ASTR-1020 Stellar Astronomy Day 26

Galaxy Classes

Galaxy Classification Lecture Tutorial – pg. 127 Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on and write complete thoughts into your LT. If you get stuck or are not sure of your answer, ask another group.

Orbits and Shapes The stars and gas in the galaxy orbit in the overall gravitational field. Spiral galaxies have a thin disk: orbits are circular and all nearly in the same direction. Stars in elliptical galaxies orbit in many different directions. Stars in the central bulge of a spiral galaxy also have orbits in many directions.

Orbits in Elliptical Galaxies

Orbits in Spiral Galaxies

Star Formation New stars are forming in spiral galaxies. No star formation in elliptical galaxies. Gas and dust are concentrated along the spiral arms. Spiral arms are waves of compression. Waves passing by trigger star formation. Hot young stars produce blue light.

Star Formation Todd Boroson/NOAO/AURA/NSF Richard Rand, University of New Mexico

Big and Small There is a wide range of sizes in each galaxy type. An elliptical galaxy can be a dwarf galaxy or a giant galaxy. Largest are several thousand times more massive than the smallest.

Dwarf and Giant Ellipticals Both images: David Malin

Galaxies are Mostly Dark Matter Orbital speeds depend on total amount of mass in the galaxy. Luminous matter (= normal matter) cannot produce all the gravity in a galaxy. There must be an additional source of gravity that does not make light, called dark matter. About 95% of the mass of a spiral galaxy is dark matter. It is located in a large dark matter halo around the galaxy.

Rotation Velocities Galaxy image courtesy David Malin

Dark Matter Halo

Active Galaxies Many galaxies have bright active galactic nuclei (AGN) that have emission lines. These regions are very small, about the size of the Solar System. Many spiral galaxies are Seyfert galaxies, with a visible AGN. Luminosity of the AGN can equal that of the rest of the galaxy.

AGN and Quasars Quasars are extremely luminous AGN. Unified model: a central supermassive black hole with an accretion disk. Jets of material shoot out from the poles of the system. A dense ring (torus) of dust blocks the central regions. What we see depends on the viewing angle.

A Radio Jet NRAO/Alan Bridle et al.

The Unified Model

Supermassive Black Holes The orbital speeds of gas near the black hole yield its mass. These are from 10 4 to over 10 9 solar masses. These are called supermassive black holes. They probably exist at the centers of all galaxies.

Distance to Quasars

A Slice of the Universe

Dancing Black Holes – 3C75

Concept Quiz – Elliptical Galaxies Which of these is not a characteristic of elliptical galaxies? A.Stellar orbits go in all directions. B.Oval shape. C.Star formation. D.Dwarf and giant types. E.Formed by galaxy collisions.

Concept Quiz – Dark Matter How do we know that galaxies are mostly dark matter? A.Stellar orbits are faster than expected from the gravity of the stars alone. B.We see dark lanes in spiral arms. C.Most galaxies have supermassive black holes.

Concept Quiz –Blue Light You do a big survey and find that extremely distant galaxies give off a lot of blue light. What is going on? 1.Interstellar dust is affecting the colors. 2.These galaxies are mainly elliptical galaxies. 3.The galaxies had a high rate of star formation when their light was emitted. 4.You must have made a mistake, since the stars in these galaxies would all be very old.

Chapter 19 The Milky Way – A Normal Spiral Galaxy

Building Galaxies

Colliding Galaxies

The Local Group

Galaxy Formation The Milky Way offers clues to galaxy formation. All halo stars have some heavy elements, so at least one prior generation of stars must have existed. Halo objects were formed before interstellar gas was all concentrated into the disk. Later star formation was all in the disk.

Satellite Galaxies The Milky Way probably formed by the merger of many smaller protogalaxies. Several of these are still orbiting the Milky Way as satellite galaxies. These can contain significant amounts of gas. The gas delivered by the protogalaxies was a significant source of star formation.

Satellite Galaxies

Mapping the Milky Way It is necessary to get accurate distances to objects in the Milky Way. This is made difficult by dust. Dust blocks light, making things fainter. They look farther away. Any distance requires something with a known luminosity. Such an object is called a standard candle.

Dust in the Milky Way Axel Mellinger C. Howk and B. Savage (University of Wisconsin); N. Sharp (NOAO)/© WIYN, Inc

Structure of the Milky Way Milky Way stars are in three groups: –A large flat disk with spiral arms. The Sun is a disk star. –A bright central bulge. –A diffuse, extended halo of stars. About ¼ of the globular clusters are in the disk, with the rest in the halo. The center of the Milky Way is 27,000 ly from the Sun.

The Milky Way

A Normal Spiral Galaxy

Top View

Model of the Halo, Current

Milky Way Scales Lecture Tutorial pg. 123 Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on and write complete thoughts into your LT. If you get stuck or are not sure of your answer, ask another group.