Trace 13 billion years of history in the light of ancient quasars

Astrophysicists in Australia have shed new light on the state of the universe 13 billion years ago by measuring the density of carbon in the gases surrounding ancient galaxies.

The study, published in Monthly Notices of the Royal Astronomical Societyadds another piece to the puzzle of the history of the universe.

“We found that the fraction of charcoal in hot gas increased rapidly around 13 billion years ago, which may be linked to large-scale heating of gas associated with the phenomenon known as the epoch of reionization,” said Dr. Rebecca Davies, ASTRO 3D Postdoctoral Fellow at Swinburne University of Technology, Australia and lead author of the paper describing the discovery.

The study shows that the amount of hot coal suddenly increased by a factor of five over a period of just 300 million years – an instant on astronomical time scales.

While previous studies have suggested an increase in hot coal, much larger samples—the basis of the new study—were needed to provide statistics to accurately measure the rate of this growth.

“That’s what we’ve done here. And so we present two potential interpretations of this rapid evolution,” says Dr Davies.

The first is that there is an initial increase in carbon around galaxies simply because there is more carbon in the universe.

“During the period when the first stars and galaxies are forming, many heavy elements are formed because we never had carbon before we had stars,” says Dr Davies. “And so one possible reason for this rapid rise is just that we’re seeing the products of the first generations of stars.”

However, the study also found evidence that the amount of cold coal decreased over the same period. This suggests that there may be two different phases in the development of the coal – a rapid increase while reionization occurs, followed by an equalization.

The epoch of reionization, which took place when the universe was “only” a billion years old, was when the lights were turned back on after the cosmic dark ages after the Big Bang.

Before this, the universe was a dark, dense fog of gas. But when the first massive stars formed, their light began to shine through space and reionize the cosmos. This light may have led to rapid heating of the surrounding gas, causing the increase in hot carbon observed in this study.

Studies of reionization are critical to understanding when and how first stars was formed and began to produce the basic elements that exist today. But the measurements have been notoriously difficult.

“The research led by Dr Davies built on an exceptional sample of data obtained during 250 hours of observations on the Very Large Telescope (VLT) at the European Southern Observatory in Chile,” said Dr Valentina D’Odorico of the Italian Institute of Astrophysics, principal investigator of the observational program . “This is the largest amount of observing time allocated to a single project performed with the X-shooter spectrograph.

“Thanks to the 8m VLT, we were able to observe some of the most distant quasars, which act like flashlights, illuminating galaxies along the path from the early universe to Earth.”

As quasar light passes through galaxies on its 13-billion-year journey through the universe, some photons are absorbed, creating distinct barcode-like patterns in the light, which can be analyzed to determine the chemical composition and temperature of the gas in the galaxies.

This provides a historical picture of the evolution of the universe.

“These ‘barcodes’ are picked up by detectors at the VLT’s X-Shooter spectrograph,” explains Dr Davies. “This instrument splits the galaxy light into different wavelengths, like putting light through a prism, which allows us to read the barcodes and measure the properties of each galaxy.”

The study led by Dr Davies captured more barcodes from ancient times galaxies than ever before.

“We increased from 12 to 42 the number quasars for which we had high-quality data, which finally enabled a detailed and precise measurement of the evolution of carbon density, says Dr. D’Odorico.

This major advance was made possible by the ESO VLT, one of the most advanced telescopes on Earth, and a strategic partner of Australia.

“The study provides an older data set that will not be significantly improved until 30 m class telescopes come online towards the end of this decade,” said Professor Emma Ryan-Weber, Principal Investigator at the ARC Center of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and other authors of the study. “High-quality data from even earlier in the universe will require access to telescopes such as the Extremely Large Telescope (ELT) now under construction in Chile.”

Astronomers use many different types of data to build a history of the universe.

“Our results are consistent with recent studies showing that the amount of neutral hydrogen in intergalactic space is rapidly decreasing around the same time,” said Dr Davies.

“This research also paves the way for future investigations with the Square Kilometer Array (SKA), which aims to directly detect emissions from neutral hydrogen during this key phase of the universe’s history.”

Professor Ryan-Weber says the research goes to the heart of ASTRO 3D’s mission to understand the evolution of elements, from the Big Bang to today: “It addresses this key goal: How did the building blocks of life – in this case carbon – spread across the universe ?

“As humans, we strive to understand ‘Where did we come from?’ It is incredible to think that the barcode of these 13 billion year old carbon atoms was imprinted on photons at a time when […] Earth didn’t even exist. These photons traveled through the universe, into the VLT, and were then used to develop a picture of the evolution of universe.”