galaxy physics

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Galaxy Physics: 

Galaxy Physics Mark Whittle University of Virginia

Outline : 

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

1. Galaxy Basics : 

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

Galaxies are huge : 

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 : 

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 : 

Galaxy Components Nucleus Bulge Disk Halo

Bulges & disks : 

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: 

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: 

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


GC more stars fewer stars <V> less than Vc 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: 

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: 

Enhanced star formation

Blowout : environmental pollution via superwinds: 

Blowout : environmental pollution via superwinds

Cosmic star formation history: 

Cosmic star formation history



3. Nuclear Black Holes & Activity: 

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: 

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 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: 

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,…)

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