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Galaxy Physics

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  1. Galaxy Physics Mark Whittle University of Virginia

  2. Outline • Galaxy basics : scales, components, dynamics • Galaxy interactions & star formation • Nuclear black holes & activity • (Formation of galaxies, clusters, & LSS) Aim to highlight relevant physics and recent developments

  3. 1. Galaxy Basics • Scales & constituents • Components & their morphology • Internal dynamics

  4. Galaxies are huge • Solar sys = salt crystal • Galaxy = Sydney • Very empty • Sun size = virus (micron) • @ sun : spacing = 1m • @ nucleus : spacing = 1cm • Collisionless • Average 2-body scattering ~ 1 arcsecond • Significant after 10^4 orbits = 100 x age of universe • Stars see a smooth potential

  5. Constituents • Dark matter • Dominates on largest scales • Non-baryonic & collisionless • Stars • About 10% of total mass • Dominates luminous part • Gas • About 10% of star mass • Collisional  lose energy by radiation • Can settle to bottom of potential and make stars • Disk plane : gas creates disk stars (“cold” with small scale height) • Nucleus/bulge : generates deep & steep potentials • Historically ALL stars formed from gas, so behaviour important

  6. Nucleus Bulge Disk Halo Galaxy Components

  7. Bulges & disks • Radically different components • Ratio spread ( E – S0 – Sa – Sb – Sc – Sd ) • Concentrations differ (compact vs extended) • Dynamics differ (dispersion vs rotation) • Different histories (earlier vs later)

  8. Disks : Spiral Structure • Disk stars are on nearly circular orbits • Circular orbit, radius R, angular frequency omega • Small radial kick  oscillation, frequency kappa • View as retrograde epicycle superposed on circle • Usually, kappa = 1 – 2 omega  orbits not closed • (Keplerian exception : kappa = omega  ellipse with GC @ focus) • Near the sun : omega/kappa = 27/37 km/s/kpc • Consider frame rotating at omega – kappa/2 • orbit closes and is ellipse with GC at centre • Consider many such orbits, with PA varying with R

  9. Depending on the phase one gets bars or spirals • These are kinematic density waves • They are patterns resulting from orbit crowding • They are generated by : • Tides from passing neighbour • Bars and/or oval distortions • They can even self-generate (QSSS density wave) • Amplify when pass through centre (swing amplification) • Gas response is severe  shocks  star formation

  10. Disk & Bulge Dynamics • Both are self gravitating systems • Disks are rotationally supported (dynamically cold) • Bulges are dispersion supported (dynamically hot) • Two extremes along a continuum • Rotation  asymmetric drift  dispersion • What does all this mean ? • Consider circular orbit, radius R speed Vc • Small radial kick  radial oscillation (epicycle) • Orbit speeds : V<Vc outside R, V>Vc inside R • Now consider an ensemble of such orbits

  11. <V> less than Vc GC more stars fewer stars • Consider stars in rectangle • Mean velocity  mean rotation rate (<V>) • Variation about mean  dispersion (sig) • In general <V> less than Vc • For larger radial perturbations, <V> drops and sig increases • Vc^2 ~ <V>^2 + sig^2 • This is called asymmetric drift(clearly seen in MW stars) • Extreme cases : • Cold disks <V> = Vc and sig = 0  pure rotation • Hot bulges <V> = 0 and sig ~ Vc  pure dispersion

  12. More complete analysis considers : • Distribution function = f(v,r)d^3v d^3r • This satisfies a continuity equation (stars conserved) • The collisionless Boltzmann equation • Difficult to solve, so consider average quantities • <Vr>, <sig>, n (density), etc • This gives the Jean’s Equation (in spherical coordinates) • Which mirrors the equation of hydrostatic support : dp/dr + anisotropic correction + centrifugal correction = Fgrav • Hence, we speak of stellar hydrodynamics

  13. 2. Interactions & Mergers • Generate bulges (spiral + spiral = elliptical) • Gas goes to the centre (loses AM) • Intense star formation (starbursts) • Supernova driven superwinds • Chemical pollution of environment • Cosmic star formation history

  14. Spiral mergers can make Ellipticals

  15. During interactions : • Gas loses angular momentum • Falls to the centre • Deepens the potential • Forms stars in starburst

  16. stars Gas/SFR

  17. Enhanced star formation

  18. Blowout : environmental pollution via superwinds