Galaxy Physics

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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)
• _at_ sun spacing 1m
• _at_ nucleus spacing 1cm
• Collisionless
• Average 2-body scattering 1 arcsecond
• Significant after 104 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 _at_ 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 VltVc outside R, VgtVc inside R
• Now consider an ensemble of such orbits

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ltVgt less than Vc
GC
more stars
fewer stars

• Consider stars in rectangle
• Mean velocity ? mean rotation rate (ltVgt)
• Variation about mean ? dispersion (sig)
• In general ltVgt less than Vc
• For larger radial perturbations, ltVgt drops and
  sig increases
• Vc2 ltVgt2 sig2
• This is called asymmetric drift (clearly seen in
  MW stars)
• Extreme cases
• Cold disks ltVgt Vc and sig 0 ? pure
  rotation
• Hot bulges ltVgt 0 and sig Vc ? pure
  dispersion

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• More complete analysis considers
• Distribution function f(v,r)d3v d3r
• This satisfies a continuity equation (stars
  conserved)
• The collisionless Boltzmann equation
• Difficult to solve, so consider average
  quantities
• ltVrgt, ltsiggt, n (density), etc
• This gives the Jeans 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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