The age of the quasars

摘要

Ask an astronomer when quasars were at their peak and they will probably tell you it was about 10 billion years ago, when the Universe was about one-third of its current size. Before then, the quasar population was still growing along with other large structures in the young Universe; there has since been a steady decrease in quasar numbers. However, in a paper published in The Astrophysical Journal, Vardanyan et al. present results suggesting that this widely accepted picture may not be correct - or at least that it does not tell the whole story. That story started in 1963 with the discovery of a new type of astronomical object, referred to variously as quasi-stellar objects or quasars, the name that is generically used today. Their physical nature was initially unknown, but it was gradually deduced that a quasar is a glowing disk of hot, dense material that can form around the supermassive black hole at the centre of a large galaxy, often the result of a collision with a second galaxy. Although such accretion disks are 'only' about the size of the Solar System, they can outshine all the stars in the host galaxy by a factor of a thousand or so. Quasars can hence be seen comparatively easily at great distances, which makes it possible to trace their evolution back to the first billion years after the Big Bang.