1 erg = 1 photon! Must be bloody hard gamma rays!
Actually, equating phota with any specific amount of energy is rather like specifying one’s length standard as “one piece of string”. The energy of phota is dependent upon their frequency.
Photon energies are often measured in electron-volts (eV), which, as the Teeming Thousands will know, is the amount of energy needed to move one electron through a potential difference of one volt. Harder phota, such as short UV, X-rays or gamma rays, may have their energies quoted in terms of thousands of electron-volts (keV) or millions of electron-volts (MeV).
One electron-volt, according to my handy table of conversions, is 1.60207x10[sup]-12[/sup] erg. A photon with an energy would be…well, I won’t that say such a photon has never existed, but you won’t find them in sunlight.
Incidentally, an electron has a mass, IIRC, of about 511 keV; a proton or neutron of about 938 MeV (the neutron is slightly more massive). Two phota of energy one erg colliding would undoubtedly create a whole spray of particles (and antiparticles).
“Kings die, and leave their crowns to their sons. Shmuel HaKatan took all the treasures in the world, and went away.”