13 March, 2013
Astronomers are having to rewrite a major part of the history of the Universe. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) show that the most vigorous bursts of star birth in the cosmos took place much earlier than previously thought. The results are published in a set of papers to appear in the journal Nature on 14 March 2013, and in the Astrophysical Journal. The research is the most recent example of the discoveries coming from the new international ALMA observatory, which celebrates its inauguration tomorrow.
The most intense bursts of starbirth are thought to have taken place in the early Universe, in massive, bright galaxies containing lots of cosmic dust. By looking far into space, at galaxies which are so distant that their light has taken many billions of years to reach us, astronomers can observe this busy period of the Universe’s youth.
“The more distant the galaxy, the further back one is looking, so by measuring their distances we can piece together a timeline of how vigorously the Universe was making new stars at different stages of its 13.7 billion year life,” said Joaquin Vieira (California Institute of Technology, USA), lead author of the paper in the journal Nature.
The international team of researchers used ALMA to explore this stellar baby boom in the young Universe, and were surprised to find that many of these distant dusty star-forming galaxies are even further away than expected. This means that, on average, their bursts of starbirth took place 12 billion years ago, when the Universe was just under 2 billion years old — a full billion years earlier than previously thought.
In fact, two of these galaxies are the most distant of their kind ever seen — so distant that their light began its journey when the Universe was only one billion years old. What’s more, in one of these record-breakers, water is among the molecules detected, marking the most distant observations of water in the cosmos.
The team used the unrivalled sensitivity of ALMA to capture light at wavelengths around 3 millimeters from 26 of these galaxies, selected from an initial survey using the South Pole Telescope. Light at characteristic wavelengths is produced by gas molecules in these galaxies, and the wavelengths are stretched by the expansion of the Universe over the billions of years that it takes the light to reach us. By measuring this stretch, one can calculate how long the light’s journey has taken, and place each galaxy at the right point in cosmic history.
“ALMA’s sensitivity and wide wavelength range mean we could make our measurements in a few minutes per galaxy — about one hundred times faster than previous telescopes,” said Axel Weiss (Max-Planck-Institut für Radioastronomie in Bonn, Germany), who led the work to measure the distances to the galaxies. In the majority of cases, the ALMA observations alone could pinpoint the distances, but for a few galaxies the team combined the ALMA data with measurements from other telescopes such as the Atacama Pathfinder Experiment (APEX) and the ESO Very Large Telescope.
The astronomers were using only a partial array of 16 of ALMA’s full complement of 66 giant antennas, as the observatory was still under construction on its 5000-meter-altitude plateau in the Chilean Andes. When complete, ALMA will be even more sensitive, and will be able to detect even fainter galaxies, but for now the astronomers targeted the brightest of them. They took advantage of a helping hand from nature, too: an effect known as gravitational lensing, predicted by Einstein’s general theory of relativity, where light from a distant galaxy is distorted by the gravitational effect of a nearer foreground galaxy, which acts like a lens and amplifies the brightness of the faint target.
To understand precisely how much this gravitational lensing brightened the view of the galaxies, the team made sharper images of them using more ALMA observations at wavelengths around 0.89 millimeters.
“These beautiful pictures show the background galaxies warped into multiple arcs and rings of light, known as Einstein rings, which encircle the foreground galaxies. It’s like looking through a cosmic kaleidoscope,” said Yashar Hezaveh (McGill University, Montreal, Canada), who led the study of the gravitational lensing.
Analysis of the distortion reveals that the distant star-forming galaxies have brightnesses equivalent to as many as 40 trillion (40 thousand billion) Suns, and that gravitational lensing has magnified this by up to 22 times.
“Only a few gravitationally lensed galaxies have been found before at these submillimeter wavelengths, but now ALMA’s found dozens of them. This kind of science was previously done mostly at visible-light wavelengths with the Hubble Space Telescope, but this shows that ALMA is a very powerful new player in the field,”said Carlos De Breuck (ESO), a member of the team.
The research is described in the paper “High-redshift, gravitationally lensed, dusty, starburst galaxies revealed by the South Pole Telescope and ALMA”, by J. Vieira et al., in the journal Nature. The work to measure the distances to the galaxies is detailed in the paper “ALMA redshifts of millimeter-selected galaxies from the SPT survey: The redshift distribution of dusty star-forming galaxies”, by A. Weiss et al., in the Astrophysical Journal. The study of the gravitational lensing is detailed in the paper “ALMA observations of strongly lensed dusty star-forming galaxies”, by Y. Hezaveh et al., also in the Astrophysical Journal.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
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