Transcript Galaxy Physics - TAPIR at Caltech

Galaxy Physics
Mark Whittle
University of Virginia
Outline
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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
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Nucleus
Bulge
Disk
Halo
Bulges & disks
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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
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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 :
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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
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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
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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
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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,…)