Warm Carbon Increased Suddenly in the Early Universe. Made by the First Stars?

According to the most widely-accepted model of cosmology, the Universe began roughly 13.8 billion years ago with the Big Bang. As the Universe cooled, the fundamental laws of physics (the electroweak force, the strong nuclear force, and gravity) and the first hydrogen atoms formed. By 370,000 years after the Big Bang, the Universe was permeated by neutral hydrogen and very few photons (the Cosmic Dark Ages). During the “Epoch of Reionization” that followed, the first stars and galaxies formed, reoinizing the neutral hydrogen and causing the Universe to become transparent.

For astronomers, the Epoch of Reionization still holds many mysteries, like when certain heavy elements formed. This includes the element carbon, a key ingredient in the formation of planets, an important element in organic processes, and the basis for life as we know it. According to a new study by the ARC Center of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), it appears that triply-ionized carbon (C iv) existed far sooner than previously thought. Their findings could have drastic implications for our understanding of cosmic evolution.

ASTRO 3D is a collaborative effort led by Australia National University (ANU), comprising six Australian universities and many international partners. They were joined by researchers from the Astronomical Observatory of Trieste, the Institute for Fundamental Physics of the Universe (IFPU), the Max Planck Institute for Astronomy (MPIA), the MIT Kavli Institute for Astrophysics and Space Research, the Kavli Institute for Cosmology, the Leibniz Institute for Astrophysics Potsdam (AIP), the Gemini Observatory, and the Scuola Normale Superiore. The paper that described their research recently appeared in the Monthly Notices of the Royal Astronomical Society,

A 13.8-billion-year cosmic timeline showing the eras observed by the Planck satellite, HERA, and NASA’s James Webb Space Telescope. Credit: HERA

During the early Universe, young stars fused hydrogen and helium in their interiors to create heavier elements (like metals). As these stars collapsed and went supernova, these elements were dispersed throughout the cosmos and became part of the warm gas and dust clouds surrounding galaxies (aka. galactic halos). When observing the cosmos, astronomers use the C iv in these clouds (“warm carbon”) to trace the metal-rich content of these halos to better understand how galaxies evolved.

In previous studies, astronomers observed that the density of carbon heated by galactic radiation (“warm carbon”) decreased slowly between roughly 1.3 and 4 billion years after the Big Bang, then began declining rapidly. However, the cause of this sudden downturn has never been understood. For their study, the team relied on 260 absorber samples from 42 spectra measurements obtained using the XShooter intermediate-resolution spectrograph on the ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile.

Most of these measurements were part of the XQR-30 legacy survey, a campaign that observed 30 high-redshift quasars roughly 13 billion light-years away. As the light from these quasars traveled 13 billion years to reach us, it passed through the halos surrounding intervening galaxies. Some of this light is absorbed in the process, producing spectra that reveal things about the halos’ temperature and chemical composition. This allows astronomers to track the historical development of the Universe.

These measurements allowed the team to measure the density of carbon in the gases surrounding ancient galaxies that existed about 1 billion years after the Big Bang. From this, they found that the amount of “warm carbon” suddenly increased by a factor of five over just 300 million years. One possibility, they suggest, is that the initial increase around galaxies is simply because there was more in the early Universe.

The number density of C iv absorbers showing the abundance of “warm carbon” revealed by the XQR-30 survey. Credit: ASTRO3D/Davies et al. (2023)

Dr. Rebecca Davies, an ASTRO 3D Postdoctoral Research Associate at the Swinburne University of Technology, was the paper’s lead author. As she said in a recent ASTRO 3D press release:

“We found that the fraction of carbon in warm gas increased rapidly about 13 billion years ago, which may be linked to large-scale heating of gas associated with the phenomenon known as the ‘Epoch of Reionization.’ That’s what we’ve done here. And so, we present two potential interpretations of this rapid evolution. During the period when the first stars and galaxies are forming, a lot of heavy elements are forming because we never had carbon before we had stars.”

As astronomers have understood for some time, the first stars in our Universe were composed of only hydrogen and helium because heavier elements did not exist until after the first generation of stars (Population III) went supernova. Subsequent generations (Population I and II) formed from gas clouds containing these elements, leading to new stars with greater levels of “metallicity,” which astronomers use to measure the age of stars. Based on their results, Davies and her team considered that the same light used to characterize the galactic halos also caused rapid heating, leading to the observed increase in

However, Davies and her team also found that the amount of “cool carbon” decreased over the same period. This suggests that carbon experienced two phases of evolution, including a rapid rise during the Reionization Epoch, followed by a leveling off. These findings could have significant implications for the study of reionization, which is vital to understanding how and when the first stars produced the elements from which the planets and all life is composed. Said Professor Ryan-Weber, Chief Investigator of ASTRO 3D and second author of the study, this research goes to the heart of the mission:

“It addresses this key goal: how did the building blocks of life – in this case carbon – proliferate across the Universe? As humans we strive to understand ‘where did we come from?’ It’s incredible to think that the barcode of those 13-billion-year-old carbon atoms were imprinted on photons at a time when the Earth didn’t even exist. Those photons travelled across the Universe, into the VLT, and then were used to develop a picture of the evolution of the Universe.”

Artist’s impression of GNz7q, a galaxy Hubble observed roughly 13 billion light-years away. Credit: NASA/ESA

This study also increased the number of quasars for which high-quality data exists from 12 to 42, finally allowing for a detailed and accurate measurement of carbon density. It also demonstrated the effectiveness of the Paranal Observatory’s telescopes and their advanced suite of spectrographs. But perhaps most interesting is the way these findings anticipate what astronomers will see when next-generation telescopes begin probing the early Universe to determine when and how all its building blocks emerged.

“The study provides a legacy data set which will not be significantly improved until 30m-class telescopes come online towards the end of this decade,” said Prof. Ryan-Weber. “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.”

“Our results are consistent with recent studies showing that the amount of neutral hydrogen in intergalactic space decreases rapidly around the same time,” added Davies. “This research also paves the way for future investigations with the Square Kilometre Array (SKA), which aims to directly detect emission from neutral hydrogen during this key phase of the Universe’s history.”

Further Reading: Astro3D, MNRAS