Tracing 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 universe’s history.

“We found that the proportion of carbon in hot gas increased rapidly around 13 billion years ago, which may be associated with large-scale heating of gas associated with the phenomenon known as the epoch of deionization,” says Dr. Rebecca Davies, ASTRO 3D Postdoctoral Research Associate at Swinburne University of Technology, Australia and lead author of the paper describing the discovery.

The study shows that the amount of hot carbon suddenly increased by a factor of five over a period of just 300 million years – a blip on astronomical timescales.

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

“That is what we have done here. And so we present two potential interpretations of this rapid development,” 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 form, a lot of heavy elements are formed because we never had carbon before we had stars,” says Dr. Davies. “And so one possible reason for this rapid increase is just that we’re seeing the products of the first generations of stars.”

However, the study also found evidence that the amount of cool carbon decreased during the same period. This suggests that there may be two distinct phases in the evolution of the carbon—a rapid rise while reionization takes place, followed by a flattening.

The reionization epoch, which occurred when the universe was “only” a billion years old, was when the lights came back after the cosmic dark ages after the Big Bang.

Before this, the universe was a dark, dense nebula 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 crucial to understanding when and how the first stars formed and began producing the elements that exist today. But measurements have been notoriously difficult.

“The research led by Dr. Davies was built on a unique sample of data obtained during 250 hours of observations on the Very Large Telescope (VLT) at the European Southern Observatory in Chile,” says Dr. Valentina D’Odorico of the Italian Institute of Astrophysics, the principal investigator of the observing 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 as 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 characteristic barcode-like patterns in the light that can be analyzed to determine the chemical composition and temperature of gas in the galaxies.

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

“These ‘barcodes’ are captured by detectors on the VLT’s X-Shooter spectrograph,” explains Dr. Davies. “This instrument splits the galaxy light into different wavelengths, like putting light through a prism, so we can read the barcodes and measure the properties of each galaxy.”

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

“We increased from 12 to 42 the number of quasars for which we had high-quality data, finally allowing a detailed and accurate measurement of the evolution of the 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 for Australia.

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

Astronomers use many different types of data to build a story about 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,” says Dr. Davies.

“This research also paves the way for future investigations with the Square Kilometer Array (SKA), which aim to directly detect emission 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 the present day: “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 the 13 billion year old carbon atoms was imprinted on photons at a time when (…) Earth did not even exist. These photons traveled across the universe, into the VLT, and then became used to develop a picture of the evolution of the universe.”

Provided by the ARC Center of Excellence for All Sky Astrophysics in 3D (ASTRO 3D)