ESO/M.Kornmesser

Astronomers spot planet-forming gas and dust 600 million years after the big bang

The big bang only provided the universe with a few basic building blocks: mostly hydrogen, some helium, and traces of other light elements. Everything else needed to make planets and life is created by stars burning, which fuses hydrogen into heavier elements. (Some of the very heaviest elements are thought to require the energy of a supernova to forge them.) A team of astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA), a collection of 66 radio dishes high in the mountains of northern Chile, to discover a very distant galaxy called A2744_YD4 (pictured in an artist’s impression, above) in the very early days of the universe—when it was just 4% of its current age. They were surprised to find it was already full of dust—microscopic grains of most probably silicon, carbon, and aluminum. The team reports in a paper published today in Astrophysical Journal Letters that they were viewing A2744_YD4 about 600 million years after the big bang, only 200 million years after the current best estimate of when the universe’s very first stars lit up. Even though they estimate that the galaxy was forming stars at a rate of 20 solar masses per year (compared with 1 solar mass in today’s Milky Way), to have produced all that dust in just 200 million years, the galaxy’s first stars must have burned hard and fast. That’s valuable information because astronomers are intensely interested in this first generation of stars, in an era often called cosmic dawn. At that time, almost all the neutral gas pervading the whole universe underwent a profound change: It had electrons stripped away to transform it into the ionized gas we see today. What caused the ionization is not known for certain but astronomers suspect light from the first stars is the culprit. Studies such as this one from ALMA will help astronomers answer the question of whether there were enough stars, shining brightly enough at the right wavelength, to bring about this transformation.