The image above shows some detail of cluster of galaxies known as MACS J1149.5+2223, which is located almost at the edge of the observable Universe. The inset shows a member of the cluster known as MACS1149-JD1, in which the distribution of oxygen is shown in green. At a distance of 13.28 billion light years, this galaxy contains the oldest known oxygen in the Universe.
It has long been known that successive generations of relatively massive stars started changing the chemical composition of primitive galaxies not long after the Big Bang. However, finding the earliest known traces of the Universes’ chemical evolution was hampered by the lack of instruments that are sensitive enough to detect the subtle chemical changes that took place during the Universes’ extreme youth.
In a recent study performed by a team of researchers under the leadership of Takuya Hashimoto, from the Osaka Sangyo University and the National Astronomical Observatory of Japan, using the Atacama Large Millimetre/submillimeter Array (ALMA), the researchers found clear evidence of the presence of oxygen in a galaxy that was chemically mature when the Universe was only 500 million years old, or just 4% of its current age.
However, according to standard models of the evolution of the Universe, stars must have started forming in the galaxy as early as only 250 million years after the Big Bang, which if true, implies that rich chemical environments within galaxies evolved much faster that was previously thought.
To understand the implications of this discovery, it is necessary to understand that the baryonic matter created during, or soon after the Big Bang consisted of nothing but hydrogen, helium, and minuscule amounts of lithium. Speaking generally, these elements were the building blocks of the first stars, which created progressively heavier elements when they died as supernovas.
In the case of the galaxy MACS1149-JD1 however, it appears that the first stars were sufficiently massive for their violent deaths to have driven the heavy molecules created by less violent supernova events from the galaxy, which stopped further star formation in the galaxy. Over time, the expelled molecular clouds settled back into the galaxy, which triggered a second burst of star formation, and it is this second burst of star formation that ionized the oxygen between the stars that the ALMA instrument had detected.
During this epoch of ionisation, the galaxy MACS1149-JD1 was less than 500 million years old, and the ionised oxygen molecules were radiating strongly in the infrared. However, as the Universe expanded, the infrared frequencies were stretched until they fell into the millimetre-range the Alma instrument was specifically designed to detect. Based on exact measurements of the amount of stretching the light frequency had undergone, the distance to the galaxy was fixed with unprecedented accuracy to 13.28 billion light years.
While nobody has yet found actual, surviving examples of first-generation of stars, this discovery of some of the oldest known second-generation stars has profound implications for our understanding of galactic evolutionary processes, about which very little is known for certain. According to Nicolas Laporte, a researcher at University College London/Université de Toulouse and a member of the research team:
“The mature stellar population in MACS1149-JD1 implies that stars were forming back to even earlier times, beyond what we can currently see with our telescopes. This has very exciting implications for finding ‘cosmic dawn’ when the first galaxies emerged.”
More details of the discovery of the oldest known oxygen in the Universe will soon be published in the scientific journal Nature, in a paper entitled “The onset of star formation 250 million years after the Big Bang”, by T. Hashimoto et al.