Ancient stardust sheds light on the first stars

An astronomer from Queen Mary University of London (QMUL) is part of a team that have discovered new insights into the birth and death of the very first stars in the Universe.

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Spectacular view of the rich galaxy cluster Abell 2744 with A2744_YD4 highlighted (c) ALMA (ESO/NAOJ/NRAO), NASA, ESA, ESO and D. Coe (STScI)/J. Merten (Heidelberg/Bologna)
Spectacular view of the rich galaxy cluster Abell 2744 with A2744_YD4 highlighted (c) ALMA (ESO/NAOJ/NRAO), NASA, ESA, ESO and D. Coe (STScI)/J. Merten (Heidelberg/Bologna)

The team have detected an unexpectedly huge mass of glowing stardust in a galaxy seen when the Universe was only four per cent of its present age.

Writing in The Astrophysical Journal Letters, the team describe how they used the ALMA telescope (Atacama Large Millimeter/submillimeter Array) and the X-shooter instrument on the European Southern Observatory's Very Large Telescope to observe A2744_YD4, the youngest and most remote galaxy ever seen by ALMA.

Co-author Dr David Quénard from QMUL’s School of Physics and Astronomy contributed to planning and executing the ALMA observations that revealed the extent of the dust in what is the most distant galaxy in which dust has been detected.

Dr Quénard said: “It’s impressive that ALMA was able to detect such a faint object at such a huge distance – this indicates how sensitive this telescope is at low resolution.”    

Cosmic Dust

The team were surprised to find that this youthful galaxy contained an abundance of interstellar dust — dust formed by the deaths of an earlier generation of stars.

Lead author Dr Nicolas Laporte from University College London, said: “Not only is A2744_YD4 the most distant galaxy yet observed by ALMA, but the detection of so much dust indicates early supernovae must have already polluted this galaxy.”

Cosmic dust is composed of silicon, carbon, and aluminium, in grains as small as a millionth of a centimetre across. These grains are forged inside stars and are scattered across the cosmos when the stars die, most spectacularly in supernova explosions, the final fate of short-lived, massive stars. Today, this dust is plentiful and is a key building block in the formation of stars, planets and complex molecules; but in the early Universe — before the first generations of stars died out — it was scarce.

The observations of the dusty galaxy A2744_YD4 were made possible by pointing the ALMA array through a massive galaxy cluster called Abell 2744 also known as Pandora’s Cluster. Because of a phenomenon called gravitational lensing, the cluster acted like a giant cosmic “telescope” to magnify the more distant A2777_YD4 by about 1.8 times, allowing the team to peer far back into the early Universe.

This NASA/ESA Hubble Space Telescope image shows a spectacular view of the rich galaxy cluster Abell 2744. Far beyond this cluster, and seen when the Universe was only about 600 million years old, is a very faint galaxy called A2744_YD4. QMUL astronomer Dr David Quenard was part of a team that used ALMA to make new observations of this galaxy, shown in the inset, that have demonstrated that it is rich in dust.

A star is born

Follow-up observations undertaken with the Very Large Telescope in Chile confirmed the enormous distance to A2777_YD4. The galaxy appears to us as it was when the Universe was only 600 million years old, during the period when the first stars and galaxies were forming.

The detection of dust in the early Universe provides a new probe of when the first supernovae exploded and hence the time when the first hot stars bathed the Universe in light. Determining the timing of this “cosmic dawn” is one of the holy grails of modern astronomy, and it can be probed through the study of early interstellar dust.

The team estimates that A2744_YD4 contained an amount of dust equivalent to 6 million times the mass of our Sun, while the galaxy’s total stellar mass — the mass of all its stars — was 2 billion times the mass of our Sun. The team also measured the rate of star formation in A2744_YD4 and found that stars are forming at a rate of 20 solar masses per year — compared to just one solar mass per year in the Milky Way.

Co-author Professor Richard Ellis from ESO and UCL, said: “This rate is not unusual for such a distant galaxy, but it does shed light on how quickly the dust in A2744_YD4 formed.

“Remarkably, the required time is only about 200 million years — so we are witnessing this galaxy shortly after its formation.”

Origins of the Universe?

This means that significant star formation began approximately 200 million years before the epoch at which the galaxy is being observed.

Observations like this allow astronomers to study the era when the first stars and galaxies “switched on” — the earliest epoch yet probed. Our Sun, the Earth and our existence are the products — 13 billion years later — of this first generation of stars. By studying their formation, lives, and deaths, we are exploring our origins.

Dr Laporte added: “Further measurements of this kind offer the exciting prospect of tracing early star formation and the onset of chemical enrichment, even further back into the early Universe.”

More information

• “Dust in the Reionization Era: ALMA Observations of a z =8.38 Gravitationally-Lensed Galaxy” by Laporte et al. is published in The Astrophysical Journal Letters.

• Find out more about studying Astrophysics BSc and Astrophysics MSc at QMUL’s School of Physics and Astronomy.

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