medium, is a good indicator of the spatial extent of star-forming regions. The study showed that most galaxies (about 80 %) display a tight correlation between these two infrared indicators, independent of their distance to us, or of their total brightness. On the other hand, the contribution of merging events to the build-up of stars in the Universe has been minor as they only make up about 20 % of the sources of all probed patches of the sky. 'Herschel was conceived to study the history of star formation across cosmic time', comments Göran Pilbratt, Herschel project scientist at ESA. The debate on the role of merging events in star formation has been ongoing for decades, and 'these new observations now change our perception of the history of the Universe', he concludes.For more information, please visit: European Space Agency (ESA):http://www.esa.int/ Astronomy & Astrophysics (A&A): http://www.aanda.org/
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