Big Galaxies Are Rare! Cepheid Distance Measurement. Clusters of Galaxies. The Nature of Galaxies

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Sharlene Pearl Hawkins
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common Dwarf ellipticals & irregulars are most common Faint, hard to see Mostly studied in galaxy

Hubble observed Cepheid Variables in the Andromeda galaxy Found that the Andromeda galaxy is over 2

Cepheid Distance Measurement The Hubble Space Telescope (Named for Hubble) has now measured the

called the Local Group Milky Way Using The Cepheid Method NGC 6822 Small Magellanic Cloud Large

1 Big Galaxies Are Rare! Potato Chip Rule: More small things than large things Big, bright spirals are easy to see, but least common Dwarf ellipticals & irregulars are most common Faint, hard to see Mostly studied in galaxy clusters The Nature of Galaxies The answer to the great debate came in the 1920s from Edwin Hubble Hubble observed Cepheid Variables in the Andromeda galaxy Found that the Andromeda galaxy is over 2 million light years away! Cepheid Distance Measurement The Hubble Space Telescope (Named for Hubble) has now measured the distances many galaxies M33 Andromeda Galaxy The Milky Way is part of a small cluster of galaxies called the Local Group Milky Way Using The Cepheid Method NGC 6822 Small Magellanic Cloud Large Magellanic Cloud Clusters of Galaxies Coma Cluster The Local Group of galaxies consists of: The Milky Way, Andromeda & 30 small irregular galaxies. Other galaxies are found in Clusters, just as stars can be found in clusters Some rich clusters contain hundreds of galaxies! The Virgo Cluster 1

Superclusters of Galaxies Not only do galaxies group together to form clusters, but

The Local Group, (containing the Milky Way), and the Virgo Cluster are in the Local

Interacting Galaxies Occasionally Galaxies Collide: Gravity distorts their shapes,

Gas clouds in each galaxy collide, triggering star formation Superclusters appear to be

Tidal Tails Example for galaxy interaction with tidal tails: The Mice NGC 4038/4039

2 Superclusters of Galaxies Not only do galaxies group together to form clusters, but clusters of galaxies group together to form superclusters. The Local Group, (containing the Milky Way), and the Virgo Cluster are in the Local Supercluster. Interacting Galaxies Occasionally Galaxies Collide: Gravity distorts their shapes, sometimes producing an irregular galaxy. Gas clouds in each galaxy collide, triggering star formation Superclusters appear to be arranged in walls or bubbles which surround huge empty voids These are the largest structures in the universe. Pof10 Interacting Galaxies Cartwheel Galaxy Galaxy collisions can produce ring galaxies Tidal Tails Example for galaxy interaction with tidal tails: The Mice NGC 4038/4039 Long tidal tails. The collision of two galaxies often triggers star formation. NGC 4676 Two galaxies called The Mice with long tails. Questions: How do we measure the distance to a galaxy? How do we detect a galaxy s motion? How many galaxies are there in the universe? How do we measure the mass in a galaxy? 2

Extragalactic Distances Galaxies are typically millions or billions of parsecs from our galaxy.

Matter from large companion falls onto a white dwarf, causing its mass to exceed 1.4 M sun The resulting explosion is a Type Ia supernova.

ordinary supernovae caused by an exploding massive stars Supernovae are Good Standard Candle They are all the same brightness They can be seen at very large distances

Spectrum will be redshifted if it is moving away, blueshifted if it is moving toward us.

3 Extragalactic Distances Galaxies are typically millions or billions of parsecs from our galaxy. Type Ia Supernovae Need a standard candle other than Cepheid variable stars: Supernovae! Matter from large companion falls onto a white dwarf, causing its mass to exceed 1.4 M sun The resulting explosion is a Type Ia supernova. Supernovae Types Typical distance units: Kpc = kiloparsec = 1,000 parsecs Mpc = megaparsec = 1 million parsecs Type Ia: Exploding White Dwarf in Binary Type II: ordinary supernovae caused by an exploding massive stars Supernovae are Good Standard Candle They are all the same brightness They can be seen at very large distances (1000x farther than Cepheids) Galaxies in Motion Motion of galaxies is measured using the Doppler effect. Spectrum will be redshifted if it is moving away, blueshifted if it is moving toward us. Non-moving galaxy spectrum Redshifted Spectrum Supernova in galaxy NGC4526 (HST Image) Hubble Law Hubble Law & Expansion E. Hubble measured the distances to some galaxies using Cepheid variables Redshifts were also measured from spectra He found that the redshift of a galaxy is proportional to its velocity, came up with Hubble s Law: Hubble s Original Data v = H o x d Expansion of pool balls. The balls farthest from the center are moving the fastest. Velocity is proportional to distance. 3

4 Hubble Law and Distance The fact that the universe is expanding can be used to determine the distances to galaxies which are far, far away. Hubble Law d = v r / H 0 v r = H 0 x d d = v r / H 0 d = distance to galaxy (Mpc) v r = radial velocity of galaxy (km/s) H 0 = Hubble constant (70 km/s/mpc) If we measure v r we can calculate the distance. Hubble Law Example: Find the distance to a galaxy that has a radial velocity of 35,000 km/s. d = v r / H 0 d = 35,000 km/s 70 km/s/mpc d = 500 Mpc 500 Mpc is about 1.5 billion light years! Techniques for Measuring Distances Review (nearby stars) (nearest galaxies) 1. Parallax Measure angle, use d = 1/p 2. Cepheid method (standard candle) Measure Period, get luminosity (distant 3. Type Ia Supernovae (standard candle) galaxies) 4. Hubble s Law (whole universe!) Measure velocity V r. Use: V r = H o x D How Many Galaxies are There? To add up the total number of galaxies in the universe, the Hubble Space Telescope took a picture of an empty region of space. The exposure time was 11 days! This is a small portion of the sky, just the size of a grain of sand held at arm s length! 4

Looking at Distant Objects Lecture Tutorial: Page 131 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now!

If you get really stuck, raise your hand and I will come around.

5 Looking at Distant Objects Lecture Tutorial: Page 131 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Discuss each question and come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. Masses of Galaxies Estimate mass: Brightness Rotation curve method Cluster method Velocity dispersion method Measured masses of galaxies are way too big! Rotation Curve Method How fast are stars near the edge of the galaxy moving? How much mass is that star orbiting around? Only works for nearest galaxies Cluster Method Measure motions of galaxies in clusters How are they orbiting each other? Same method as used in multiple star systems Velocity Dispersion Method If the galaxy is rotating, one side is moving away from us and one side is moving towards us Dark Matter The rotation curves do not decrease with distance; instead they are flat. Mass is not concentrated in the center: evenly distributed in the disk But most of this mass cannot be seen; it gives off no light. 5

Dark Matter 90% of the mass of the universe is dark matter!

throughout a galaxy, not clumped in the disk May also exist in between galaxies in rich clusters

Can t be: Neutrinos Black Holes Very faint stars or brown dwarfs Only interacts with normal

Lensing One method used to detect dark matter Light passing by a massive object will bend.

6 Dark Matter 90% of the mass of the universe is dark matter! Known from rotation curves and gas in galaxy clusters Dark matter is probably spread out evenly throughout a galaxy, not clumped in the disk May also exist in between galaxies in rich clusters What do we know about Dark Matter? Can t be: Neutrinos Black Holes Very faint stars or brown dwarfs Only interacts with normal matter via gravity If so much of the universe is made of dark matter, then what is it?? We don t really know! The nature of dark matter is one of the great unsolved mysteries of astronomy Gravitational Lensing One method used to detect dark matter Light passing by a massive object will bend. (Examples: Starlight near the Sun, light near a black hole) X-rays & Hot Gas More evidence for dark matter: We see hot, fast-moving gas particles in between galaxies in clusters There must be more mass in between the galaxies that gives off no light Chapter 18: Cosmology in the 21st Century 6

Cosmology Cosmology attempts to answer big questions: What was the origin of the Universe? What is fate of the Universe? Is the Universe infinite or limited?

Thus there is no edge or wall to the universe A wall or edge divides inside from outside, but there is nothing outside the universe!

7 Cosmology Cosmology attempts to answer big questions: What was the origin of the Universe? What is fate of the Universe? Is the Universe infinite or limited? The Universe means everything: all matter, all energy, and even all of space. Thus there is no edge or wall to the universe A wall or edge divides inside from outside, but there is nothing outside the universe! Everywhere you look you see a tree/star Olbers s Paradox Is the Universe infinite? In the 1820 s Heinrich Oblers noted that if the universe is infinite, then every line of sight should end on the surface of a star eventually. The night sky should be as bright as the surface of stars! So, why is the sky dark at night? Solution to Olbers s Paradox Why is the sky dark at night? Isotropy If the universe had a beginning, then we can only see light from galaxies that has had time to travel to us since the beginning of the universe. The speed of light is not infinite The age of the universe is not infinite No matter where we look, the Universe looks roughly the same No preferred or special direction The Universe is isotropic We can only see part of the universe: The Observable Universe Homogeneity No matter where we are, the Universe contains the same stuff No preferred or special location The Universe is homogeneous Isotropic + Homogeneous = Cosmological Principle The Cosmological Principle states that any observer in the Universe should see the same basic features This reinforces the no edge, no center This applies on a large scale 7

8 Expansion of the Universe Almost all galaxies are moving away from us, and more distant galaxies are moving away faster (Hubble s Law). We can explain this with an expanding universe model The Expanding Universe Space itself is a significant part of the universe. In the universe, space is expanding, carrying the galaxies along! Expansion of the Universe Lecture Tutorial: Page 133 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Discuss each question and come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. 8

Galaxies and Cosmology

Galaxies and Cosmology 4/28/17 The Discovery of Galaxies Up to the 1920 s, astronomers were not sure exactly how far away galaxies were, and thus didn t know how big they are! Spiral Nebulae could be assumed to be inside our