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AS1001:Extra-Galactic Astronomy Stars and Gas in Galaxies

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AS1001:Extra-Galactic Astronomy Stars and Gas in Galaxies Stars and Gas in Galaxies AS1001:Extra-Galactic Astronomy • Stars are born from gas in high-density regions. • Compressing gas (e.g. in collisions, or spiral arms) triggers gravitational collapse to form stars. Lecture 5: Dark Matter – Ellipticals: very little gas => M GAS ~ 0.01" 0.1 • ~ all gas converted into stars M STARS M – Spirals: some gas => GAS ~ 0.1"1.0 • most gas converted MSTARS MGAS – Irregulars: lots of gas => >1.0 • little gas converted MSTARS ! Distribution of Gas and Stars Dark Matter Gas extends well outside the star distribution. • STARS + GAS account for only M82 ~10% of a galaxy’s total mass. • The rest is DARK MATTER. • The orbit velocity of the STARS + GAS M81 is too large - they should fly away! • Not enough gravity to hold the galaxy together, unless there is DARK MATTER (or unless our theory of gravity is wrong). NGC 3077 Starlight Gas (H I) • Lets examine the evidence … λ = 21 cm radio emission (spin-flip of electron in H I atoms) Spiral Galaxy Rotation Rotational Equilibrium • Gravity force: • Galaxies form via collapse due to gravity. m G M m r • During collapse, rotation increases: F = Conservation of angular momentum : IN r2 v Velocity x Radius = constant – M = mass interior to radius r – !m = mass of the orbiting star (or gas cloud) • Centrifugal force: • In Spiral Galaxies, rotation halts the collapse: m v2 F = GRAVITATIONAL CENTRIFUGAL OUT r FORCE (inward) = FORCE (outward) ! Virial Theorem “Virial Mass” of a Galaxy • The “Virial Theorem” (generalised Kepler’s Law) A star at the edge of a distant galaxy has a velocity applies when a galaxy is in rotational equilibrium: about the galaxy’s centre of 200 km/s. Its distance from the centre of the galaxy is 15 kpc. What is F = F m IN OUT r the mass of the galaxy ? 2 G M m m v v 2 = G M r r v = r G M 5 2 4 16 v = v2r 2 "10 m/s " 1.5 "10 pc " 3"10 m/pc r M = = ( ) ( ) ( ) G (6.67 "10#11m3kg-1s-2 ) • Observe: v = velocity of rotation at radius r. 41 Calculate: M = mass interior to r. 2.7 "10 kg 11 = 10 =1.2 "10 M$ 2.0 "10 kg/M$ ! ! The Mass Distribution M(r) Measuring Rotation Curves • Stars are centrally concentrated. Take spectra at different locations in the galaxy • Do stars trace the mass ? Flux • If so, then stars at the edge should “feel” Doppler shift gives the velocity difference almost all the mass : between the centre and G M the edge of the galaxy. v = r Δλ λ A B "# v(r) = c M M , so r r v v #BULGE • If stars trace mass: A " B A > B # A < B ! Test: Measure v as a function of r => “Rotation curve” λ = 21 cm radio emission line from H I (neutral Hydrogen) is used to measure velocity of gas outside the star distribution. ! ! Rotation Curves Galaxy Rotation Curves are Flat • Stars (hence mass?) VELOCITY centrally concentrated, B A HI velocities VelocitiesV2 = G M(from r ) / Hr I gas -0.5 flat rotation so expect: v ∼ r M( r ) = V2 r / G curves outside the stars. G M Dark Matter v = Haloes r RADIUS VELOCITY VELOCITY • Measured rotation ! curves are flat B A M/L~30 overall (outside the stellar M/L~100 outskirts distribution). RADIUS Stars mainly inside => Additional Mass Component 10 kpc Flat Rotation Curves Implication for Dark Matter • At large radii: 2 G M v = = constant r • Hence mass is proportional to radius: ! v2r M = " r G • Density proportional to 1/ r2 M r 1 ! " = # # Volume r3 r2 • => a large spherical “halo” of Dark Matter. ! Revised Galaxy Model Dark Matter in Galaxy Clusters GLOBULAR CLUSTERS H I GAS DISK • Found by Fritz Zwicky (1930s). • Pre-dates rotation curve observations and COMPANIONS analysis (1975). • Galaxies in clusters have very large observed velocities ( v ~ 1000 km/s ). • Galaxy clusters should be unbound! • But clusters ARE bound, so more mass must be present than the luminous matter. DARK MATTER HALO • Dark Matter needed to bind galaxy clusters. STELLAR DISK BULGE Dark Matter in Galaxy Clusters Gravitational Lensing 200 Kpc • Luminous arcs seen in galaxy clusters. • Multiple images of some quasars. • Background sources are magnified and distorted by gravitational lensing as the light passes through an intervening galaxy or cluster of galaxies. Chandra X-ray Image of Abell 2029 The galaxy cluster Abell 2029: thousands of galaxies enveloped in a gigantic cloud of hot T~107 K gas. Bound by Dark Matter equivalent to 1014 Suns. The enormous elliptical galaxy at the center has formed by mergers of many smaller galaxies. Lensing by a point mass Masses from Gravitational Lensing Light from background source deflected by lens mass 1 Perfect alignment gives an Einstein Ring. Imperfect alignment = Luminous arc 2 Angular size of the Einstein Ring: " %1/2 4 G M DS ! DL !E = $ 2 ' # c DL DS & 2 ! $ Two distorted/magnified images of background source Mass of the Lens: M ! !E $ DL DS 11 = # & # & 10 M SUN "1 arcsec% "(1 Gpc) (DS ' DL )% 1 2 D = Distance to the Lens Observer’s view: L DS = Distance to the background Source Einstein ring AS 4022 Cosmology AS 4022 Cosmology Summary: DARK MATTER candidates • Large spiral galaxies have flat rotation curves. • Normal (i.e., Baryonic) • Stars do not trace the mass. – Ionised gas – Cold dust • Stars are a minor mass component, about 10%. Ruled out by • DARK MATTER is needed to hold galaxies – Planets observations. (and clusters of galaxies) together. – White dwarfs – Black Holes • Dark Matter forms a large halo with density – MACHOS (Massive Compact Halo Objects) falling as 1/(radius)2 • Exotic (i.e., non-Baryonic) • Alternatively, our theory of Gravity may be wrong. – WIMPS (Weakly Interacting Massive Particles) – Neutrinos The Large Hadron Collider is hunting for WIMPS. MACHO survey using LMC stars Microlens Surveys rule out MACHOs LMC stars would be lensed by MACHO contribution < 5-10 % MACHOs in the Milky Way’s brightness vs time Dark Matter Halo. MACHOs predicted to magnify dozens of LMC stars each year. Only a 1 or 2 are seen. MACHO contribution to Dark Matter Halo Slow events -> high mass MACHO mass AS 4022 Cosmology Fast events -> low mass AS 4022 Cosmology Alternative Gravity Theory ? • Is our theory of gravity wrong ? • Newtonian gravity failed to explain all solar system observations (e.g., Mercury’s orbit precesses too fast). • Einstein’s General Relativity improved on Newton, but is now failing to explain how galaxies rotate … • Will an observational breakthrough “discover” Dark Matter? • Or will a convincing alternative theory of gravity emerge? .
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