Why stardust is magical for astronomy

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Mar 16, 2017

Stardust may not come from fairy tales or Hollywood, but the stuff those glowing orbs in the sky are made of is giving us an unprecedented glimpse into the early universe.

An international team of astronomers using the Atacama Large Millimeter Array (ALMA) recently studied interstellar dust and gas in a galaxy which is the youngest and furthest away ALMA has ever found. Galaxy A2744_YD4 is swirling with the remains of long-dead stars, which ALMA has been able to identify on a molecular level (which would be otherwise impossible to observe) because it can image at millimeter and submillimeter wavelengths. From Earth, we see the galaxy as it was during the era of star and galaxy formation when the universe was just 600 million years old.

A2744_YD4 appears as it did that far back in time because it has a redshift of 8.38, meaning it's incredibly remote. What redshift measures is how much the expansion of the universe has stretched the light given off by an object in space. Something floating near us has zero redshift, while observing high-redshift objects is like looking through a portal into the distant past — in this case, all the way back to the Epoch of Reionization, when the first luminous objects in the universe ionized neutral hydrogen atoms that had previously absorbed short wavelengths of light struggling to shine through. This was the first time light was able to travel across the vastness of space.

The phenomenon of gravitational lensing also made it possible for the research team to bring this previously undiscovered galaxy to light. Pandora's Cluster (aka Abell 274) acted as a convenient cosmic magnifying glass that enlarged the image of A2744_YD4, which was hiding behind it, enough for astronomers to zero in.

A2744_YD4 is the faint red blob magnified on this image of galaxy cluster Abell 274. 

Supernovae, the explosive death throes of short-lived massive stars, shrouded the galaxy in immense amounts of cosmic dust that dispersed after these stars violently scattered their insides all over the universe. That same stardust ended up being an essential component of the stars and planets that now exist, including the sun. The early universe did not have an abundance of this dust because not enough time had elapsed for stars to burn out and scatter it via supernova; these processes are thought to have already occurred in A2744_YD4 when they hadn't yet in most galaxies. It now produces stars at the rate of 20 solar masses per year — 20 times more than 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 astronomer Richard Ellis of the European Southern Observatory and University College London and explains why, according to Ellis, "we are witnessing this galaxy shortly after its formation."

Observations of this faraway stardust, along with estimates of the age of A2744_YD4, are helping astronomers illuminate the life cycles of the first stars to emerge in the universe, from their birth to the last breaths as supernova explosions that lit up the vast darkness of space. The study could eventually spark more revelations about the earliest and possibly most most massive stars that ever blazed. ALMA also detected the oldest and most distant ionized oxygen in this stellar find of a galaxy.

"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," say Ellis and team lead Nicolas Laporte, also of University College London. "Determining the timing of this 'cosmic dawn' is one of the holy grails of modern astronomy."

(via Astronomy Magazine)