Galaxy Physics Mark Whittle University of Virginia.
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Galaxy Physics
Mark Whittle
University of Virginia
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Outline
1. Galaxy basics : scales, components, dynamics
2. Galaxy interactions & star formation
3. Nuclear black holes & activity
4. (Formation of galaxies, clusters, & LSS)
Aim to highlight relevant physics and recent developments
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1. Galaxy Basics
• Scales & constituents
• Components & their morphology
• Internal dynamics
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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
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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
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Galaxy Components
• Nucleus
• Bulge
• Disk
• Halo
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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)
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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
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• 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
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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
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GC
morestars
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
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• 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
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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
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Spiral mergers can make Ellipticals
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During interactions : – Gas loses angular momentum
– Falls to the centre
– Deepens the potential
– Forms stars in starburst
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stars
Gas/SFR
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Enhanced star formation
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Blowout : environmental pollution via superwinds
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Cosmic star formation history
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HDF
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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
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Example #1 : the milky way
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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)
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Pure rotation – gas or cold star disk
isotropic dispersion
anisotropic dispersion
* Gas &/or star disks are best
* Bulge stars are poor, unless isotropy known
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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
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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
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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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