Quasars and Active Galactic Nuclei Quasi-stellar objects Redshifts Variability Supermassive black holes Active Galaxies

If we could see the last few seconds of the collapse of a star to form a black hole, we would see the star grow steadily redder. Why? The star moves away from us at an increasing speed. The star grows steadily cooler. The star's gravitational redshift increases. The star becomes obscured by more and more interstellar dust.

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.

Optical picture of a quasar

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.

PKS 2000-030 has a distance of more than 11 billion light years

Quasars vary

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

A quasar varies in brightness by a factor of 2 in 10 days A quasar varies in brightness by a factor of 2 in 10 days. What does this tell us about the quasar? It has a large magnetic field. It is quite small. It must be highly luminous. It cannot emit radio waves.

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 lie at the center of double radio sources.

Active galaxies lie at the center of double radio sources.

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

Black holes in normal galaxies 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.

Review Questions Why is it thought that quasars are at very large distances? How can we place an upper bound on the size of a quasar even if we can’t resolve it with a telescope? Where are quasars found? Do most galaxies have supermassive black holes?