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Collisions of Galaxies
• Outside of Clusters (the field), most galaxies are spiral or irregular
• In dense clusters, the most massive galaxies are ellipticals
• Spirals in clusters tend to inhabit the periphery
Formation of Galaxies
• Initially: masses ~ 106 M
• Protogalaxies collided and merged to form larger galaxies.
• If there is a net angular momentum, a disk forms, giving a spiral galaxy
Active Galactic Nuclei
• The center of the Milky Way is a 2x106 M black hole.
• Bigger galaxies have more massive central black holes (up to 109 M)
• Some just sit there
• Others are actively accreting matter (gas, stars)
Active Galactic Nuclei
• Accretion proceeds through a disk
• Accreting gas orbits near the Schwarzchild radius (event horizon) of the black hole
• The gas heats up as potential energy is converted to kinetic energy (heat)
• We see bright X-ray sources
Relativistic Jets
• Accreting black holes emit jets of radiation at velocities near the speed of light
Radio Jet: M87
Quasars• In 1963 Martin Schmidt puzzled over
the spectrum of a 12th magnitude star-like radio source (a quasi-stellar object, or QSO)
3C 273• Redshift z = 0.16; Distance 2.4 x 109 ly
• It is one of the most luminous objects in the universe