Quasar

thumb|An artist’s impression showing the surroundings of a supermassive black hole, typical of that found at the heart of many galaxies. thumb|A growing black hole, called a quasar, can be seen at the center of a faraway galaxy in this artist's concept. 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: 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 2010 7184–7189 0027-8424 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: On the selection of high-z quasars using LOFAR observations Frontiers in Astronomy and Space Sciences 5 2018 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 – ... where is the spectral index. = 0 corresponds to a flat spectrum, whereas = 1 describes a spectrum in which the same energy is emitted in every logarithmic frequency interval. Peter Schneider: Extragalactic Astronomy and Cosmology: An Introduction https://books.google.com/books?id=uP1Hz-6sHaMC&pg=PA178 9 October 2006 178 Springer Science & Business Media 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 6 May 2019 YouTube https://www.youtube.com/watch?v=fnC1hFpjKmw (quote at 11:28 of 1:04:11) Also see Cosmology General relativity External links Category:Astronomy Category:Cosmology