Friday, 12 July 2013

Active Galaxies- Of Quasars and Kin

Something weird is going on in the centres of many galaxies. Often, intense of aggregates of 'blue' light with characteristics distinct from the radiation associated with stars or gas are produced. Galaxies possesing these centers are 'active galaxies' and their central sources are active galactic nuclei (AGN). The optical spectra of a typical galaxy is a composite of contributions from H II regions and stars; elliptical galaxies mirror the spectrum of a star while spiral galaxies are akin to both a star and H II region (partially ionised gas clouds). The optical spectrum of an active galaxy is a combination of the spectra of a typical galaxy and extra radiation that features strong emission lines. Since the common denominator of all active galaxies is an AGN, there are many such types of active galaxy; namely Seyferts, which are spiral galaxies containing very bright point nuclei which have brightness variation. Quasars look like far-way Seyferts with bright nuclei while radio galaxies are made distinctive by their massive radio lobes powered by relativistic jets. Blazars are just quasars that appear differently when observed from varying angles but have a stellar appearance and produce continuous spectra. The central question that concerns astrophysics is how a volume so small can generate such intense luminosities; the central engine of an AGN is thought to be driven by a supermassive black hole around which an accretion disk forms by falling material that converts gravitational energy to radiating heat. Jets are believed to to be discharged orthogonally to the accretion disk. Such a paradigm leads to a standard model of an AGN (pictured), summarised as an accreting supermassive black hole (central engine) encircled by a broad-line region contained inside a obscuring torus of infrared emitting dust and a narrow-line region. Unification is an emerging means of modelling AGN according to the viewers position relative to the axis of the accretion disk; one unification regime links so called Type 1 and Type 2 AGN depending on whether the observer has a clear view of the black hole (Type 1) or is prohibited from viewing it by an opaque dusty torus (Type 2). In Type 2 AGN, the observer can't see the source of ultraviolet radiation or even emission lines but only 'mirror images' of such properties on adjacent clouds of gas. Another unification regime is applicable to around a tenth of AGN that have intense jets (radio loud); an observer viewing along the axis of the jet will see a blazar while looking aside from the axis will make the AGN appear much less intense and would be discerned as either a radio-loud quasar or radio galaxy.

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