Key Questions:

1. What happens when galaxies collide?
2. What are "active galaxies" and how do they form?
3. What are radio galaxies and quasars?
4. How do we know they host a supermassive black hole?
• How do we know they are compact in size?
• How do we know they are really massive for their size?

Exam 2 Results

Galaxy Evolution by Collisions and Mergers

• Galaxy interactions or collisions are quite common, even in galaxies that seem isolated.


• Major Cosmic Train Wrecks are also seen!


• Radical transformations
• collisions and mergers involving a satellite companion
• collisions and mergers involving two large galaxies
• Interactions and Mergers are the main drivers of the galaxy evolution?
• collisions re-arrange the distribution of stars and dark matter
• collisions re-arrange the angular momentum of stars and dark matter
• collisions re-arrange the distribution of gas, where collisions and concentrations lead to starbursts (formation of new stars)
• collisions can heat the gas and expel some of the gas

Active Galaxies: Starburst Galaxies

• Definition: galaxies forming stars at an unsustainable rate
• Luminous: >10-100 times more young massive stars forming than in the Milky Way galaxy
• Rare: only one in 100 to 1000 normal galaxies
• Rich in molecular gas and dust (which converts UV light from young stars into infrared light)
• Triggered by tidal interactions and/or mergers?


• Example: M82

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Active Galaxies: Galaxies Hosting Supermassive Black Holes

Supermassive Black Hole at the Galactic Center

• See stars fly around the Milky Way center (by R. Genzel)
• Ionized gas is seen circling and falling toward the Milky Way center (see picture on the right side)
• The Milky Way's Heart of Darkness
• The Shadow of a Black Hole at the center of the MW

Seyfert Galaxies

• Seyfert Galaxies: all types of galaxies with an extremely luminous nucleus showing broad optical and UV spectral lines

Radio Galaxies

• Radio Galaxy: usually an elliptical galaxy associated with one or more large synchrotron emission jet of relativistic charged particles

Quasars

• Quasars: "quasi-stellar radio sources". These are the most extreme forms of active galaxies, with presumably the most massive central black holes.

Active Galactic Nucleus, Supermassive Black Hole, and Nuclear Activity

• How do you know the central object is very small in size? The shortest time it takes for a light source to change its brightness is the light-crossing time (size divided by the speed of light) because no information can travel faster than the speed of light. Quasars can change their brightness on days to months time scales, suggesting the source size of light days to light months.



• How do you know the central object is extremely massive? Stars and gas orbit around the central object at extremely high speed. Mass can be calculated using the generalized version of Kepler's Law.


• Quiz 21A: weighing a supermassive blackhole....


• How do we know these active nuclei are powered by a supermassive black hole?


• Quiz 21B: how can we see these activities if they are powered by a black hole?


• How is a supermassive black hole formed and fed?

Bigger BHs in More Massive Galaxies?

It is observationally shown that the more massive galaxies host the larger mass black holes in their centers. This indicates that the process leading to the formation and the growth of the host galaxy and that of the central black hole are closely related.

Reading assignment for next lecture: Unit 78