Astronomy

Stars formed much earlier than previously thought

Print edition : June 22, 2018

Caption: Image shows the galaxy cluster MACS J1149.5+2223 taken with the NASA/ESA Hubble Space Telescope; the inset image is the very distant galaxy MACS1149-JD1, seen as it was 13.3 billion years ago and observed with ALMA. Here, the oxygen distribution detected with ALMA is depicted in red.

A team of international astronomers, led by groups at the University College London (UCL) and Osaka Sangyo University, Japan, has found that stars in a galaxy 13.28 billion light years away—called MACS1149-JD1—were formed only 250 million years after the Big Bang.

The discovery implies that stars in MACS1149-JD1 formed at an unexpectedly early stage in the age of the universe. The new observations also break the team’s own record for detecting the most distant known source of oxygen.

The team confirmed the distance of the galaxy through observations undertaken with the Atacama Large Millimetre/Submillimetre Array (ALMA) and the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT). The galaxy distance corresponds to looking back to a time when the universe was only 500 million years old, which is 3.5 per cent of its present age. The finding has been reported in a recent issue of the journal ‘Nature’.

Although the presence of galaxies at such an early epoch itself is not surprising, the detection of oxygen indicates that a previous generation of stars had already formed and died at an even earlier time. Oxygen is only created in stars and then released into the gas clouds in galaxies when those stars die. “This is an exciting discovery as this galaxy is seen at a time when the universe was only 500 million years old and yet it already has a population of mature stars,” says Nicolas Laporte, second author and a postdoctoral researcher at UCL who led the VLT observing campaign.

Using ALMA to observe MACS1149-JD1, the Japanese team, led by Takuya Hashimoto and Akio Inoue of the Osaka Sangyo University, detected a signal from ionised oxygen from the galaxy whose infrared light was stretched tenfold to microwave wavelengths by the expansion of the universe.

Using the VLT, Laporte independently confirmed the inferred distance of 13.28 billion light years by detecting emissions of hydrogen. These signals also stretched to near-infrared wavelengths, making MACS1149-JD1 the most distant known galaxy with a precise distance measurement.

In 2016, Inoue and his colleagues detected oxygen emission at 13.1 billion light years. Several months later, Laporte used ALMA to detect oxygen at 13.2 billion light years away. Both teams now collaborated in their efforts to achieve this latest record.

The team reconstructed the earlier history of MACS1149-JD1 using infrared data taken with the NASA/ESA Hubble Space Telescope and NASA Spitzer Space Telescope. The observed brightness of the galaxy is well explained by a model where the onset of star formation corresponds to a time only 250 million years after the universe began.

By establishing the age of MACS1149-JD1, the team has effectively demonstrated the existence of early galaxies to times earlier than those where we can currently directly detect them. More pertinently, the maturity of the stars seen in it raises the question of when the very first galaxies emerged from total darkness, an epoch called “cosmic dawn”.

“Determining when cosmic dawn occurred is akin to the ‘Holy Grail’ of cosmology and galaxy formation,” Richard Ellis, astronomer at UCL and co-author, said. “With MACS1149-JD1, we have managed to probe history beyond the limits of when we can actually detect galaxies with current facilities... [and] we are getting closer and closer to witnessing directly the birth of starlight. Since we are all made of processed stellar material, this is really finding our own origins.”

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