ACTIVE GALAXIES AND QSOS

AGN

Some galaxies have "active" black holes in their bugles. These galaxies are called AGN, or Active Galactic Nuclei. One type of AGN are called Seyfert galaxies. Often AGN are near other galaxies, suggesting that they are interacting with other galaxies or just merged with another galaxy. It is believed that this may be what causes the black hole to become active, eating its surroundings.

The Central Engine

How do we know they are black holes? Again, we measure how fast stuff rotates around the center of mass and how close it is to the mass. We can them measure the mass and it is huge for such a small area! Sometimes, the giant elliptical galaxies in rich clusters have active black holes. We can see jets of material beaming out of the galaxies!

The jets form as a disk of material is accreting onto the black hole. This is called an accretion disk. The disk of material is spinning rapidly around the black hole at the center. Lots and lots and lots of energy is released as the material spirals into the hole and heats up. The jets are made of electrons that are confined by magnetic fields.

Sometimes we see that the jets stop at a "hot spot". This is where the jet energy is released sue to a collision with the intergalactic medium, a very tenuous gas between galaxies. Other times, we see the jets bend like they are flowing in a wake (much like a boat wake). This also suggests that the galaxies are moving in this tenuous intergalactic medium.

QSOs

Quasars are extreme versions of AGN. They are much more powerful. Some quasars are at great distances, and therefore, because of Hubble's Law, their spectra are highly redshifted. They are so redshifted that to compute their velocity from us, we need to correct for Einstein's Relativity relationships. Also, sometimes we see gravitational lensed quasars. This is due to the bending of light by massive objects (galaxies) between us and the quasars.

Gravitational Lensing

Gravitational Lensing is quite common. The space around a massive object, like a large galaxy slightly warped. If there is a distant object behind this large galaxy, then sometimes we see multiple images of it as shown in the diagram in the lecture.