Living on the Edge

Living on the Edge

During our mountaineering trips I am always the cautious one, carefully charting the safest routes to our destinations. Not so much my wife. Whenever we get to the edge of a chasm, she loves to dangle her legs over the cliff’s edge and gaze for several minutes at the vista.

There are times, however, when living on the edge is the safest place to be. When on a steep mountain face during a lightning storm, the best place to avoid being burnt by the current from lightning strikes is to get as far out on a ledge as possible. Such was the case during one of our hikes in the mountain range just west of Banff National Park (see figure 1).

Figure 1: Narrow Ledge on Mt. Tivoli above the Tivoli Glacier in Eastern British Columbia. During a lightning storm you would be much safer standing near the edge of this ledge, rather than next to the mountain wall. Photo credit: Hugh Ross

The latest example of the benefit of living on the edge appears in a research paper published by five Korean astronomers.1 The paper announced the discovery of the origin of ultrahigh-energy cosmic rays.

Ultrahigh-energy cosmic rays (UHECRs) are cosmic rays that are above the Greisen–Zatsepin-Kuzmin energy level of 5.7 x 1019 electron volts (eV).2 This level is the theoretical limit on the energy of a proton traveling from other galaxies more distant than 50 megaparsecs (163 million light-years) through the intergalactic medium to our galaxy. In the case of an iron nucleus, the energy limit is 2.8 x 1021 eV. The reason for the 50 megaparsecs distance limit is that interaction of UHECRs with the cosmic microwave background radiation photons limits their propagation distance.

The first observation of such a UHECR was by the physicist John Linsley in 1962.3 By 2000, over a dozen such cosmic rays had been observed.4 Since 2004, the Pierre Auger Observatory in Argentina (with a collecting area of 3,000 square km) and, since 2008, the Telescope Array in Utah (with a collecting area of 800 square km) have been detecting a UHECR about once every four weeks.5 The highest energy UHECR yet observed had kinetic energy equal to 3.2 x 1020 eV,6 roughly equal to that of a baseball moving at 100 km per hour (60 mph) and about 30 million times more energetic than the top particle energy achieved at CERN’s Large Hadron Collider.

Figure 2: One of the Solar-Powered Detectors of the Telescope Array. Image credit: John Matthews, University of Utah

For more than two decades, astronomers believed that UHECRs must originate from beyond our Milky Way Galaxy since they cannot be confined by our galaxy’s magnetic field, nor can they be accelerated within our galaxy. The first definitive proof that UHECRs must arrive from galaxies other than our own was a report in 2017 from the Pierre Auger Collaboration team that the UHECRs they were observing showed a slightly dipolar distribution.7

Now, the five Korean astronomers reported from their analysis of five years of observational data from the Telescope Array that UHECRs are arriving from a hot spot that is centered on the Virgo Cluster of galaxies (see figure 3). They also describe “a new finding that there are filaments of galaxies connected to the Virgo Cluster around the hotspot.”8 Specifically, they and other Korean astronomers found six filaments that have galaxies infalling (moving closer to an object due to gravity) toward the Virgo Cluster, thereby establishing that these filaments are dynamically connected to the cluster.9 The five Korean astronomers deduced from the hotspot and the filaments of galaxies that the UHECRs they detected are “produced at sources in the Virgo Cluster, and escape to and propagate along filaments, before they are scattered toward us.”10

Figure 3: The Central Core of the Virgo Cluster of Galaxies.
The Virgo Cluster is the nearest large cluster of galaxies. It contains about 2,000 galaxies. The black circles blot out foreground stars. Three supergiant galaxies are visible in this image: M87 to the lower left and M86 and M84 to the central right. Well below the bottom of this image would be M49, the largest galaxy in the Virgo Cluster. Image credit: ESO/Chris Mihos, Case Western Reserve University

The sources in the Virgo Cluster that are candidates for producing the observed UHECRs are (1) one or more powerful radio galaxies, (2) transient gamma-ray burst events, and (3) cluster-scale shock waves. The most probable source is the first, especially since the Virgo Cluster contains the exceptionally powerful radio supergiant galaxy Messier 87, or M87.

At 5.7 trillion times the mass of the Sun,11 M87 ranks as one of the most massive galaxies in the local universe (see figure 4). To give some idea of its size, M87 possesses over 12,000 known globular clusters compared to the 157 known globular clusters orbiting our Milky Way Galaxy. It also contains one of the most massive supermassive black holes known in the universe. The supermassive black hole in its nucleus has a mass equal to 7.22 billion times the Sun’s mass.12

Figure 4: M87, a Supergiant Galaxy near the Core of the Virgo Cluster.
Image credit: NASA/ESA/STScI(AURA)

Thanks to the enormous mass of M87’s supermassive black hole and all the matter that it is continually consuming, a powerful jet of matter and energy is being blasted out at relativistic velocities (velocities close to the speed of light) from M87’s core (see figure 5). This jet most likely explains the majority of the UHECR incidents on Earth.

Figure 5: Nucleus of the M87 Galaxy Showing the Relativistic Jet Blasting Out from the Vicinity of M87’s Supermassive Black Hole.
Image credit: NASA/Hubble Heritage Team (STScI/AURA)

M87 lies 54 million light-years away from us. We are living on the edge: Our Milky Way Galaxy is situated on the extreme outer edge of the Virgo Supercluster of galaxies. Because we are so far away from the Virgo Cluster core, and because our Milky Way Galaxy, as large as it is, has such a tiny supermassive black hole—only 4 million times the Sun’s mass—we humans are exposed to so few UHECRs that they have no measurable impact on our health or well-being.

This condition of extreme low exposure to UHECRs is rare in the universe. We are living in the only probable advanced-life-conceivable location in the universe where the exposure to UHECRs is so extraordinarily low. As real estate agents are quick to point out, the key factor in the desirability of a home is “location, location, location.” Thanks to the research efforts of the five Korean astronomers, we have yet more evidence that Somebody intervened to place us in the most desirable location within our vast universe.

Featured image: A Giant Elliptical Galaxy and a Giant Spiral Galaxy on the Eastern Side of the Virgo Galaxy Cluster. Featured image credit: NASA/ESA/Hubble Heritage Team (STScI/AURA)

  1. Jihyun Kim et al., “Filaments of Galaxies as a Clue to the Origin of Ultrahigh-Energy Cosmic Rays,” Science Advances 5 (January 2, 2019): eaau8227, doi:10.1126/sciadv.aau8227.
  2. Kenneth Greisen, “End to the Cosmic-Ray Spectrum?” Physical Review Letters 16 (April 25, 1966): 748–50, doi:10.1103/PhysRevLett.16.748; G. T. Zatsepin and V. A. Kuzmin, “Upper Limit of the Spectrum of Cosmic Rays,” Journal of Experimental and Theoretical Physics Letters 4 (August 1966): 78–80, English translation of the paper published in ZhETF Pis’ma 4, No. 3 (August 1, 1966): 114–117,
  3. John Linsley, “Evidence for a Primary Cosmic-Ray Particle with Energy 1020 eV,” Physical Review Letters 10 (February 15, 1963): 146–68, doi:10.1103/PhysRevLett.10.146.
  4. M. Nagano and A. A. Watson, “Observations and Implications of the Ultrahigh-Energy Cosmic Rays,” Reviews of Modern Physics 72 (July 1, 2000): 689–732, doi:10.1103/RevModPhys.72.689.
  5. Laura J. Watson, Daniel J. Mortlock, and Andrew H. Jaffe, “A Bayesian Analysis of the 27 Highest Energy Cosmic Rays Detected by the Pierre Augur Observatory,” Monthly Notices of the Royal Astronomical Society 418 (November 21, 2011): 206–13, doi:10.1111/j.1365-2966.2011.19476.x.
  6. D. J. Bird et al., “Detection of a Cosmic Ray with Measured Energy Well beyond the Expected Spectral Cutoff Due to Cosmic Microwave Radiation,” Astrophysical Journal 441 (March 1, 1995): 144–50, doi:10.1086/175344.
  7. The Pierre Auger Collaboration, A. Aab et al., “Observation of a Large-Scale Anistropy in the Arrival Directions of Cosmic Rays above 8 x 1018 eV,” Science 357 (September 22, 2017): 1266–70, doi:10.1126/science.aan4338; The Pierre Auger Collaboration, A. Aab et al., “Large-Scale Cosmic-Ray Anisotropies above 4 EeV Measured by the Pierre Auger Observatory,” Astrophysical Journal 868 (November 20, 2018): id. 4, doi:10.3847/1538-4357/aae689.
  8. Jihyun Kim et al., “Filaments of Galaxies,” page 1.
  9. Suk Kim et al., “Large Scale Filamentary Structures around the Virgo Cluster Revisited,” Astrophysical Journal 833 (December 20, 2016): id. 207, doi:10.3847/1538-4357/833/2/207.
  10. Jihyun Kim et al., “Filaments of Galaxies,” page 1.
  11. Jeremy D. Murphy, Karl Gebhardt, and Joshua J. Adams, “Galaxy Kinematics with VIRUS-P: The Dark Matter Halo of M87,” Astrophysical Journal 729 (March 10, 2011): id. 129, doi:10.1088/0004-637X/729/2/129.
  12. L. J. Oldham and M. W. Auger, “Galaxy Structure from Multiple Tracers—II. M87 from Parsec to Megaparsec Scales,” Monthly Notices of the Royal Astronomical Society 457 (January 20, 2016): 421–39, doi:10.1093/mnras/stv2982.