Quasar

An artist’s impression showing the surroundings of a supermassive black hole, typical of that found at the heart of many galaxies.

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: Template:Cite journal

• 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: Template:Cite journal

• 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: Template:Cite book

• 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: Template:Cite web (quote at 11:28 of 1:04:11)

Also see

• Cosmology
• General relativity

External links

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