Discovery of a star nursery in the primordial universe

A look 13 billion years into the past

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 taken billions of years to reach us. 

Now, an international team of astronomers led by Tom Bakx at Chalmers University of Technology 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 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.

A "star-making factory"

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 cosmological scales. 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,” Laura Sommovigo concludes.

About this study

Theses results are presented in the article "A warm ultraluminous infrared galaxy just 600 million years after the Big Bang", published in the Monthly Notices of the Royal Astronomical Society (first author: Tom Bakx, Chalmers University of Technology, Sweden).
Link to article: https://doi.org/10.1093/mnras/staf1714 

The galaxy, listed under catalog number MACS0416_Y1, is located so far away that its light is strongly redshifted (redshift 8.3) due to the expansion of the universe.

Previous observations by the same team had already established that Y1 holds the record for the most distant detection of light from cosmic dust.

This study opens up a new avenue for the quest for our cosmic origins, a quest that will take a major turn at the dawn of the 2030s with the opening of the world's largest optical telescope in Chile: the Extremely Large Telescope (ELT). The Laboratoire d'Astrophysique de Marseille is heavily involved in the construction of two ELT instruments: HARMONI and MOSAIC. These two cameras will shed new light on stardust in distant galaxies.