Detection of the Cosmic Gamma Ray Horizon

Fri, 05/24/2013 - 10:38am
University of California High-Performance AstroComputing Center


This figure illustrates how energetic gamma rays (dashed lines) from a distant blazar strike photons of extragalactic background light (wavy lines) and produce pairs of electrons and positrons. The energetic gamma rays that are not attenuated by this process strike the upper atmosphere, producing a cascade of charged particles which make a cone of Čerenkov light that is detected by the array of imaging atmospheric Čerenkov telescopes on the ground. Credit: Nina McCurdy and Joel R. Primack/UC-HiPACC; Blazar: Frame from a conceptual animation of 3C 120 created by Wolfgang Steffen/UNAMHow much light has been emitted by all galaxies since the cosmos began? After all, almost every photon (particle of light) from ultraviolet to far infrared wavelengths ever radiated by all galaxies that ever existed throughout cosmic history is still speeding through the Universe today. If we could carefully measure the number and energy (wavelength) of all those photons—not only at the present time, but also back in time—we might learn important secrets about the nature and evolution of the Universe, including how similar or different ancient galaxies were compared to the galaxies we see today.






Now, the big news—announced in today's Astrophysical Journal paper—is that the evolution of the EBL over the past 5 billion years has been measured for the first time. That's because looking farther out into the Universe corresponds to looking back in time. Thus, the gamma ray attenuation spectrum from farther distant blazars reveals how the EBL looked at earlier eras.




Comparing the calculations of the unattenuated gamma rays to actual measurements of the attenuation of gamma rays and X-rays from blazars at different distances allowed Dominquez et al. to quantify the evolution of the EBL—that is, to measure how the EBL changed over time as the Universe aged—out to about 5 billion years ago (corresponding to a redshift of about z = 0.5). "Five billion years ago is the maximum distance we are able to probe with our current technology," Domínguez said.

"Sure, there are blazars farther away, but we are not able to detect them because the high-energy gamma rays they are emitting are too attenuated by EBL when they get to us—so weakened that our instruments are not sensitive enough to detect them." This measurement is the first statistically significant detection of the so-called "Cosmic Gamma Ray Horizon" as a function of gamma-ray energy. The Cosmic Gamma Ray Horizon is defined as the distance at which roughly one-third (or, more precisely, 1/e – that is, 1/2.718 – where e is the base of the natural logarithms) of the gamma rays of a particular energy have been attenuated.


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