the continuous spectrum. They exhibit Doppler broadening corresponding to mean speed of several percent of the speed of light. Fast motions strongly indicate a large mass. Emission lines of hydrogen (mainly of the Lyman series and Balmer series), helium, carbon, magnesium, iron and oxygen are the brightest lines. The atoms emitting these lines range from neutral to highly ionized, leaving it highly charged. This wide range of ionization shows that the gas is highly irradiated by the quasar, not merely hot, and not by stars, which cannot produce such a wide range of ionization. Like all (unobscured) active galaxies, quasars can be strong X-ray sources. Radio-loud quasars can also produce X-rays and gamma rays by inverse Compton scattering of lower-energy photons by the radio-emitting electrons in the jet. Iron quasars show strong emission lines resulting from low-ionization iron (Fe ), such as IRAS 18508-7815. Spectral lines, reionization, and the early universe Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest known quasars (z = 6) display a Gunn–Peterson trough and have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas. More recent quasars show no absorption region, but rather their spectra contain a spiky area known as the Lyman-alpha forest; this indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds. The intense production of ionizing ultraviolet radiation is also significant, as it would provide a mechanism for reionization to occur as galaxies form. Despite this, current theories suggest that quasars were not the primary source of reionization; the primary causes of reionization were probably the earliest generations of stars, known as Population III stars (possibly 70%), and dwarf galaxies (very early small high-energy galaxies) (possibly 30%). Quasars show evidence of elements heavier than helium, indicating that galaxies underwent a massive phase of star formation, creating population III stars between the time of the Big Bang and the first observed quasars. Light from these stars may have been observed in 2005 using NASA's Spitzer Space Telescope, although this observation remains to be confirmed. Quasar subtypes The taxonomy of quasars includes various subtypes representing subsets of the quasar population having distinct properties. Radio-loud quasars are quasars with powerful jets that are strong sources of radio-wavelength emission. These make up about 10% of the overall quasar population. Radio-quiet