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Cosmology and extragalactic astronomy Mat Page Mullard Space Science Lab, UCL 1. Overview and introduction Slide 2 1. Overview • This lecture: • What is going to be covered in the course • Before we get too extragalactic or cosmological! – Our place in the Galaxy. – Motions and dark matter in the Galaxy. Slide 3 What is to be covered in the course: • Galaxies: – what they are, the different types, clustering – high redshift quasars, giant starbursts and evolution • The expanding Universe: – doppler shifts and Hubble’s law – cosmological redshift and expansion – The cosmic distance ladder • Cosmology: – – – – concepts, principles and competing theories the big bang, nucleosynthesis, inflation the cosmic microwave background dark matter, growth of large scale structure Slide 4 Our place in the Galaxy • For `early*’ Cosmologists our Galaxy was the Universe. • It’s a good place to start our travels. *up until about 100 years ago! Slide 5 Our Galaxy Our galaxy itself contains a hundred billion stars. It’s a hundred thousand light years side to side. It bulges in the middle, sixteen thousand light years thick, But out by us it’s just three thousand light years wide. We’re thirty thousand light years from Galactic centre point. We go round every two hundred million years, And our galaxy is only one of millions of billions In this amazing and expanding Universe. – Monty Python Slide 6 What does our galaxy look like? The word galaxy comes from the greek word for milk Slide 7 But where are we in the Galaxy? • Very hard to see such a lot of the galaxy in such fantastic contrast as the last picture (especially today from London..) • Herschel and Kapteyn tried to determine where we are in the Galaxy from star counts. Slide 8 They decided that we are in the middle of the galaxy! William Herschel’s star map • They saw the same number and brightness of stars in all directions! Slide 9 They were wrong! • Interstellar dust in the Galactic plane absorbs light. – the uniformity is because we can see about the same distance in all directions through the plane. Slide 10 So where are we? • Harlow Shapley (1920) • Looked out of the plane at globular clusters. • Apparent magnitude of standard candle gives distance (inverse square law) • Globular clusters orbiting the galaxy • RR Lyrae variables all ~100 LO • Periods < 1 day. • Standard candle! gives distance (inverse square law) Slide 11 So where are we? But Shapley was off by a factor of 2 because of extinction by interstellar dust -(Still pretty good!) Slide 12 The Milky Way in the infrared IRAS at 100 mm: emission from dust concentrated in plane. Cobe near IR: fairly transparent to dust –starlight Slide 13 What about spiral structure? • Need to see through the dust. • Need to locate material along the line of sight. • 21cm line from spin-flip in atomic hydrogen is ideal to trace cool gas Slide 14 Mapping the arms in 21cm Use velocity in the spectral line to separate the arms. Slide 15 The observed spiral structure Slide 16 But what are the spiral arms? Are they specific material rotating differentially? Well, we know how fast they are rotating, so lets see what happens… Slide 17 But what are the spiral arms? Are they specific material rotating differentially? Well, we know how fast they are rotating, so lets see what happens… Slide 18 But what are the spiral arms? Are they specific material rotating differentially? Well, we know how fast they are rotating, so lets see what happens… Slide 19 But what are the spiral arms? Are they specific material rotating differentially? Well, we know how fast they are rotating, so lets see what So the arms would be all wound up happens… after just a few orbits! Slide 20 Density wave model So the arms must be moving slower than the actual material! Best model is that they are density waves in the disc. Fast moving gas and dust arrives at a region of higher density. The gas is slowed and compressed causing star formation. Bright new stars move out of dense region and travel on. O and B stars form; they are very luminous but short lived – so mark the positions of the arms. Slide 21 Slide 22 Rotation of the galaxy We can learn something else very important from the motions in spiral galaxies. The stars and gas are orbiting about the mass interior to their orbits. If we measure how the velocity changes with radius we can see the radial distribution of matter. Most of the visible mass is concentrated in the bulge. Slide 23 Observation vs expectation: Slide 24 Idealised motion in the galaxy disc • Assume all the mass M is concentrated in the bulge • Circular motion under gravity (Newton and Kepler): • MTOT=r v2/G v= GMTOT r Slide 25 Idealised motion in the galaxy bulge • Assume uniform density r, spherical bulge. • MINT=4prr3/3=rv2/G • (of course r isn’t constant in reality) v = r 4prG 3 Slide 26 So there is a problem! • Orbital velocities rise OK but then don’t fall off. • Something wrong – Either gravity not a 1/r2 (!!) – or there is more mass than we can see. – This is “dark matter” Slide 27 Some key points: • The Universe extends well beyond our Galaxy. • Dust can have a profound effect on cosmological observations. • We live in the disc of a spiral galaxy – not at the centre. • Spiral arms must be density waves rather than circulating material. • Our Galaxy contains a lot of dark matter