Quasars, the expanding Universe and large-scale structure

Most galaxies have a supermassive black hole (10⁶–10¹⁰ M☉) at the centre; when matter falls onto it (accretion) and enormous energy is released, the galaxy’s nucleus is called active. Radio galaxies are galaxies that radiate very strongly in the radio range: two jets of plasma hundreds of thousands of light-years long and radio “lobes” emerge from the nucleus (Cygnus A). A quasar (quasi-stellar object) is the very luminous, star-like nucleus of a distant galaxy; the nature of the first quasar, 3C 273, was recognised in 1963 by M. Schmidt. A quasar’s luminosity can reach ≈ 10⁴⁰ W (hundreds to thousands of times a normal galaxy) while its size is about that of the Solar System: because the brightness varies within days, the source must be only light-days across. The most distant quasars have redshifts above 7, from when the Universe was ≈ 700 million years old. Nuclear activity plays an important role in galaxy evolution.