  The active galaxy Markarian 501 was discovered with air-Cerenkov telescopes
at photon energies of 10 tera-electron volts. Such high energies may indicate
that the gamma rays from Markarian 501 are due to the acceleration of protons
rather than electrons. Furthermore, the observed absence of gamma ray
attenuation due to electron-positron pair production in collisions with cosmic
infrared photons implies a limit of 2 to 4 nanowatt per squaremeter per
steradian for the energy flux of an extragalactic infrared radiation background
at a wavelength of 25 micrometers. This limit provides important clues on the
epoch of galaxy formation.
