Researchers from the Fermi Gamma-ray Space Telescope Collaboration, including scientists at the U.S. Naval Research Laboratory (NRL), used Fermi to measure the rate of star formation through interactions of gamma rays with extragalactic background light, gaining insight into the star formation history of the universe. Research findings were published in the 29 November 2018 issue of Science.
The universe is 13.8 billion years old, with stars being born, living, and dying for most of that time. The stars create a relic light that fills the universe, known as the extragalactic background light, or EBL. The EBL is made up of light from all the stars that have ever lived in the observable universe, even long after they have ceased to shine.
In 2010, Dr. Justin Finke of NRL's High Energy Space Environment Branch was part of a team of NRL researchers that created a model for EBL. This model and a similar model created by collaborator Dr. Kari Helgason of the Max Planck Institute for Astrophysics (now at the University of Iceland) were used to convert the EBL measurement into a measurement of star formation history. The results using both models are similar, increasing confidence in the measurement.
Using these results, the scientists were able to measure how many stars were formed over most of cosmic history.
The key to measuring EBL is through "blazars," according to Finke.
"Blazars are super-massive black holes that generate gamma rays," Finke said. "These gamma rays interact with background light from stars that converts the gamma rays into pairs of particles."
Gamma rays are the most energetic form of light. Each individual gamma-ray photon, or particle of light, seen by Fermi has energy billions of times more than the photons seen with the human eye.
"By determining how many gamma rays are missing from our observations and how many get transformed by interacting with starlight, we can measure how much of the starlight there is," Finke said.
This process was done for a very large blazar sample, spread out over many different distances. Fermi's Large Area Telescope is especially suited to measuring the blazars' gamma rays and interactions with EBL.
"There is nothing else that can measure gamma rays like Fermi can in this energy range," Finke said of the telescope's capabilities.
This was the first measurement of star formation history with gamma rays over such a large range of cosmic time. The recent measurement also was able to constrain the formation of the first stars in the universe, opening the door for future research projects, according to Finke.
"The first stars formed in the very early universe were thought to re-ionize the universe, making the hydrogen in the universe go from mostly neutral to being mostly ionized," Finke said. "Understanding that process is a major goal of the upcoming mission for NASA's James Webb Space Telescope."
The study shows that mapping the history of the stars helps space scientists from NRL and other institutions learn more about the universe's past, while providing valuable information for space exploration in the future.
NRL's Space Science Division was one of the principal developers of the Fermi Large Area Telescope instrument. Fermi is supported by NASA and the Department of Energy, with important contributions from institutions in France, Germany, Italy, Japan, and Sweden. The current research was led by Marco Ajello, Vadehi Paliya, and Abeshik Desai at Clemson University; Alberto Dominguez at the Universidad Complutense de Madrid in Spain; Kari Helgason at the University of Iceland; and Finke at NRL.