Fermi Detects Cosmic Fog


Even space-based telescopes have problems with fog. An ultra-thin mist of photons fills the universe and creates what’s called the extragalactic background light, or EBL. The EBL spans the far-infrared (redder than visible light) to the ultraviolet (bluer than visible light). The question is, where does the light come from?

Fermi observations in context
The Fermi Gamma-ray Space Telescope detected drop-offs (purple curves) in the number of high-energy gamma rays received from distant sources. These missing photons allowed scientists to measure how thick the cosmic fog is. Click for a larger view.
Astronomers’ most recent attempt to answer that question uses powerful, high-energy cosmic headlights, whose light is dimmed by interactions with the EBL fog. An international team working with the Fermi Gamma-ray Space Telescope has used these headlights to detect the EBL and determine where it comes from. The team’s results, published online in Science, suggest that the ultraviolet portion of the EBL that suffused the cosmos when the universe was about 4 billion years old came almost entirely from galaxies.

The team looked at gamma-ray emission from 150 blazars, galaxies in which the central supermassive black hole belches a powerful jet straight at us. (Blazars are a subset of objects called active galactic nuclei, the catch-all term for galactic cores in which a leviathan gobbles gas and produces blisteringly bright radiation as it feeds.) The blazars Fermi observed sit at a variety of cosmic times, from redshifts of 0.03 (basically yesterday) to 1.6 (about 9 billion years ago). As the distance between us and a blazar increases, more of its high-energy gamma rays collide with the cosmic fog. That means that, for farther blazars, there’s a more noticeable drop-off in the gamma-ray emission.

The Fermi team used this drop-off to estimate how thick the EBL fog is. In particular, the scientists estimated an upper limit for the intensity of the fog’s ultraviolet component, which they conclude is about 3 nanowatts per square meter per steradian. Don’t worry about the units; what’s important is that this number is very close to the lower limit given by Hubble Space Telescope observations of galaxies, 2.9-3.9 nW m-2sr-1. The convergence implies that more or less all of the ultraviolet EBL at this cosmic epoch comes from galaxies, leaving little room for other sources. (By little, I mean maybe 10-20% more, max.)

One of these “other sources” is the universe’s first stars. These stars, monstrously large balls of gas probably a couple hundred times the Sun’s mass, emitted ultraviolet radiation that helped ionize the neutral hydrogen pervading the early universe. That epoch, called the reionization era, happened (as a very rough estimate) less than one billion years after the Big Bang and forever changed the cosmic landscape.

Because the reionization era occurred at least 3 billion years prior to the oldest sources Fermi observed, we wouldn’t expect to see light from the first stars in the EBL that Fermi detected, explains Felix Aharonian (Dublin Institute for Advances Studies, Ireland, and the Max Planck Institute for Nuclear Physics, Germany). That’s because the universe’s expansion had already stretched the stars’ light to near-infrared and optical wavelengths by the time the blazars pumped out their emission.

Study coauthor Anita Reimer (Leopold-Franzens-University Innsbruck, Austria) says that the Fermi null result therefore supports the reionization epoch’s early date. To explain the correlation between the Fermi and Hubble results, the team concludes that the peak of first-star formation happened less than 500 million years after the Big Bang (in astronomers’ parlance, before redshift 10).

While not directly related, it's worth noting that a separate study recently pointed out a potentially overlooked source of the near-infrared fog that could mask signals from the first stars. So for now, it looks like the first stars are staying hidden.

Below, you'll find an animation of the gamma-ray photons' journey from blazar to Earth, and how they're destroyed when they collide with an EBL photon, creating an electron and positron in the process.

Reference: M. Ackermann et al. “The Imprint of the Extragalactic Background Light in the Gamma-Ray Spectra of Blazars.” Science, 1 November 2012.

One thought on “Fermi Detects Cosmic Fog

  1. svuille

    EBL light pollution. Interesting how the effect is being quantified. Scientists findings and interpretation make anticipation of the James Webb even greater.

    Great article and writing Camille! Thank you.