1. GENS4001 - S1 and S2Galaxies1 Dr Michael Burton

2. GENS4001 - S1 and S2Galaxies2 The Structure of our Galaxy How did we find out? Herschel’s 18 th Century map Analogy: lighthouses above the fog Globular Clusters RR Lyrae stars (pulsating red giants) Flattened disk in shape of Spiral 100,000 light years across, 300,000 million stars Dust lanes in disk (young stars) Central Bulge (old stars) Halo (globular clusters)

3. GENS4001 - S1 and S2Galaxies3 Galactic Rotation Rotation curve is constant with distance V ~ 250 km/s There must be ‘Dark Matter’ (unseen matter exerting a gravitational pull) Winding of spiral arms dilemma Density Wave (cf. traffic jam on highway) Star formation induced by compression of gas clouds as they pass through the spiral arm

4. GENS4001 - S1 and S2Galaxies4 The Realm of Galaxies Island universes or nebulae in our Galaxy? The Great Debate on the Scale of the Universe Spirals galaxies – active star formation (young and old stars). Elliptical galaxies – no gas, star formation has finished (all old stars). Irregular galaxies – ill-defined shape

5. GENS4001 - S1 and S2Galaxies5 The Expansion of the Universe Measure distance of galaxy –e.g. apparent size, brightness of supernova Measure velocity of galaxy –Doppler shift Hubble’s Law –Speed = H 0 x Distance –H 0 is Hubble’s Constant –65 +/- 10 kilometres per second per megaparsec (megaparsec = million parsecs = 3 million light years) –1 / H 0 is age of Universe (12-15 billion years)

6. GENS4001 - S1 and S2Galaxies6 Galaxy Clusters Milky Way is in the ‘Local Group’ –a small cluster of galaxies Galaxies are clustered. Clusters combine to form superclusters. Large Scale Structure (e.g. the Great Wall) –2dF instrument on the AAT has measured ~250,000 galaxies.

7. GENS4001 - S1 and S2Galaxies7 Galaxy Mergers and Cannibalism Galaxy Mergers –Stars pass right by one another (do not collide!) –Gas collides, induces star formation Galactic Cannibalism –Forms the dominant cluster member Galaxies today built up by mergers over first 1–2 billion years of the Universe.

8. GENS4001 - S1 and S2Galaxies8 Galaxy Formation Hubble Deep Field: a pencil beam to the early universe We see the building blocks of galaxies, merging together and undergoing star formation Theories for Galaxy Formation Collapse of a single huge gas cloud? Coalescence of many smaller clouds? Perhaps a bit of both??

9. GENS4001 - S1 and S2Galaxies9 Miscellaneous Galactic Facts Flat Rotation curves for Spiral Galaxies suggests that Dark Matter is everywhere! Gravitational lens magnifies distant, background galaxies A probe of the most distant galaxies Quasars and Active Galactic Nuclei Central engine – a ‘supermassive black hole’ Surrounded by accretion disk Emitting jets of plasma at relativistic speeds