1ASTR 1020: Stars & GalaxiesApril 4, 2008• Reading: Chapter 22, sections 22.1 –22.2.• MasteringAstronomy Homework on Galaxies and Hubble’s Law is due April 7th.• Extra credit observing at SBO & for attending Fiske Planetarium shows!Astronomy In the NewsRosey StuurmansMany, Perhaps Most, Nearby Sun-Like Stars May Form Rocky PlanetsToday• Galaxy Evolution• Active Galaxies & QuasarsLast Time: Galaxy Collisions• Galaxy collisions destroy disks• Burst of star formation uses up all the gas• Leftovers: train wreck• Ellipticals more common in dense galaxy clustersComputerSimulationsof GalaxyMergers2Starburst Galaxies• Milky Way forms about 1 new star per year• Starburst galaxies form 100’s of stars per year• Heats dust to very hot temps: glows strongly in the infraredMuch evidence for giant supernova-driven winds?Triggered by galaxy collisions?“Active Galactic Nuclei = Active Galaxies”• Galaxies with strange stuff going on in their cores• Bright “nuclei” as bright as the rest of the galaxyQuasars• Quasi-stellar Radio Source• Nuclei so bright that the rest of the galaxy is not easily seen• First discovered as radio sources- then they were found to have high redshifts!Clicker Question: What is the most likely source of the light from bright nuclei (radio, visible, X-rays) in active galaxies?a) Thermal radiation from a massive star clusterb) Emission lines from hot gasc) 21 cm from hydrogend) Synchrotron radiation from a black hole• D: SynchrotronOnly Synchrotron light is bright at both radio and X-ray wavelengths. Æ Active nuclei suggest a massive black hole in the center of the galaxy!3Galactic Jets• Many show jets of optical and radio emission from accretion disks• Billion solar mass black holes!Centaurus A: The Closest Active GalaxyCygnus A Radio JetsMore radio observationsArtist’s Conception• Accretion disk around a massive black hole• Disk itself may or may not be obscured by dust• If bright nucleus is not visible, we’d call it a radio galaxy, but not a quasarA real example• Doppler shifts from orbiting material• Suggest billion solar mass black hole4Another example• 400 light year wide disk of material in core of an elliptical galaxy with radio jetsDo ALL galaxies have huge black holes?• As of 2008: probably yes!• Part of normal galaxy formation?• More quasars seen in the distant (early) universe than now• They grow, but can run out of available fuel and become relatively invisible (like in the Milky Way)Resurrected by galaxy collisions?• Many galaxies with bright nuclei show signs of being disturbed• Expect more such collisions in denser early universe: explain why fewer quasars todayProto-galactic Clouds• Looking for gas between the galaxies• Cold, invisible, too dim even at 21cmUse quasars as beacons- see absorption lines from gasSurveys for absorption by inter-galactic gas• Hydrogen absorption at different redshifts- all less than the quasar!• General rise in heavy elements as time goes on- fountains from
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