GALAXIES

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Active Galaxies	Active Galaxies - Radio
Active Galaxies-Quasar	Active Galaxies - Seyfert
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Active Galaxies Questions

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Q1.    Compare the active energy output of normal galaxies, quasars, radio galaxies, and Seyfert galaxies. How do we explain the different levels of energy output?  Answer

Q2.    Describe the continuum of activity that occurs in galaxies. What is responsible for this activity and what controls the level of activity?  Answer

Q3.    How is the "extra" energy of an active galaxy produced?   Answer

Q4.    Describe the mechanism for the energy production in active galaxies. Why are normal (quiet) galaxies not active?  Answer

Q5.    What is meant by the term active galaxy? Describe the observed properties of the three different types of active galaxies.  Answer

Active Galaxies-Quasar Questions

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Q1.    How do we know that the active regions of a quasar are very small, by galactic standards? Answer

Q2.    How is a double image of a quasar produced?  Answer

Q3.    How do we know that the light emitting region of a quasar is very small?  Answer

Active Galaxies - Radio Questions

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Q1.    Describe the radio appearance of a typical radio galaxy. Why do some of them appear twisted?  Answer

Q2.    What is meant by the term "active galaxy"? Describe the observed properties of a radio galaxy.  Answer

Active Galaxies - Seyfert Questions

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Q1.    How do we know that the active part of a Seyfert galaxy is incredibly small?  Answer

Active Galaxies Answers

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A1.    Normal galaxies have very little output from the active centers, radio galaxies emit as much as 10 times the stellar energy from their centers, Seyfert galaxies up to 100 times their normal stellar output, and quasars more than 1000 times the output of a normal galaxy. The different energy levels are the result of both different masses for the supermassive black holes that power the activity and different amounts of matter falling into the black holes at different times in the lives of galaxies.

A2.    All normal galaxies are observed to have at least a little bit of non-stellar activity in their centers. For most galaxies, like ours, the amount of energy produced by non-stellar activity in their center is so small as to be inconsequential unless very careful observations are taken. At the other extreme, the activity in quasars completely overwhelms the normal stellar output of the galaxy such that the non-stellar energy is all that is noticed unless extremely careful observations are taken. Other galaxies show all possible ratios of stellar to non-stellar output in between these two extremes. The energy emitted in this way comes from matter falling toward a supermassive black hole in the center of the galaxy. The level of activity is determined both by the mass of the black hole and the rate at which matter falls toward it.

A3.    The extra energy emitted by active galaxies comes from matter being pulled into a supermassive black hole in the center of the galaxy. At it swirls around the black hole, but before it falls into the event horizon, it begins to move very rapidly. Collisions heat the material to high temperature and it emits a large amount of energy.

A4.    Active galaxies are believed to have supermassive black holes in their centers. As matter is pulled into these black holes, it is heated to great temperatures and glows intensely, making the center of the galaxy very bright. Normal galaxies usually also harbor supermassive black holes, but they are not attracting matter into them and so they do not cause any excess light emission.

A5.    An active galaxy is one that emits a significant amount of light from non-stellar sources. Radio galaxies have a small intense source of radio waves in their centers, often have jets of radio-emitting material leading from their center, and have giant lobes of radio-emitting material on either side of the visible galaxy. Seyfert galaxies are spiral galaxies with unusually bright nuclei. Quasars are distant galaxies whose centers are so bright that that is usually all we see.

Active Galaxies-Quasar Answers

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A1.    Quasars are observed to varying in brightness in just a few days or weeks. Anything that changes its brightness in a few weeks can be no more than a few light weeks across. Otherwise, the change in brightness would be spread out by the time it takes light to travel across the object. That is, light from the back of the object would take longer to get to us than does light from the front of the object. An immediate change in the object would be spread out by this travel time.

A2.    f a galaxy lies between the quasar and us, the gravity of the galaxy will bend the quasar’s light and produce two (or more) images of the quasar on either side of the (often invisible) galaxy.

A3.    Quasars are observed to vary in brightness over a time span of seconds to months. Let’s take as an example an object that changes brightness in one week. If it were more than one light week in radius, then the change in brightness would be spread over a longer interval of time as light from more distant parts of the object reached us at later times. If we see the change in one week, that tells us that the object can be no more than one light week in radius.

Active Galaxies - Radio Answers

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A1.    A typical radio galaxy has two large regions of radio emission, called lobes, on either side of the optical galaxy. There is usually a small central source of radio emission at the center of the galaxy, and often one or two jets of radio emitting matter leading toward the lobes. If the jets do not lie along the spin axis of the galaxy, they will become twisted by the rotation of the galaxy.

A2.    An active galaxy is one which produces a significant fraction of its light from non-stellar processes. A typical radio galaxy emits radio waves from three areas: a small point source at the center of the galaxy; one or two jets of matter being ejected from the center of the galaxy; and one or two huge lobes or bubbles of matter at the end of the jets. The jets may be either straight or twisted by the rotation of the galaxy.

Active Galaxies - Seyfert Answers

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A1.    The active part of Seyfert galaxies varies in brightness over the course of months. Any object that changes in brightness can be no larger than the time light would need to travel across the object, since the change we see is spread out by the light travel time across the object. Hence, if we see the change in a month, the object responsible for the change can be no larger than one light month is size.