Using new technologies to study and estimate extragalactic background light can lead to discoveries about galaxies and the evolution of the Universe.
The bulk of optical light in the Universe comes from stars. Stars also synthesize most of the elements found in galaxies, and the cosmic star formation history (SFH) summarizes the history of stellar formation, investigating the stellar birth rate as a function of the age of the Universe. The rate of star formation is most commonly estimated by measuring emission of light from massive stars, typically ultraviolet (UV) and infrared (IR) light. Estimates of SFH rely on the detection of many galaxies in deep surveys, but because not even the most powerful telescope can detect all the galaxies in a region of interest, estimating the amount of “missed” light is one of the largest sources of uncertainty. SFH measurements are uncertain to the extent that verification with multiple independent methods is ideal. Light from the stars that escapes galaxies is pretty much never destroyed but becomes a part of the extragalactic background light (EBL), which is the total light accumulated by all sources in the Universe across all time. An alternative method to measuring SFH is based on the attenuation produced by the EBL in the gamma-ray spectra of distant sources. Gamma rays at high energy annihilate when they collide with photons of the EBL, which produce electron-positron pairs. This reaction is effectively absorption, and the gamma-ray photon would never reach detectors closer to Earth. In the high energy range, the attenuation depends on the density of EBL photons along the line of sight. Observations of gamma-ray sources at different redshifts, a measure of distance, can be used to measure EBL density.
The EBL is a fundamental observational quantity in cosmology and the second most important radiation field in the universe, after only the cosmic microwave background (CMB). It consists of all the accumulated near-infrared, far-infrared, optical, and ultraviolet radiation in the universe produced by stars, dust, and active galactic nuclei (AGNs). All emissions from extragalactic sources and the light from any diffuse background, excluding the CMB, contribute to its intensity and spectral energy distribution (SED). Because the EBL is the repository of all thermal energy released by nuclear and gravitational processes since the epoch of recombination, its intensity and spectral shape hold important information about the formation and evolution of galaxies and their contents.
The EBL plays an important role in the propagation of high energy gamma-rays that are predominantly emitted by blazars, a subgroup of active galactic nuclei. AGNs include roughly one-fifth of supermassive black holes found at the center of non-dwarf galaxies. These are considered “active,” because they are in the process of accreting superheated matter into their accretion disk. Roughly one-fifth of AGNs also have relativistic jets, beams of ionised matter ejected perpendicular to the plane of the accretion disk, moving near the speed of light. AGNs are considered blazars when their relativistic jets are aligned with the line of sight of the observer (from Earth or Earth orbit).
Blazars can be used to probe the EBL because they are some of the brightest objects in the universe, visible from billions of light years away. High energy photons emitted by blazars are attenuated by photon-photon interactions with the EBL. Comparing the EBL-absorbed spectrum of a blazar with its intrinsic spectrum allows us to effectively probe the EBL. This process can be used to set limits on both the intrinsic spectra of blazars and the intensity of the EBL.
Estimates of the EBL with gamma-rays have been made since the late 1990s by ground-based instruments. The present generation of ground-based telescopes includes the High Energy Stereoscopic System (H.E.S.S.), the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC), and the Very Energetic Radiation Imaging Telescope Array System (VERITAS). They observe gamma-rays at energies higher than those observed by the Fermi Large Area Telescope (LAT) and are therefore more suited to probe the optical-to-infrared part of the EBL, complementing the Fermi-LAT measurement in the UV-optical band.
The longest gamma-ray observations have been made by the Fermi Gamma-ray Telescope, a satellite that makes gamma-ray observations from low-Earth orbit. The satellite has two instruments, the Large Area Telescope (LAT) and Gamma-ray Burst Telescope (GBT). While the former is more useful for all-sky surveys, the latter is used to study gamma-ray bursts.
When working with gamma-ray spectra, it’s important to perform a likelihood analysis to find the EBL attenuation experienced by a single blazar source. In a given view of the LAT, each incoming gamma-ray photon is attributed to a coordinate but not a source, because the sources are too far and a small error in angle would assign photons to the wrong source. To rectify this, the total flux of a region is recorded, and a likelihood analysis would attempt to fit the total flux by energy bin and attribute photons to sources. All sources in the region of interest are fitted simultaneously, which provides a good estimation of the contamination from nearby sources and retrieves the real spectrum of the target source.
Studying the EBL is important for almost all cosmological studies of data. EBL influences measurements, and a comprehensive model for EBL would be helpful to correct for these influences. Currently, EBL is assumed to be isotropic, or uniform everywhere. It is generally accepted but unproven, that there are anisotropies, or variations. A broad EBL density map would allow for the correction of absorption due to the EBL. The gamma-ray spectra at different cosmological distances measured from satellite and ground-based telescopes offer the opportunity to search beyond the standard model of particle physics. Two effects of interest are the possible oscillation of gamma-rays into exotic particles called axions (dark matter particle candidates) and a dependence of photon propagation on its energy, an effect called the Lorentz invariance violation. Both effects would give rise to anomalies in the spectra once corrected for the EBL absorption. These studies will be made possible by measurements of the optical and infrared light from the deep Universe, providing an independent, more direct estimate of the EBL and its evolution over cosmic time.