Astronomers have uncovered a previously unknown, extreme kind of star factory by taking the temperature of a distant galaxy using the ALMA telescope. The galaxy is glowing intensely in superheated cosmic dust while forming stars 180 times faster than our own Milky Way. The discovery indicates how galaxies could have grown quickly when the universe was very young, solving a long-standing puzzle for astronomers.

The first generations of stars formed under conditions very different from anywhere we can see in the nearby universe today. Astronomers are studying these differences using powerful telescopes that can detect galaxies so far away their light has travelled towards us for billions of year.

Now, an international team of astronomers led by Tom Bakx at Chalmers University of Technology in Sweden has measured the temperature of one of the most distant known star factories. The galaxy, known as Y1, is so far away that its light has taken over 13 billion years to reach us.

“We’re looking back to a time when the universe was making stars much faster than today. Previous observations revealed the presence of dust in this galaxy, making it the furthest away we've ever directly detected light from glowing dust. That made us suspect that this galaxy might be running a different, superheated kind of star factory. To be sure, we set out to measure its temperature,” says Tom Bakx.

Stars like our Sun are forged in huge, dense clouds of gas in space. The Orion Nebula and the Carina Nebula are two examples of such star factories. They shine brightly in the night sky, powered by their youngest and most massive stars, which light up clouds of gas and dust in many different colours.

At wavelengths longer than the human eye can see, star factories shine brightly thanks to huge numbers of tiny grains of cosmic dust, heated by starlight.

To be able to probe the galaxy's temperature, the scientists needed the superior sensitivity of ALMA. One of the world's largest telescopes, ALMA’s dry, high-altitude location in Chile made it possible to image the galaxy in just the right colour, at a wavelength of 0.44 millimetres using its Band 9 instrument.

“At wavelengths like this, the galaxy is lit up by billowing clouds of glowing dust grains. When we saw how bright this galaxy shines compared to other wavelengths, we immediately knew we were looking at something truly special,” says Tom Bakx.

The detection showed the galaxy’s dust glowing at a temperature of 90 Kelvin – around -180 degrees Celsius.

“The temperature is certainly chilly compared to household dust on Earth, but it’s much warmer than any other comparable galaxy we’ve seen. This confirmed that it really is an extreme star factory. Even though it’s the first time we’ve seen a galaxy like this, we think that there could be many more out there. Star factories like Y1 could have been common in the early universe,” says team member Yoichi Tamura, astronomer at Nagoya University, Japan.

Y1 is manufacturing stars at the extreme rate of over 180 solar masses per year, an unsustainable pace that cannot last long. On average, our galaxy, the Milky Way, creates only about one solar mass per year. Brief, hidden bursts of star formation, as seen in Y1, may have been common in the early universe, the scientists suspect.

"We don't know how common such phases might be in the early universe, so in the future we want to look for more examples of star factories like this. We also plan to use the high-resolution capabilities of ALMA to take a closer look at how this galaxy works," says Tom Bakx.

Bakx’s team believes that galaxy Y1 may help solve another cosmic mystery. Earlier studies have shown that galaxies in the early universe appear to have far more dust than their stars could have produced in the short time they have been shining.

Astronomers have been puzzled by this, but Y1’s unusual temperature points to a solution. Team member Laura Sommovigo, astrophysicist at the Flatiron Institute and Columbia University, USA, takes up the story.

“Galaxies in the early universe seem be too young for the amount of dust they contain. That’s strange, because they don’t have enough old stars, around which most dust grains are created. But a small amount of warm dust can be just as bright as large amounts of cool dust, and that’s exactly what we’re seeing in Y1. Even though these galaxies are still young and don’t yet contain much heavy elements or dust, what they do have is both hot and bright,” she concludes.

The research is presented in the paper A warm ultraluminous infrared galaxy just 600 million years after the Big Bang in Monthly Notices of the Royal Astronomical Society, lead author Tom Bakx (Chalmers University of Technology, Sweden).

All authors: Yoichi Tamura (Nagoya University, Japan), Renske Smit (Liverpool John Moores University, UK), Andrea Ferrara (Scuola Normale Superiore, Italy), Hiddo Algera (Academia Sinica, Taiwan), Susanne Aalto (Chalmers University of Technology, Sweden), Duncan Bossion (University of Rennes, France), Stefano Carniani (Scuola Normale Superiore, Italy), Clarke Esmerian (Chalmers University of Technology, Sweden), Masato Hagimoto (Nagoya University, Japan), Takuya Hashimoto (University of Tsukuba, Japan; Tomonaga Center for the History of the Universe, University of Tsukuba, Japan), Bunyo Hatsukade (National Astronomical Observatory of Japan, Japan; SOKENDAI, Japan; University of Tokyo, Japan), Edo Ibar (Universidad de Valparaíso, Chile; Millenium Nucleus for Galaxies, Chile), Hanae Inami (Hiroshima University, Japan), Akio K. Inoue (Waseda University, Japan; Waseda University, Japan), Kirsten Knudsen (Chalmers University of Technology, Sweden), Nicolas Laporte (Aix Marseille Université, France), Ken Mawatari (Waseda University, Japan; Waseda University, Japan), Juan Molina (Universidad de Valparaíso, Chile; Millenium Nucleus for Galaxies, Chile), Gunnar Nyman (University of Gothenburg, Sweden), Takashi Okamoto (Hokkaido University, Japan), Andrea Pallottini (Scuola Normale Superiore, Italy; Università di Pisa, Italy), W. M. C. Sameera (Chalmers University of Technology, Sweden), Hideki Umehata (Nagoya University, Japan; Nagoya University, Japan), Wouter Vlemmings (Chalmers University of Technology, Sweden), Naoki Yoshida (University of Tokyo, Japan)

The galaxy is known by its catalogue number, MACS0416_Y1. It lies so far from Earth that its light is stretched out by the expansion of the universe; astronomers refer to its distance as redshift 8.3. It was discovered behind a cluster of galaxies called MACS0416, which itself lies only 4 billion light years away in the direction of the constellation Eridanus, the River.

Previous observations by the same team showed that the galaxy holds the record for the furthest away detection of light from cosmic dust. (Press release at ALMA:s web page)