1. GALAXY FORMATION AND EVOLUTION - 1

2. The Expansion of Galaxies In the 1920’s, Edwin Hubble demonstrated that all the `nebulae’ observed in the sky, which were previously thought to reside in our own Galaxy, were actually `Island Universes’, I.e., other galaxies. They also have a recession velocity relative to us (they `run away’ from us): How did he prove it?

3. Emission spectra V=0 V=1000 Light becomes redder

4. Redshift Redshift is defined as the fraction by which the light’s wavelength changes. 1 2 z =      /   (  /

5. How to calculate a redshift [NII] H  [NII] [SII] H , one of the hydrogen recombination lines, has a well known wavelength of emission:  = 6563 A In the galaxy NGC7714, H  is measured at wavelength:  = 6624 A z = (  / z = (6624-6563) / 6563 = 0.0093 Very small for this galaxy

6. How to convert a redshift into a recession velocity If the redshift of galaxies is a Doppler shift (I.e., the shift in light is caused by the galaxies moving away from us, and the faster their velocity from us, the stronger the shift) [think of an ambulance first approaching (high pitch) and then receding (low pitch) from you] z = (  / V = c (  / Speed of light

7. How to calculate a recession velocity [NII] H  [NII] [SII] H , one of the hydrogen recombination lines, has a well known wavelength of emission:  = 6563 A In the galaxy NGC7714, H  is measured at wavelength:  = 6624 A v = c (  / v = 300,000 * (6624-6563) / 6563 = = 2788 km/s

8. Hubble noticed that dimmer (farther away) galaxies had larger recession velocities Distance (Mpc) Recess Velocity (km/s) v = H o d (H o = 71 km/s/Mpc) The Hubble Law H o is the Hubble constant

9. How to measure distances Very difficult, and until recently astronomers were in disagreement on the value of the Hubble constant (some said 100 km/s/Mpc, some said 50 km/s/Mpc). Some methods used to measure distances involve supernovae, and Cepheids (pulsating stars) The HST solved the issue of the value of the Hubble constant.

10. Use the Hubble Law to know distances! Once the Hubble Constant is known, you can use the Hubble Law to derive distances to galaxies: d = V/H o = 49,000 km/s / (71 km/s/Mpc) = 690 Mpc

11. All observers see the same expansion You: V=0 km/s 700 km/s 700 1400 2100, an alien V=2100-2100=0 V=2100-1400=700 V=2100-0=2100 The alien will observe the same Hubble Law; Cosmological Principle 10 Mpc 20 3040 2800 V=2800-2100=700

12. The Hubble expansion is expansion of space between galaxies Think of buttons strung across a rubber band: Stretch the rubber band and the buttons will separate from each other.

13. Because light has finite speed, a further away galaxy is also younger Thus redshift can also be used to `tell time’, and by how much the size of the Universe has changed: Z = (change in ) /  = (change in size of Universe) / (size of Universe)

16. What is in the Cosmos? 4% baryons (ordinary matter) 27% dark matter 69% dark energy