Scientific results of the highest resolution observations yet attempted by the Atacama Large Millimeter/submillimeter Array (ALMA) telescope are described in four papers to be published this month in Astrophysical Journal Letters. The observations were part of a campaign to test ALMA’s capabilities when run in its long-baseline configuration, and the result is stunning images and data for five astronomical objects. These include the planet-forming disc of HL Tau, the tumbling asteroid Juno, and a lensed galaxy 12 billion light-years away. These results are not only rich with new insights, but they also give us a sense of the incredible capabilities that ALMA will provide in the future.
ALMA is an array of 12 m- and 7 m-diameter antennas that observe the cosmos at millimetre/submillimetre wavelengths. The antennas work together to function as an interferometer, and this allows the signals from each of the antennas to be combined to simulate a telescope the size of the distance between the individual units. The array works like a zoom lens on a camera: the antennas can be repositioned so that the baseline of the simulated telescope is as small as 150 m or as large as 15 km across.
Expecting the unexpected
ALMA’s capabilities in its wavelength regime are revolutionary. The array is the largest and most sensitive millimetre/submillimetre instrument in the world, and its resolution is 10 times better than even the Hubble Space Telescope. As Anneila Sargent, chair of the ALMA board while the array was being built, predicted in 2008, “We know that every time in the past that a new wavelength region has been opened up, as ALMA will do, we have been surprised by entirely unexpected discoveries that significantly changed our understanding of the universe. We also expect the unexpected from ALMA.”
In its smaller configurations, ALMA can study the large-scale structure of cold gas and dust in the universe – and this is how the array has been used since it began its first early-science operations in 2011. But now, ALMA is beginning to test its long-baseline configuration, in which it is able to make its highest resolution observations and study the small-scale structure of objects in detail.
ALMA’s Long Baseline Campaign, which ran in late 2014, observed five targets using 22–36 antennas arranged with a baseline of up to the maximum 15 km. The targets were specifically selected to push the limits of ALMA’s capabilities: each target has a small angular size (less than two arcseconds) with a fine-scale structure that had been largely unresolved in previous observations made with other telescopes. Two of the targets, the variable star Mira and the active galaxy 3C138, were primarily used for calibration and comparisons of ALMA data with those of other telescopes. The remaining three targets not only demonstrated ALMA’s capabilities, but also resulted in new science discoveries.
The first discovery involves HL Tau, which is a young star surrounded by a protoplanetary disc – a disc of gas and dust from which planets can be born. ALMA’s detailed observations of this region revealed a remarkable structure within the disc: a series of light and dark concentric rings indicative of planets caught in the act of formation. Studying this system is allowing scientists to better understand how multi-planet solar systems, like our own, form and evolve.
Juno, which is one of the largest asteroids in our solar system’s main asteroid belt, is the subject of the second discovery. ALMA’s observations of Juno were made when the asteroid was approximately 295 million kilometres from Earth. The 10 images ALMA took have been stitched together into the brief animation shown below, which shows the asteroid tumbling through space as it orbits the Sun. ALMA’s observations are not of reflected light, but rather of the millimetre-wavelength light emitted by the asteroid itself. The resolution of these images is good enough to study the shape and even some surface features of the asteroid – something that is unprecedented for this wavelength.
The final discovery concerns the star-forming galaxy SDP.81, which is so far from Earth that the light we see was emitted when the universe was only 15% of its current age. The galaxy is only visible because of a fortuitous alignment between it and a nearby foreground galaxy. The gravity of the foreground galaxy acts as a lens and bends the light from SDP.81 into a highly magnified cosmic ring. The combination of this lucky alignment and ALMA’s high resolution gives scientists a spectacularly detailed view of this distant galaxy, allowing them to study the actual shape of the galaxy and motion within it. This is ALMA’s highest resolution observation so far, and is described by Alma astronomers as being “about the same as seeing the rim of a basketball hoop atop the Eiffel Tower from the observing deck of the Empire State Building”.
The observations from ALMA’s first test of its long baseline clearly demonstrate that exciting times are ahead as scientists gear-up for the next cycle of observations. “It takes a combination of ALMA’s high resolution and high sensitivity to unlock these otherwise hidden details of the early universe,” says ALMA director Pierre Cox. “These results open a new frontier in astronomy, and prove that ALMA can indeed deliver on its promise of transformational science.”
