An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.
Seyfert galaxies are one of the two largest groups of active galaxies, along with quasar host galaxies. They have quasar-like nuclei with very high surface brightnesses whose spectra reveal strong, high-ionisation emission lines, but unlike quasars, their host galaxies are clearly detectable.
The Eddington luminosity, also referred to as the Eddington limit, is the maximum luminosity a body can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward. The state of balance is called hydrostatic equilibrium. When a star exceeds the Eddington luminosity, it will initiate a very intense radiation-driven stellar wind from its outer layers. Since most massive stars have luminosities far below the Eddington luminosity, their winds are driven mostly by the less intense line absorption. The Eddington limit is invoked to explain the observed luminosities of accreting black holes such as quasars.
NGC 4395 is a nearby low surface brightness spiral galaxy located about 14 million light-years from Earth in the constellation Canes Venatici. The nucleus of NGC 4395 is active and the galaxy is classified as a Seyfert Type I known for its very low-mass supermassive black hole.
In astronomy and astrophysics, an ultraluminous X-ray source (ULX) is less luminous than an active galactic nucleus but more consistently luminous than any known stellar process (over 1039 erg/s, or 1032 watts), assuming that it radiates isotropically (the same in all directions). Typically there is about one ULX per galaxy in galaxies which host them, but some galaxies contain many. The Milky Way has not been shown to contain an ULX, although SS 433 is a candidate. The main interest in ULXs stems from their luminosity exceeding the Eddington luminosity of neutron stars and even stellar black holes. It is not known what powers ULXs; models include beamed emission of stellar mass objects, accreting intermediate-mass black holes, and super-Eddington emission.
Andrew Robert King, is a British astrophysicist and Professor of Astrophysics in the Department of Physics and Astronomy at the University of Leicester. His previous institutions include University College London and the Institute for Theoretical Physics at the University of Hamburg and a visiting position at the Observatoire de Paris. He currently holds visiting positions at the Astronomical Institute of the University of Amsterdam, and he is a visiting professor at Leiden University. He has served as Editor and now is Deputy Editor-in-Chief of the international astronomy journal Monthly Notices of the Royal Astronomical Society.
NGC 5643 is an intermediate spiral galaxy in the constellation Lupus. Based on the tip of the red-giant branch distance indicator, it is located at a distance of about 40 million light-years. NGC 5643 has an active galactic nucleus and is a type II Seyfert galaxy.
NGC 7213 is a lenticular galaxy located in the constellation Grus. It is located at a distance of circa 70 million light-years from Earth, which, given its apparent dimensions, means that NGC 7213 is about 75,000 light-years across. It was discovered by John Herschel on September 30, 1834. It is an active galaxy with characteristics between a type I Seyfert galaxy and LINER.
NGC 4278 is an elliptical galaxy located in the constellation Coma Berenices. It is located at a distance of circa 55 million light years from Earth, which, given its apparent dimensions, means that NGC 4278 is about 65,000 light years across. It was discovered by William Herschel on March 13, 1785. NGC 4278 is part of the Herschel 400 Catalogue and can be found about one and 3/4 of a degree northwest of Gamma Comae Berenices even with a small telescope.
NGC 4074 is a peculiar lenticular galaxy located 310 million light-years away in the constellation Coma Berenices. It was discovered by astronomer William Herschel on April 27, 1785 and is a member of the NGC 4065 Group.
NGC 7172 is a spiral galaxy located in the constellation Piscis Austrinus. It is located at a distance of about 110 million light years from Earth, which, given its apparent dimensions, means that NGC 7172 is about 100,000 light years across. It was discovered by John Herschel on September 23, 1834.
NGC 5506 is a spiral galaxy located in the constellation Virgo. It is located at a distance of about 75 million light years from Earth, which, given its apparent dimensions, means that NGC 5506 is about 80,000 light years across. It was discovered by William Herschel on April 15, 1787. It is a Seyfert galaxy.
PG 1543+489, also known as QSO B1544+4855 and PGC 2325245, is a quasar located in the constellation of Boötes. At the redshift of 0.399, the object is located 4.5 billion light-years away from Earth. It was first discovered in 1983, by researchers who presented 114 objects in the Palomar-Green bright quasar survey, as one of the best studied samples of active galactic nuclei (AGN).
NGC 6051 is a giant elliptical galaxy located in the constellation of Serpens. The galaxy lies 453 million light-years from Earth, which means given its apparent dimensions, the galaxy is around 250,000 light-years across. It is the brightest cluster galaxy inside a relaxed poor cluster called AWM 4, a fossil system, making up of at least 30 galaxy members.
Markarian 463 known as UGC 8850, is a galaxy merger located in the constellation Boötes. It is located 706 million light years from Earth. It is classified a double nucleus Seyfert galaxy.
ESO 148-2 known as ESO 148-IG002 and IRAS 23128-5919, is a galaxy merger located in the constellation of Tucana. It is located 642 million light years from Earth and is classified a Wolf-Rayet galaxy as well as an ultraluminous infrared galaxy.
J1000+1242 known as SDSS J1000+1242 or J1000+12 is a radio-quiet type-2 quasar, located in the constellation Leo. It is located 2 billion light years from Earth and is classified as a Seyfert galaxy.
IRAS 01003-2238 also known as IRAS F01004-2237 or simply F01004-2237, is a galaxy located in the constellation of Cetus. It is located 1.65 billion light years away from Earth and is a Seyfert galaxy and an ultraluminous infrared galaxy. IRAS 01003-2238 is also classified as a Wolf-Rayet galaxy, making the object one of the most distant known.
1H 0323+342 known as 2MASX J032441.19+341045.9, is a galaxy located in the constellation of Perseus. It is located 831 million light years from Earth. It is classified a gamma-ray emitting narrow-line Seyfert galaxy, the nearest known example of this subtype.
RX J1633.3+4718 known as RXS J16333+4718 according to VLBI Network observations, is a narrow-line Seyfert 1 galaxy, located in the constellation of Hercules. It has a redshift of (z) 0.116 and is located 1.75 billion light years from Earth. The first known reference to this galaxy comes from a radio source which was identified in 1995 in the IRAS catalogue as F16319+4725.
