Quiz on Galaxy Clusters and AGN

Galaxies are often found in groups called clusters of galaxies. Clusters with few members (tens or so) are called "poor" and clusters with many members (hundreds or more) are called "rich."

The Milky Way belongs to a cluster with about 40 members.

1.Does the Milky Way belong to a rich or poor cluster?

2. What is the name of the galaxy cluster to which the Milky Way belongs?
A)...The Local Group.
B)... M31.
C)...The Sagittarius Cloud.
D)...The Scorpius Group.
E)...The Virgo Cluster.

3. What holds galaxy clusters together?
A)... The pressure from the cosmic background radiation.
B)...The cosmological constant force.
D)... The Strong force.
E)...Magnetic forces.

Galaxies in clusters are separated by distances only about ten times larger than their diameters. Stars within a galaxy are separated by distances about 10 million times their diameters. This makes collisions between stars extremely unlikely but collisions between galaxies moderately likely. you can get some sense of the likelihood of galaxy collisions if you put a dozen or so pennies on a table spread about 10 inches apart. Now flick pennies through the group in random directions. How many flicks before one penny hits another? Not very many, is it?

In the Local Group, the galaxy M31 is heading toward the Milky Way at about 60 km/sec. It is about 2.2 million light years away.

4. If it continues in a straight line at the same speed, how long until it hits the Milky Way?
A)...12,000 yrs.
B)... 1.2 million yrs.
C)... 1.1x109 yrs.
D)...1.1x1010 yrs.
E)...6 billion yrs.

5. Will this collision pose a problem for you personally? Why?
A)...Yes. It will hurt like crazy.
B)...No, but it will be a menace to my great-grandchildren.
C)...No. The Sun will have burnt out long before this happens.
D)...No. It will mean I won't have to worry about paying off my college debts.
E)... Yes. I may not get to collect social security.

When two galaxies collide, the gravitational attraction of one on the other disturbs the orbits of their stars. This is especially true if at the collision the galaxies merge into a single system. For example, if stars are originally moving in neat circular orbits in a disk, their orbits may be scrambled by the collision so that they no longer lie in a disk. As a result, a galaxy that before a collision is a spiral may after a collision look more like an elliptical system. In rich galaxy clusters the most common kind of galaxy is elliptical, while in poor clusters and for galaxies not in clusters, spiral types dominate.

6. What part of our discussion above might explain this result?
A)... The gravity in rich clusters is so strong that disk galaxies are pulled into balls.
B)... In rich clusters collisions between galaxies are more common than in poor clusters and such collisions can change spirals into ellipticals.
C)...In rich clusters there is more mass so we expect galaxies that form to be mor massive. This will make them ellipticals.
D)...In rich clusters the large number of galaxies present blocks our view of disk galaxies, but ellipticals are harder to cover up, so we see more.
E)...In rich clusters there are more Seyfert galaxies. These systems eject so much gas that they puff up their disks into ellipticals.

Observations made with x-ray telescopes reveal that many galaxy clusters contain gas.

7. If the gas emits strongly at x-ray wavelengths, how hot is? Hint: Use Wien's law. (The wavelength of the x-rays observed is about 1.0 nm.)
A)... 1 K
B)...300 K
C)... 3 K
D)...3x106 K
E)... 3x104 K

When astronomers measure the speed of the motion of galaxies in a rich cluster, they find that the galaxies typically move at a speed of several 1000 km/sec. When they calculate the escape velocity from the cluster using the mass they can see in the form of member galaxies, they find an escape velocity very different from this value.

For example, let's calculate the escape velocity of a cluster of galaxies whose radius is 2 Mpc and that contains 1000 member galaxies, each with a mass of 1011 solar masses. What is the escape velocity for this cluster?

A few hints to help you solve this problem. First, convert the radius in Mpc to a radius in meters. To do this, look up the number of meters in a parsec and then multiply by the number of parsecs in a Mpc.

8.How many meters in a Mpc?

Next, figure out the mass of the cluster in kilograms. To figure this out, multiply the number of galaxies by the mass of each galaxy in solar masses and then multiply by the number of kilograms in 1 solar mass (2x1030). When you do this out you should get 2x1044 kg.

Now put the values for the mass and radius that you found into the escape velocity formula, v=(2GM/R) 1/2. To save you needing to look it up, G is 6.67x10-11m3-kg-1-sec-2. Notice that with the units as given, the velocity you find will be in meters/sec. Thus, you need to make one last conversion to get this in km/sec. To make that conversion, recall that 1 km = 103 meters. Thus, divide your answer by 1000 to get the escape velocity in km/sec.

9. What is the escape velocity for this cluster?
A)...66 km/s.
B)...6600 km/s.
C)... 10,000 km/s.
D)... 660 km/s.

When you compare this speed with the observed speed of the galaxies in the cluster, what do you find?

10. The escape velocity of the cluster is
A)...bigger than the speed of the galaxies.
B)...less than the speed of the galaxies.
C)...About the same as the speed of the galaxies.

We can infer from this that the cluster should therefore disperse. However, it hasn't, so something must be wrong in what we have done. You may recall that we found a similar problem in discussing the motion of stars within the Milky Way: namely, the stars in the outer part of the Milky Way (and other galaxies) are moving "too fast" to be held in orbit by the mass that we can observe.

This mass discrepancy led us to believe that the Milky Way (and other galaxies) are embedded in a halo of dark matter. Astronomers conclude similarly that galaxies clusters contain large amounts of dark matter.

Astronomers find additional evidence for dark matter in clusters because they see strange arcs of light within clusters (see figure below). Such arcs are the image of distant galaxies bent and distorted by the mass within the galaxy cluster. That is, the gravity of the cluster acts like a lens - a gravitational lens.

About 1% of all galaxies show signs of activity in their cores.

11. What are some signs of an active galaxy?
A)...The core is very bright.
B)...The core is very small.
C)...Spectra of the core show that it contains hot gas moving at high speeds.
D)... Spectra of the core show it contains lots of uranium.
E)... All of the above except D.

12. Which of the following is not an active galaxy?
A)...Seyfert Galaxies.
B)...Radio Galaxies.
D)...Attractor voids.
E)...None of the above are active galaxies.

13. What theory do astronomers think explains the activity in galaxies ?
A)...The explosion of a supermassive star.
B)...A chain reaction of supernovas
C)...A cosmic worm-hole.
D)...A supermassive black hole.
E)...Astronomers have no ideas at all to explain active galaxies.

14. How does the prevailing model for activity explain the hot, ejected gas seen in active galaxies?
A)... It is shot out of a black hole.
B)...It is material trapped between two colliding stars and squirts out.
C)...It is gas evaporating from an accretion disk around a massive black hole.
D)...It is stars forming as matter is funneled into a black hole.
E)...It is a rip in space-time from which the vacuum energy is leaking out.

QSO's are among the most distant objects we know.

15.How do we know they are so far away?
A)...Their spectrum shows a very high blueshift.
B)...Their spectrum shows a very high redshift.
C)...Their parallax is extremely small.
D)... We can detect no radio emission from them.
E)...None of the above are relevant.

16. Given that QSO's are so far away, what can we conclude about their power output?
A)...They must be very luminous to be visible at such great distances.
B)...Their luminosity must be very feeble.
C)...They must be expanding and contracting very rapidly.
D)...They must be spinning very fast.
E)...None of the above.

Your Answers
Your answer to the question was