Quasar

A quasar (also called quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of stellar masses (denoted M☉) is surrounded by a gaseous accretion disk. Active galactic nuclei are the most luminous persistent sources of electromagnetic radiation in the universe.

Quotes

 * We seem to live in a remarkably economical X-ray universe, in that the observed cosmic X-ray background (CXRB) is produced with almost the least cosmic effort possible. It is not dominated by luminous obscured quasars thundering out huge amounts of power at z ≈ 2–4 but rather by moderate-luminosity, obscured AGNs at z ≈ 0.5–2.
 * William N. Brandt and David M. Alexander:


 * LOFAR is a new European radio interferometer operating at frequencies 15–240 MHz (van Haarlem et al., 2013) and represents a milestone in terms of radio survey speed compared to existing telescopes. The LOFAR Surveys Key Science Project aims to carry out a tiered survey. ... These surveys will open the low-frequency electromagnetic spectrum for exploration, allowing unprecedented studies of the radio population across cosmic time and opening up new parameter space for searches for rare, unusual objects such as high-z radio quasars in a systematic way. Perhaps, one of the most tantalizing prospects are the 21 cm absorption line measurements using LOFAR along sight lines toward z > 6 radio quasars.
 * Edwin Retana-Montenegro and Huub Röttgering:


 * The continuum spectrum of a quasar can often be described, over a broad frequency range, by a power law of the form $$S$$$\nu$ $$\propto $$ $$\nu$$–$\alpha$ ... where $$\alpha$$ is the spectral index. $$\alpha$$ = 0 corresponds to a flat spectrum, whereas $$\alpha$$ = 1 describes a spectrum in which the same energy is emitted in every logarithmic frequency interval.
 * Peter Schneider:


 * Quasars were several hundred times more numerous when the universe was much younger. They were most numerous when the universe was about twenty percent of its current age, a time in the history of the universe sometimes called "cosmic noon".
 * Scott Tremaine: (quote at 11:28 of 1:04:11)

Also see

 * Cosmology
 * General relativity