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.

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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: (2010). "Supermassive black-hole growth over cosmic time: Active galaxy demography, physics, and ecology from Chandra surveys". Proceedings of the National Academy of Sciences 107 (16): 7184–7189. ISSN 0027-8424. DOI:10.1073/pnas.0914151107.

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: (2018). "On the selection of high-z quasars using LOFAR observations". Frontiers in Astronomy and Space Sciences 5. DOI:10.3389/fspas.2018.00005.

The continuum spectrum of a quasar can often be described, over a broad frequency range, by a power law of the form $S$ $S$ _$\nu$ $\propto$ $\propto$ $\nu$ $\nu$ ^{– $\alpha$} ... where $\alpha$ $\alpha$ is the spectral index. $\alpha$ $\alpha$ = 0 corresponds to a flat spectrum, whereas $\alpha$ $\alpha$ = 1 describes a spectrum in which the same energy is emitted in every logarithmic frequency interval.
- Peter Schneider: Extragalactic Astronomy and Cosmology: An Introduction. Springer Science & Business Media. 9 October 2006. p. 178. ISBN 978-3-540-33174-2.

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: Inward Bound: Discovering and Exploring the Milky Way's Black Hole - Scott Tremaine, Institute for Advanced Study. YouTube (6 May 2019). (quote at 11:28 of 1:04:11)