Note:  This activity is taken from http://glast.sonoma.edu/teachers/agna/#act1, the html version of the first activity in the Active Galaxies Education Unit produced by the NASA GLAST Education and outreach program at Sonoma University

Activity 1: Building Perspectives with Active Galaxies  

Science Concepts:

• There are different components in active galaxies. • Different viewing angles lead to dramatic differences in the appearance of simple objects.

Duration: 60 minutes

Background Information:

The type of active galaxy we see depends on the way that we see it. If we see the accretion disk and gas torus edge on, the galaxy is called a radio galaxy. The torus of cool gas and dust blocks most of the radiation from the inner black hole and its nearby environment, so the most obvious features are the radio emitting jets and giant lobes well outside the galaxy.

If the disk is tipped slightly to our line of sight, we can see higher-energy light from the accretion disk inside the gas torus in addition to the lower energy radio waves. This kind of AG is called a Seyfert galaxy (named after American astronomer Carl Seyfert, who first catalogued these galaxies in 1943). It looks much like a normal galaxy but with a very bright core, and may be giving off high-energy photons like X-rays. If the galaxy is very far away from us, we may see the core as a star-like object even if the fainter surrounding galaxy is undetected. In this case, the galaxy is called a quasar, which is short for quasi-stellar radio source (so-named because the first ones discovered appeared to be star-like through a telescope, but emitted copious radio waves, unlike “normal” stars). The first quasar to be discovered, dubbed 3C273, was found to be a galaxy at a very large distance by astronomer Martin Schmidt in 1963.

If the tip angle is 90 degrees, we can be looking straight down a jet. This type of active galaxy is called a blazar. From blazars we see very high-energy gamma ray photons. The first blazar to be discovered, BL Lac (and after which we get the term “blazar”) was found in 1926 to change in brightness, but was thought to be a normal star! It wasn’t until the late 1970s that its galactic nature was truly revealed.

In sum, the basic components of an active galaxy are: a supermassive black hole core, an accretion disk surrounding it, and a torus of gas and dust, and in some (but not all!) highly focused jets of matter and energy. The type of active galaxy we see depends on the way we see the galaxy: radio galaxies, Seyferts, quasars and blazars.

	Fig. 3 Artist’s illustration of an active galaxy

Objective: Students will build a model of an active galaxy. From this, they will learn about the geometry of the components of the galaxy and understand that different viewing angles lead to dramatically different appearances of the galaxies.

Procedure: 1. Introduce the activity by using the information in the Introduction to Active Galaxies (page 5) and in the activity Background Information (page 6). 2. Discuss these questions with the students before starting the activity: Do objects look the same from all angles? Are they recognizable from all angles? For example, it’s not difficult to recognize a book from almost any viewing angle. But is every object like that? How do we recognize objects if viewed from unfamiliar angles? How do we categorize unfamiliar objects if seen from different angles? 3. Explain to the students that they will be building a model of an active galaxy from a styrofoam ball, construction paper, toothpicks and tape. This model will help them answer the questions on the worksheet. The model building can be done in groups of two or three. 4. Have the students make observations of the model, then sketch them as viewed from different angles. This should be done individually.

Assessment: The students’ work can be assessed using the following rubric:

Extension Activities:

Compare and contrast the students’ drawings to those on the GLAST Active Galaxies Poster for the different viewing angles. Discuss how this activity changes their perspectives about how they view and interpret what they see in the Universe.

Transfer Activities:

Have the students examine and draw everyday objects from different angles. Pass the drawings around to other students and see if they can identify the object, especially if the viewing angle is unfamiliar.

Lesson Adaptations:

Visually impaired students may have difficulty constructing the models and drawing them. Put the model in their hands, and let them note by touch how the model feels different if they can only access one part of it at a time (for example, a single cone/jet). They can examine how the model feels different if they keep their hand flat, fingers extended, and can only touch the model that way. In that example, the opening of the cone will feel like a circle, and the torus will feel flat. Have them describe how limiting their ability to touch the model limits their ability to identify its parts.
 

Answer Key for “Building Perspectives with Active Galaxies”

1.5 inches or 4 cm If the conical paper jet is properly constructed, it will obscure the styrofoam ball (black hole). For a 4 cm diameter styrofoam ball, the minimum width of the strip should be 4 cm (1.5 inches). The torus length should be 10 cm times 2π = 62.8 cm This answer will depend on the student. In general, looking straight down the jets will hide the black hole, accretion disk and torus. From an angle, the black hole, accretion, disk and torus will be visible, and the jets will be elongated. The far jet may be difficult to see. From the side, the black hole and accretion disk are again hidden, this time by the torus. Both jets will be visible and about the same size.

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