Chandra Astrophysics Institute

Investigation 6: Galaxy Clusters - Activity 1

Introduction to Galaxies

Overview: Students examine visible and X-ray light images of galaxies to motivate questions about them.

Physical resources: None

Electronic resources: Galaxy Zoo Web site, images of spiral and elliptical galaxies.

Instructor gives general introduction to galaxies:

• Typical radius = 50,000 light years = 4.5 x 10²⁰ meters
• If a galaxy was shrunk to the size of a CD (which is approximately the right shape, 100 times wider than it is thick), where would the next galaxy be located? 20 “galaxy diameters” away
• Typically, galaxies contain about 200 billion (2 x 10¹¹ stars)

Comparing and contrasting properties of galaxies:

• Challenge: Students complete tutorial and classify galaxies using “citizen science” Web site Galaxy Zoo (Galaxy Zoo, click “How to Take Part” to enter the tutorial).
• Focus question: Describe differences between elliptical and spiral galaxies in visible light, using the words “flux,” “color,” “angular width,” “angular separation.”
• Challenge: Make true color X-ray and false color visible and X-ray light images of an elliptical and spiral galaxy (half the class each).
• Spiral galaxy: M74 (obsid 2057), distance ~ 32 x 10⁶ light years (X-rays Signal Presence of Elusive Intermediate-Mass Black Hole).
• Alternate spiral: M81 (obsid 5943, examined as AGN example in Investigation 2), distance ~ 11 x 10⁶ light years (Black Holes Have Simple Feeding Habits).
• Elliptical galaxy: NGC 0507 (obsid 317), distance ~ 234 x 10⁶ light years (NASA’s Chandra Finds Black Holes Stirring Up Galaxies).
• Students should make complete descriptions of the differences between their galaxy, as seen in X-ray and visible light, using the same words as above.
• Students should switch notebooks with a partner who examined the other kind of galaxy, and then discuss the important differences, to be shared on the whiteboard with the entire class.

Wrap up discussion:

• Discuss possible models that could explain these observations (colorful XRB point sources in spiral, no AGN at center of M74, but strong AGN possibility in elliptical NGC 0507, smoothly distributed X-ray light indicates hot gas inside and surrounding the elliptical galaxy).
• Students generate questions they have about galaxies, and share on the whiteboard.

Teacher tips/tricks:

• Could have students examine other ranges of light energy for a single galaxy, beyond visible and X-ray light. Link to Chandra information about Sombrero galaxy in IR, visible, and X-ray light.
• If crunched for time, students could also investigate the difference between X-ray and visible light images of galaxy clusters, as outlined in activity 4 below.

Assessment ideas:

• Which would you say is “closer together,” a galaxy and 3 nearest neighbor galaxies, or a star and its 3 nearest neighbor stars? (Absolute distance is smaller for stars, but relative distance (i.e. compared to the size of the object) is smaller for galaxies.)
• Some astronomers say “looking at only visible light from an object is like listening to just the string section playing a piece of orchestra music.” Why would they say this?
• If galaxies are made of stars, why do they look fuzzy and not like a bunch of dots? (issues of resolution–could have them calculate the angular size of one star at the distance to one of these galaxies).