Interstellar dust generated by the first stars in the universe detected by the ALMA radio telescope

Artistic concept of the galaxy A2744_YD4 (Image ESO/M. Kornmesser)
Artistic concept of the galaxy A2744_YD4 (Image ESO/M. Kornmesser)

An article to be published in the journal “The Astrophysical Journal Letters” describes a research about the galaxy A2744_YD4, the most distant observed with the ALMA radio telescope. A team of astronomers led by Nicolas Laporte of University College London also used the X-shooter instrument on ESO’s VLT to confirm that we’re seeing A2744_YD4 as it was about six hundred million years after the Big Bang. The most interesting thing is the dust detection indicating that there were already several supernovae.

The galaxy A2744_YD4 is so distant that its light is extremely dim. Normally, even a powerful instrument such as ALMA (Atacama Large Millimeter/submillimeter Array), which was inaugurated in March 2013 as the largest radio telescope in the world, wouldn’

t be able to detect radio waves from such long distances. Luckily the galaxy is behind the galaxy cluster Abell 2744, which has an enormous overall gravity that it generated a gravitational lensing effect that allowed to observe A2744_YD4.

The ALMA radio telescope allowed to revealed and examine the characteristics of the dust in the galaxy A2744_YD4 to understand which elements were formed. The total amount of dust was estimated at about six million solar masses, while the total mass of stars in the galaxy is about two billion solar masses. The most surprising result is given by the composition of the dust, which contains elements that were formed by stars exploded in supernovae.

The various elements show specific “signatures” in the electromagnetic radiation detected and it turned out that the interstellar dust in the galaxy A2744_YD4 is primarily made up of silicon, carbon and aluminium in tiny grains with a diameter of up to one millionth of a centimeter. The detections also show the presence of ionized oxygen and it’s the most distant, and therefore the oldest, oxygen discovered in the universe.

Today it’s normal to find so many elements in interstellar dust because the grains were formed in many stars, passing sometimes through several generations of stars. In an ancient galaxy such as A2744_YD4 it’s a source of information on the age in which the first supernovae exploded and consequently on the first generation of massive and very hot stars in the universe.

The observations of the A2744_YD4 galaxy provide some interesting indications. The star formation rate is estimated at twenty solar masses per year. For comparison, today in the Milky Way it’s only one solar mass per year. Star formation in A2744_YD4 seems very high but it makes us understand how quickly interstellar dust formed. The estimate is that star formation started about two hundred million years before the time we are observing.

This research was an opportunity to contribute to the study of the period in which the first stars were born. Generations of stars were born and died in the course of billions of subsequent years forming the elements that also ended up in our own solar system and are indispensable for the Earth’s life forms.

Understanding when the first stars were born is a topic still under discussion because a number of studies provided different indications. According to a study published a few months ago based on the analysis of data collected by ESA’s Planck Surveyor space probe the first stars were born about 700 million years after the Big Bang but the exam of the A2744_YD4 galaxy suggests that they were born earlier. The research must go on to give final answers.

The galaxy cluster Abell 2744 and the galaxy A2744_YD4 magnified (Image ALMA (ESO/NAOJ/NRAO), NASA, ESA, ESO and D. Coe (STScI)/J. Merten (Heidelberg/Bologna))
The galaxy cluster Abell 2744 and the galaxy A2744_YD4 magnified (Image ALMA (ESO/NAOJ/NRAO), NASA, ESA, ESO and D. Coe (STScI)/J. Merten (Heidelberg/Bologna))

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