Researchers making use of the combined observational powers of the Herschel space observatory and the Keck telescopes have now been able to characterize a large quantity of previously unknown starburst galaxies. The research has revealed really exceptionally ‘high’ rates of star-formation throughout the Universe’s history.
A starburst galaxy is one which creates “hundreds of solar masses’ worth of stars each year in short-lived but intense events.” As a comparison, the Milky Way averages about one Sun-like star a year.
The light produced by these galaxies would far outshine our own galaxy, but because of the enormous amounts of dust they contain, the majority of their light is obscured. The light they produce is absorbed by the dust, throughout almost all of the spectrum of visible light. But because the dust is heated by the close and very hot surrounding stars, the dust reemits the energy at the far-infrared wavelength of light.
For the new research, the ESA’s infrared Herschel space observatory was used to measure the “temperature and brightness of thousands of dusty galaxies. From these, their star-formation rate could be then calculated.”
“Starburst galaxies are the brightest galaxies in the Universe and contribute significantly to cosmic star formation, so it’s important to study them in detail and understand their properties,” says Dr Caitlin Casey of the University of Hawai’i, lead author of the papers discussing the results in the Astrophysical Journal.
“Some of the galaxies found in this new survey have star-formation rates equivalent to the birth of several thousand solar-mass stars per year, constituting some of the brightest infrared galaxies yet discovered.”
To get context on how star formation rates have changed through the Universe’s 13.7 billion year history, the researchers needed to determine the distances to the galaxies.
“With Herschel signposting the way, Dr Casey’s team used spectrometers on the twin 10-metre W.M. Keck telescopes on Mauna Kea, Hawai’i, and obtained the redshifts of 767 of the starburst galaxies.”
“Redshifts provide astronomers with a measure of how long the light from each galaxy has travelled across the Universe, which, in turn, indicates when in cosmic history the light from each galaxy was emitted.”
“For most of the galaxies it was found that the light has been travelling towards us for 10 billion years or less. About 5% of the galaxies are at even greater redshifts: their light was emitted when the Universe was only 1-3 billion years old.”
“The Herschel data tell us how fiercely and prolifically these galaxies are producing stars,” says Seb Oliver from University of Sussex, UK, and Principal Investigator for the HerMES Key Programme, within which the data have been collected.
“Combining this information with the distances provided by the Keck data, we can uncover the contribution of the starburst galaxies to the total amount of stars produced across the history of the Universe.”
“How such large numbers of starburst galaxies formed during the first few billions of years of the Universe’s existence poses a vital problem for galaxy formation and evolution studies. One leading theory proposes that a collision between two young galaxies could have sparked an intense short-lived phase of star formation.”
One of the other theories questions whether in the early days of the Universe individual galaxies simply had much more gas surrounding them, available for growth. This would thus enable much higher star formation rates without the need for more collisions.
“It’s a hotly debated topic that requires details on the shape and rotation of the galaxies before it can be resolved,” adds Dr Casey.
“Before Herschel, the largest similar survey of distant starbursts involved only 73 galaxies — we’ve improved on that by over a factor of ten in this combined survey with Keck to determine the characteristics of this important galaxy population,” adds Göran Pilbratt, ESA’s Herschel project scientist.
Source: European Space Agency
Image Credits: ESA–C. Carreau/C. Casey (University of Hawai’i); COSMOS field: ESA/Herschel/SPIRE/HerMES Key Programme; Hubble images: NASA, ESA