GENS X1Galaxies1 Lecture 5: Galaxies Dr Michael Burton

GENS X1Galaxies2 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)

GENS X1Galaxies3 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

GENS X1Galaxies4 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

GENS X1Galaxies5 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 –71  4 kilometres per second per megaparsec (megaparsec = million parsecs = 3 million light years) –1 / H 0 ~ age of Universe (13.7  0.2 billion years)

GENS X1Galaxies6 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.

GENS X1Galaxies7 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.

GENS X1Galaxies8 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??

GENS X1Galaxies9 Quasars and Active Galactic Nuclei Central engine – a ‘supermassive black hole’ Surrounded by accretion disk Emitting jets of plasma at relativistic speeds

GENS X1Galaxies10 The Story of the Universe MAP satellite measures the Cosmic Microwave Background Radiation Pins down tightly several parameters that describe the nature of our Universe The Universe is 13.7  0.2 billion years old The Universe will expand for ever Ordinary matter only makes up 4% of it Dark Matter is 23%, Dark Energy is 73%