Quasars in galaxies
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Quasars and Their Host Galaxies: Key Relationships
Quasars are extremely luminous objects powered by accretion onto supermassive black holes at the centers of galaxies. Observations confirm that quasars are found within galaxies, and studying these host galaxies provides important insights into galaxy evolution and black hole growth 1245.
Types and Properties of Quasar Host Galaxies
Research shows that quasars are hosted by a variety of galaxy types. At intermediate redshifts, luminous elliptical galaxies are common hosts, often with close companion galaxies, suggesting ongoing interactions or mergers . At lower redshifts, many quasars are found in bulge-dominated, early-type galaxies, but a significant fraction also reside in disk-dominated, late-type galaxies, indicating that major mergers are not always necessary for quasar activity . At high redshifts (z > 6), quasar host galaxies are massive, compact, and often disk-like, with stellar masses up to several times 10^10 solar masses 25.
Quasar Host Galaxy Structure and Star Formation
Quasar host galaxies tend to be massive and star-forming, with structural properties that are generally similar to inactive galaxies of the same mass, though they are often more compact 49. Most quasar hosts have star formation rates that place them on or above the galaxy star-forming main sequence, especially for rapidly accreting quasars . This suggests that quasar activity and star formation can occur simultaneously, possibly fueled by centrally concentrated gas reservoirs .
Black Hole–Galaxy Mass Relationship
There is a well-established relationship between the mass of the central supermassive black hole and the stellar mass of the host galaxy. This relationship is already in place at high redshift (z > 6), less than a billion years after the Big Bang 25. However, the ratio of black hole mass to stellar mass in high-redshift quasars is higher than in local galaxies, hinting at possible evolution in this relationship over cosmic time .
Quasars, Galaxy Formation, and Mergers
Mergers between galaxies are thought to play a key role in triggering quasar activity and the growth of supermassive black holes . Models suggest that when galaxies of similar mass merge, their central black holes coalesce and accrete gas, powering quasar activity . Observations of quasar host galaxies with close companions support the idea that interactions and mergers are important in quasar and galaxy evolution 13. In some cases, radio jets from quasars may even trigger star formation in nearby galaxies, suggesting a direct role for quasars in galaxy formation .
Clustering and Environment of Quasars
Quasars are found in massive dark matter halos, and their clustering with galaxies provides information about their typical environments and lifetimes 67. At high redshift, quasars tend to reside in less massive halos compared to their low-redshift counterparts, and their activity is relatively rare, with a low duty cycle . The cross-correlation between quasars and galaxies helps constrain the processes that fuel supermassive black holes and the evolution of quasar populations over time 67.
Conclusion
Quasars are closely linked to the galaxies that host them, with evidence showing that they reside in massive, often star-forming galaxies across a range of cosmic epochs. The relationship between black hole mass and galaxy mass is established early in the universe, and galaxy interactions and mergers play a significant role in triggering quasar activity. The study of quasar host galaxies continues to provide valuable insights into the co-evolution of galaxies and their central black holes.
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