ALMA rewrites history of Universe's stellar baby boom
The most intense bursts of star birth are thought to have occurred in the early Universe, in massive, bright galaxies. These starburst galaxies convert vast reservoirs of cosmic gas and dust into new stars at a furious pace — many hundreds of times faster than in stately spiral galaxies like our own galaxy, the Milky Way. By looking far into space, at galaxies so distant that their light has taken many billions of years to reach us, astronomers can observe this busy period in the Universe’s youth.
“The more distant the galaxy, the further back in time 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 history,” said Joaquin Vieira (California Institute of Technology, USA), who led the team and is lead author of the paper in the journal Nature.
The international team of researchers first discovered these distant and enigmatic starburst galaxies with the US National Science Foundation’s 10-metre South Pole Telescope (SPT), a facility built partly by McGill astrophysicists. The researchers, including McGill doctoral candidate Yashar Hezaveh, then used ALMA to zoom in on the galaxies to explore the stellar baby boom in the young Universe. They 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 star birth took place 12 billion years ago, when the Universe was just under 2 billion years old — a full billion years earlier than previously thought.
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 published to date.
The team used the unrivalled sensitivity of ALMA, a global collaboration that includes a significant Canadian contribution, to capture light from 26 of these galaxies at wavelengths of around three millimetres. 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 the stretched wavelengths, astronomers 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 just a few minutes per galaxy -- about one hundred times faster than before,” said Axel Weiss (Max-Planck-Institut für Radioastronomie in Bonn, Germany), who led the work to measure the distances to the galaxies. “Previously, a measurement like this would have been a laborious process of combining data from both visible-light and radio telescopes.”
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, including the Atacama Pathfinder Experiment (APEX), the Australia Telescope Compact Array (ATCA), the Very Large Telescope (VLT) and the Submillimeter Array (SMA).
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 at an altitude of 5,000 metres on the remote Chajnantor Plateau in the Chilean Andes. When complete, ALMA will be even more sensitive, and will be able to detect even fainter galaxies. For now, the astronomers targeted the brighter ones. They took advantage of a helping hand from nature, too, using gravitational lensing -- an effect predicted by Einstein’s general theory of relativity, where light from a distant galaxy is distorted by the gravitational influence of a nearer foreground galaxy, which acts like a lens and makes the distant source appear brighter.
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 of around 0.9 millimetres.
"These beautiful pictures from ALMA show the background galaxies warped into multiple arcs of light known as Einstein rings, which encircle the foreground galaxies," said McGill’s Yashar Hezaveh, who led the study of the gravitational lensing. “We are using the massive amount of dark matter surrounding galaxies half-way across the Universe as a cosmic telescope to make even more distant galaxies appear bigger and brighter.”
Analysis of the distortion reveals that some of the distant star-forming galaxies are as bright as 40 trillion (40 million million) 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 submillimetre wavelengths, but now SPT and ALMA have uncovered dozens of them.” said Carlos De Breuck of the European Southern Observatory (ESO), a member of the team. “This kind of science was previously done mostly at visible-light wavelengths with the Hubble Space Telescope, but our results show that ALMA is a very powerful new player in the field.”
“This is an great example of astronomers from around the world collaborating to make an amazing discovery with a state-of-the-art facility,” said team member Daniel Marrone (University of Arizona, USA). “This is just the beginning for ALMA and for the study of these starburst galaxies. Our next step is to study these objects in greater detail and figure out exactly how and why they are forming stars at such prodigious rates.”
In addition to Hezaveh, the other McGill authors on the Nature paper are Prof. Gil Holder, Prof. Matt Dobs, and graduate student Tijmen de Haan – all members of the Astrophysics and Cosmology Group in the Department of Physics. Holder and Dobbs are also fellows in the Cosmology and Gravity research program of the Canadian Institute for Advanced Research (CIFAR).
The work at McGill is supported by the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chairs program, CIFAR and the Fonds de recherche du Québec - Nature et technologies.
The research is described in the paper “Dusty starburst galaxies in the early Universe as revealed by gravitational lensing”, by J. Vieira et al., in the journal Nature. The work to measure the distances to the galaxies is described 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 described 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.
About the South Pole Telescope
The South Pole Telescope (SPT) is a 10-meter telescope located at the National Science Foundation (NSF) Amundsen-Scott South Pole Station within 1 km of the geographic south pole. The SPT is designed to conduct low-noise, high-resolution surveys of the sky at millimeter (mm) and submillimeter (submm) wavelengths, with the particular design goal of making ultra-sensitive measurements of the cosmic microwave background (CMB). The first major survey with the SPT was completed in October, 2011 and covers 2500 square degrees of the southern sky in three mm-wave observing bands. This is the deepest large mm-wave dataset in existence and has already led to many groundbreaking science results, including the first galaxy clusters detected through their Sunyaev-Zel'dovich effect signature, the most sensitive measurement yet of the small-scale CMB power spectrum, and the discovery of a population of ultra-bright, high-redshift, star-forming galaxies. SPT is funded primarily by the Division of Polar Programs in NSF's Geosciences Directorate. Partial support also is provided by the NSF-funded Physics Frontier Center of the KICP, the Kavli Foundation, and the Gordon and Betty Moore Foundation. The SPT collaboration is led by the University of Chicago and includes research groups at Argonne National Laboratory, California Institute of Technology, Cardiff University, Case Western Reserve University, Harvard University, Ludwig-Maximilians-Universität, Smithsonian Astrophysical Observatory, McGill University, University of Arizona, University of California at Berkeley, University of California at Davis, University of Colorado at Boulder, University of Michigan, as well as individual scientists at several other institutions, including the European Southern Observatory and the Max-Planck-Institut für Radioastronomie in Bonn, Germany.
IMAGE: Light rays from a distant galaxy are deflected due to the gravity of a massive, foreground galaxy, as predicted by Einstein's theory of general relativity. This makes the background galaxy appear as multiple magnified images surrounding the foreground galaxy.
Credit: ALMA (ESO/NRAO/NAOJ), L. Calçada (ESO), Y. Hezaveh et al.