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SpaceWatch: Counting galaxies and stars

The Universe may be big, but we’re rolling out the technology to try to understand it. Here are three updates on how we’re looking and what we’re seeing.

A sort of Milky Way far away

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered an extremely distant galaxy they say looks surprisingly like the Milky Way – for now at least.

The galaxy SPT0418-47 is gravitationally lensed by a nearby galaxy, appearing in the sky as a near-perfect ring of light. Credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.

Its light has taken more than 12 billion years to reach Earth, and thus it appears as it was when the Universe was just 1.4 billion years old. It is also surprisingly unchaotic, contradicting theories that all galaxies in the early Universe were turbulent and unstable.

Because galaxies such as SPT0418-47 are so far away, detailed observations with even the most powerful telescopes are almost impossible. The researchers overcame this by using a nearby galaxy as a powerful magnifying glass – known as gravitational lensing.

In this effect, the gravitational pull from the nearby galaxy distorts and bends the light from the distant galaxy, causing it to appear misshapen and magnified. The researchers reconstructed the true shape and the motion of its gas from the ALMA data using a new computer modelling technique. 

While SPT0418-47 doesn’t appear to have spiral arms, it has at least two features typical of the Milky Way: a rotating disc and a bulge – a large group of stars packed tightly around the galactic centre.

This is the first time a bulge has been seen this early in the history of the Universe, says Francesca Rizzo from the Max Planck Institute for Astrophysics in Germany, lead author of a paper in the journal Nature.

But the big surprise, says co-author Filippo Fraternali, from the University of Groningen in the Netherlands, “was to find that this galaxy is actually quite similar to nearby galaxies, contrary to all expectations from the models and previous less detailed observations”.

In the early Universe, young galaxies were still in the process of forming, so researchers expected them to be chaotic and lacking the distinct structures typical of more mature galaxies like the Milky Way.

Rizzo and colleagues do note, however, that while SPT0418-47 has features similar to those of spiral galaxies we see today, they expect it to evolve and join the class of elliptical galaxies.

Picking the spirals from the pack

Japanese astronomers have used artificial intelligence to identify and classify about 80,000 new spiral galaxies, most located in the distant Universe more than 2500 million light-years away.

A team led by the National Astronomical Observatory of Japan (NAOJ) applied deep-learning algorithms to images of the distant Universe captured by the Subaru Telescope, detecting around 560,000 galaxies in total.

Automated processing techniques for extraction and judgment were able to sort these as S- or Z-shaped spirals, or not spirals at all, without missing a beat. In a test run using training data prepared by humans, they classified the galaxy morphologies with an accuracy of 97.5%.

The results are reported in a paper in the Monthly Notices of the Royal Astronomical Society.

Credit: Tadaki et al/NAOJ

The galaxy images were obtained in the 300-night-long large survey program (Subaru Strategic Program) using the Subaru’s ultra-wide field-of-view camera, Hyper Suprime-Cam (HSC).

The AI program uses convolutional neural networks, which do not work with raw images directly, the researchers say. They apply a series of mathematical operations multiple times, which reduces the amount of information of the images, allowing local features like outlines of distinctive objects to be detected.

NAOJ’s Ken-ichi Tadaki, who came up with the idea of applying the technique to space, says it could now be extended to classify galaxies into more detailed classes, by training the AI on the basis of a substantial number of galaxies classified by humans.

And humans are very much part of an offshoot of the research – a citizen-science project called Galaxy Cruise, which encourages people to examine Subaru images to search for features that suggest the galaxy is colliding or merging with another.

“The Subaru Strategic Program is serious big data containing an almost countless number of galaxies,” says NAOJ’s Masayuki Tanaka. “Scientifically, it is very interesting to tackle such big data with a collaboration of citizen astronomers and machines.”

TESS moves into another phase

An illustration of TESS at work. Credit: Goddard Space Flight Centre

On a seemingly more traditional note, NASA reports that its Transiting Exoplanet Survey Satellite (TESS) has finished its primary mission and moved into its extended mission, which will run until September 2022.

In two years to date, it has imaged about 75% of the starry sky and, among other discoveries, has found 66 new exoplanets and nearly 2100 candidates astronomers are working to confirm.

TESS monitors 24-by-96-degree strips of the sky called sectors for about a month using four cameras. It spent its first year observing 13 sectors comprising the southern sky, then its second imaging the northern sky. It has now turned back to the south.

In addition, a number of improvements have been made to the way the satellite collects and processes data.

Its cameras now capture a full image every 10 minutes, three times faster than during the primary mission. A new fast mode allows the brightness of thousands of stars to be measured every 20 seconds, along with the previous method of collecting these observations from tens of thousands of stars every two minutes.

TESS looks for transits – the dimming of a star caused when an orbiting planet passes in front of it from our point of view.

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