New observations using the Atacama Large Millimeter/submillimeter Array (ALMA) have demonstrated that A2744_YD4 — the youngest and most remote galaxy ever seen by ALMA — is rich in interstellar dust, material formed during explosions of an earlier generation of stars. The ALMA observations are also the most distant detection of oxygen in the Universe.
This image is dominated by a spectacular view of the rich galaxy cluster Abell 2744 from the NASA/ESA Hubble Space Telescope. But, far beyond this cluster, and seen when the Universe was only about 600 million years old, is the young galaxy A2744_YD4. New observations of this galaxy with ALMA, shown in red, have demonstrated that it is rich in dust. Image credit: ALMA / ESO / NAOJ / NRAO / NASA / ESA / D. Coe, STScI / J. Merten, Heidelberg & Bologna.
Cosmic dust is mainly composed of silicon, carbon and aluminum, in grains as small as a millionth of a centimeter across.
The chemical elements in these grains are forged inside stars and are scattered across the cosmos when the stars die, most spectacularly in supernova explosions.
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 dusty galaxy A2744_YD4 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, according to a research team led by University College London astronomer Nicolas Laporte.
The observations of A2744_YD4 were made possible because this galaxy lies behind a massive galaxy cluster called Abell 2744.
“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,” Dr. Laporte said.
The detection of dust in the early Universe provides new information on 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 indirectly probed through the study of early interstellar dust.
Dr. Laporte and his colleagues estimate 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 was 2 billion times the mass of our Sun.
The ALMA observations also detected the glowing emission of ionized oxygen from A2744_YD4.
This is the most distant, and hence earliest, detection of oxygen in the Universe, surpassing another ALMA result from 2016.
The astronomers 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.
“This rate is not unusual for such a distant galaxy, but it does shed light on how quickly the dust in A2744_YD4 formed,” said team member Dr. Richard Ellis, from ESO and University College London.
“Remarkably, the required time is only about 200 million years — so we are witnessing this galaxy shortly after its formation.”
This means that significant star formation began approximately 200 million years before the epoch at which A2744_YD4 is being observed.
This provides a great opportunity for ALMA to help study the era when the first stars and galaxies ‘switched on’ — the earliest epoch yet probed.
Details of the research will be published in the Astrophysical Journal Letters. The article is also publicly available at arXiv.org.
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Nicolas Laporte et al. 2017. Dust in the Reionization Era: ALMA Observations of a z =8.38 Gravitationally-Lensed Galaxy. ApJL, in press; arXiv: 1703.02039
