Announcements • Grades for third exam are now available on WebCT • Observing next week counts on the third exam.
Quasars and Active Galactic Nuclei • Quasi-stellar objects • Redshifts • Variability • Supermassive black holes • Active Galaxies • Reading 24.1-24.2, 23.2
Quasars • Early radio telescopes found radio emission from stars, nebulae, and some galaxies. • There were also point-like, or star-like, radio sources which varied rapidly these are the `quasi-stellar’ radio sources or quasars. • In visible light quasars appear as points, like stars.
Quasar optical spectrum Redshift shows this quasar, 3C273, is moving away from us at 16% of the speed of light Hα unshifted
3C273 The quasar 3C273 is 2.6 billion light years away. It looks dim, but must be extremely luminous to be visible as such distance. The luminosity of 3C273 is more than one trillion times the entire energy output of our Sun, or 100 times the luminosity of our entire galaxy.
Quasar size Size places a limit on how fast an object can change brightness. Conversely, rapid variations place a limit on the size of the emitting object.
Quasars vary The size of this quasar must be less than about one light year.
Quasar engine • Quasars have 100 times the luminosity of our Galaxy • The engine powering quasars is only a few light years across • The only known engine which is powerful enough and compact enough is a black hole • Quasars contain supermassive black holes
Quasar jets Optical core Radio jet
Quasars • object with a spectrum much like a dim star • high red shift • enormous recessional velocity • huge distance (from Hubble’s Law) • enormously luminous • compact physical size • powered by supermassive black hole • often produce huge jets
Quasars are the ultraluminous centers of distant galaxies. Quasars are often observed to be at the center of distant galaxies. The wispy material is likely gas that has been pulled out of the hot galaxy by gravitational interactions with nearby galaxies.
Quasars are the most extreme of a class of galaxies known as active galaxies M87 appears as an elliptical galaxy in visible light, but like a dim quasar in radio. The nucleus of the galaxy contains a weak quasar. This means that the galaxy harbors a supermassive black hole.
Centaurus A Optical Radio
NGC 1566 Spiral galaxies also sometimes contain active cores.
Active Galaxies come in several varieties • Quasars • Seyfert galaxies • luminous, star-like nuclei with strong emission lines. • BL Lacertae objects (BL Lacs) • featureless spectrum with a brightness that can vary by a factor of 15 times in a few months. • Most commonly known as a Blazar. • All contain supermassive black holes
Rotation Curves Rotation curves of stars near the centers of most galaxies show the presence of supermassive black holes with mass ranging from 106 to 109 solar masses. Essentially all galaxies contain supermassive black holes and were likely active galaxies at some point in their lives.