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Galaxy Physics

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Galaxy Physics Powered By Docstoc
					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,…)


				
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posted:4/23/2008
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