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Host galaxies and environments

Host galaxies and environments - Unified models The...

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Host galaxies and environments Radio galaxies are almost universally found hosted by elliptical galaxies , though there is one well-documented exception. [11] Some Seyfert galaxies show weak, small radio jets, but they are not radio-luminous enough to be classified as radio-loud. Such information as there is about the host galaxies of radio-loud quasars and blazars suggests that they are also hosted by elliptical galaxies. There are several possible reasons for this very strong preference for ellipticals. One is that ellipticals generally contain the most massive black holes , and so are capable of powering the most luminous active galaxies (see Eddington luminosity ). Another is that ellipticals generally inhabit richer environments, providing a large-scale intergalactic medium to confine the radio source. It may also be that the larger amounts of cold gas in spiral galaxies in some way disrupts or stifles a forming jet. To date there is no compelling single explanation for the observations.
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Unformatted text preview: Unified models The different types of radio-loud active galaxies are linked by unified models. The key observation that led to the adoption of unified models for powerful radio galaxies and radio-loud quasars was that all quasars appear to be beamed towards us, showing superluminal motion in the cores [12] and bright jets on the side of the source nearest to us (the Laing-Garrington effect : [13] [14] ). If this is the case, there must be a population of objects not beamed towards us, and, since we know the lobes are not affected by beaming, they would appear as radio galaxies, provided that the quasar nucleus is obscured when the source is seen side-on. It is now accepted that at least some powerful radio galaxies have 'hidden' quasars, though it is not clear whether all such radio galaxies would be quasars if viewed from the right angle. In a similar way, low-power radio galaxies are a plausible parent population for BL Lac objects ....
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