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Galaxy Physics Mark Whittle University of Virginia Outline 1. 2. 3. 4. 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 1. Galaxy Basics • Scales & constituents • Components & their morphology • Internal dynamics 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 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 Galaxy Components • • • • Nucleus Bulge Disk Halo 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) 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 • • • • 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 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 <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 • 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 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 Spiral mergers can make Ellipticals During interactions : – Gas loses angular momentum – Falls to the centre – Deepens the potential – Forms stars in starburst stars Gas/SFR Enhanced star formation Blowout : environmental pollution via superwinds Cosmic star formation history HDF 3. Nuclear Black Holes & Activity • • • • • • Difficulties & methods Example #1 : the milky way Other examples : gas, stars, masers Black hole demographics – links to the bulge Black hole accretion : nuclear activity Cosmic evolution – ties to mergers and SF Example #1 : the milky way Other galaxies : methods • Need tracer of near-nuclear velocity field – Defines potential M(r) – If more than M(stars) dark mass present • Obvious tracers : stars and/or gas – Doppler velocities (proper motions) – Note : both rotation &/or dispersion present – Use Jeans Equation M(r) Pure rotation – gas or cold star disk isotropic dispersion anisotropic dispersion * Gas &/or star disks are best * Bulge stars are poor, unless isotropy known Activity : accretion onto the BH • Gravitational energy near Rs ~ 50% rest mass • Accretion requires AM loss : MHD torques • Energy liberated as photons & bulk flow – Luminous across the EM spectrum – Powerful outflows, some at relativistic speeds • Accretion associated with galaxy interactions • ? Black hole formation associated with mergers ? • Quasar history linked to merger/SFR history Quasar and Galaxy Evolution • Quasar/Starburst/Galaxy evolution related ? • Major mergers – Extreme star formation rates – Elliptical/bulge formation – BH formation and feeding = QSO • Evidence – Comparable luminosity in QSO and starburst – Most luminous nearby mergers are also QSOs – QSO evolution loosely follows SFR history • Currently speculative – active area of research 4. Galaxy Formation Theory • Mature subject – semi-analytic & numerical • Two important observational constraints – Galaxy luminosity function (many small, few large) – Galaxy large scale structure (clusters, walls, voids) • Start with uniform DM (+ baryon) distribution – Add perturbations matched to CMB – Embed in comoving expansion & add gravity • Follow growth of perturbations : linear – non-linear – Semi-analytic useful but limited – Numerical follows full non-linear development + mergers – Baryon physics recently included (pressure, cooling, SF,…)