Galaxies

Galaxies Star systems like our Milky Way Contain a few thousand to tens of billions of stars. Large variety of shapes and sizes

Galaxies They all formed when the universe was 1 to 2 billion years old. They could not have formed without density variations in the early universe. These variations could start gravitational collapse!

Three types of motions now Expansion of the universe as a whole. Collapse in areas of higher density. Rotation in areas of collapse. With collapse comes an increase in temperature. With rotation comes a change in shape.

Formation of Galaxies Giant Primordial Cloud 1015-1016 x mass of the sun H, D, He Supercluster Clusters of galaxies galaxies clusters Largest matter structure in the universe, but not around today. Largest matter structure in the universe today. Within galaxies, the same process will give birth to stars.

The Formation of the Milky Way The formation of the Galaxy is believed to be similar to the formation of the solar system, but on a much larger scale:

Our address 11110 Alondra Blvd Norwalk, CA 90650 USA Earth Sol Local Group Virgo Supercluster

Clusters of Galaxies Galaxies generally do not exist in isolation, but form larger clusters of galaxies. Rich clusters: 1,000 or more galaxies, diameter of ~ 3 Mpc, condensed around a large, central galaxy Poor clusters: Less than 1,000 galaxies (often just a few), diameter of a few Mpc, generally not condensed towards the center

Our Galaxy Cluster: The Local Group Milky Way Andromeda galaxy Small Magellanic Cloud Large Magellanic Cloud Our poor, irregular cluster.

Neighboring Galaxies Some galaxies of our local group are difficult to observe because they are located behind the center of our Milky Way, from our view point. Spiral Galaxy Dwingeloo 1

Galaxy Evolution No new galaxies formed today, but there is evolution. Galaxies evolved to their current state in two ways: Dynamically – through collisions and tidal interactions Chemically – through the evolutions of the stars contained within them.

Galaxy interactions within clusters When galaxies collide, stars rarely hit. Interaction is usually at a distance through gravity. Stephen’s Quintet

Galactic Cannibalism Collisions of large with small galaxies often result in complete disruption of the smaller galaxy. Small galaxy is “swallowed” by the larger one. NGC 5194 This process is called “galactic cannibalism”

Radio image of M 64: Central regions rotating backward! Mergers of Galaxies Radio image of M 64: Central regions rotating backward! NGC 7252: Probably result of merger of two galaxies, ~ a billion years ago: Small galaxy remnant in the center is rotating backward! Multiple nuclei in giant elliptical galaxies

Magellanic Clouds The Magellanic clouds are battered due to their interactions with our galaxy. They will eventually merge with the Milky Way.

Interacting Galaxies Cartwheel Galaxy Particularly in rich clusters, galaxies can collide and interact. Galaxy collisions can produce ring galaxies and tidal tails. NGC 4038/4039 Often triggering active star formation: starburst galaxies

ultraluminous infrared galaxies Starburst Galaxies Starburst galaxies are often very rich in gas and dust; bright in infrared: ultraluminous infrared galaxies M 82 Cocoon Galaxy

Tidal Tails Computer simulations produce similar structures. Example for galaxy interaction with tidal tails: The Mice Computer simulations produce similar structures.

The “Black Eye” A merger of 2 galaxies has left M64 with an odd lock and bizarre internal motions. The gas in the outer regions of the galaxy are revolving in the OPPOSITE direction of the gas and stars that make up the majority of the galaxy.

Simulations of Galaxy Interactions Numerical simulations of galaxy interactions have been very successful in reproducing tidal interactions like bridges, tidal tails, and rings.

Classification By what they look like. By what they do. - Active Galaxies Spiral Elliptical Irregular Radio Galaxies Seyfert Galaxies Quasars

Galaxy Diversity – Types! Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies Large variety of galaxy morphologies: Spirals Ellipticals Irregular (some interacting) The Hubble Deep Field: 10-day exposure on an apparently empty field in the sky

Galaxy comparisons Spiral Elliptical M51 M87

Sombrero Galaxy

Irregular Galaxies Often: result of galaxy collisions / mergers Often: Very active star formation (“Starburst galaxies”) The Cocoon Galaxy NGC 4038/4039 Some: Small (“dwarf galaxies”) satellites of larger galaxies (e.g., Magellanic Clouds) Large Magellanic Cloud

Irregular Galaxy M82

Barred Spirals Some spirals show a pronounced bar structure in the center They are termed barred spiral galaxies Sequence: SBa, …, SBc, analogous to regular spirals

Sub-Classification Normal Spirals Barred Lenticular (SO) Ellipticals E0 E7 Irregular Irr

Galaxy Classification Large nucleus; tightly wound arms; less gas/dust E0, …, E7 Sa E0 = Spherical E1 Sb Sc Small nucleus; loosely wound arms; more gas/dust E7 = Highly elliptical E6

Spirals

Barred Spirals

Ellipticals

Lenticular Galaxy NGC 5866

Hubble Tuning-Fork Diagram

Andromeda – which classification?

So, what is this? Centaurus A Merger of 2 galaxies

Milky Way From Earth, see few stars when looking out of galaxy (red arrows), many when looking in (blue arrows). Milky Way is how our Galaxy appears in the night sky (b).

Milky Way

There are about 200 billion (2 × 1011) stars in the Galaxy Our Galaxy has a disk about 50 kpc (160,000 ly) in diameter and about 600 pc (2000 ly) thick, with a high concentration of interstellar dust and gas in the disk The Sun orbits around the center of the Galaxy at a speed of about 790,000 km/h It takes about 220 million years to complete one orbit

Milky Way Structure This artist’s conception shows the various parts of our Galaxy, and the position of our Sun:

Milky Way Structure The Galactic halo and globular clusters formed very early; the halo is essentially spherical. All the stars in the halo are very old, and there is no gas and dust. The Galactic disk is where the youngest stars are, as well as star formation regions – emission nebulae, large clouds of gas and dust. Surrounding the Galactic center is the Galactic bulge, which contains a mix of older and younger stars.

Milky Way Structure Stellar orbits in the disk are in a plane and in the same direction; orbits in the halo and bulge are much more random.

Structure of the Milky Way Revealed Distribution of dust Sun Distribution of stars and neutral hydrogen Bar Ring

Milky Way Structure This infrared view of our Galaxy shows much more detail of the Galactic center than the visible-light view does, as infrared is not as much absorbed by gas and dust.

Quiz Questions 1. Galaxies with active star formation also have which of the following? a. Plenty of gas and dust. b. Hot stars. c. Emission nebulae d. A bluish tint. e. All of the above.

Quiz Questions 2. How does a Sa galaxy differ from a Sc galaxy? a. The Sa galaxy has a smaller nuclear bulge. b. The Sa galaxy has more loosely wound spiral arms. c. The Sa galaxy has less gas and dust. d. Both a and c above. e. Both b and c above.

Quiz Questions 3. What gives elliptical galaxies a redder color than spiral galaxies? a. Elliptical galaxies are more massive, and thus have a larger gravitational red shift. b. Elliptical galaxies are more distant, and thus have more interstellar reddening. c. Elliptical galaxies are more distant, and thus have larger red shifts. d. Elliptical galaxies have a higher percentage of iron. e. Elliptical galaxies have less gas and dust and older stars.

Answers 1. e 2. c 3. e