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The Formation of the First Stars and GalaxiesThe Formation of the First Stars and Galaxies
ITA ColloquiumITA ColloquiumApril 2008April 2008
Thomas Thomas GreifGreif
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The Formation of the First Stars and Galaxies ITA Colloquium April 2008
IntroductionIntroductionIntroduction
Collaborators:
• Potsdam: Simon Glover
• Austin: Volker Bromm, Jarrett Johnson
• Lonestar: ~ 5,000 CPU‘s
• Numerical simulations of cosmological structure formation with GADGET
What do we do?
• Heidelberg: Paul Clark, Ralf Klessen
Resources:
• Ranger: ~ 50,000 CPU‘s, world‘s largest open science system
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First Galaxies: z ~ 10
Cosmic TimelineCosmic TimelineCosmic Timeline
First Stars (Pop III): z ~ 20
Larson & Bromm 01
Dark Ages
CMB observations
“Direct” observations
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Review: First StarsReview: First StarsReview: First Stars
How do the first stars form?
• Primordial quantum fluctuations grow over time
• Dark matter clumps decouple from the Hubble flow once their overdensity approaches unity
• Formation of minihaloeswith Mvir ~ 105 - 106 Mo
Larson & Bromm 01
non-linear
linear
collapsed
moves up with time
critical overdensity
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Review: First StarsReview: First StarsReview: First Stars
What does the gas do?
• Jeans criterion prevents gas from settling into haloes below ~ 105 Mo
• Once densities become high enough: formation of H2 and onset of cooling
• Gas initially follows the potential set by the dark matter
Adiabatic heatingCooling
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Review: First StarsReview: First StarsReview: First Stars
The final phases:
• Extremely efficient accretion:dM/dt α cs
3 α T3/2
• Present-day universe: ~ 10 K
• Accretion rate ~ 100 times higher!
• Formation of a single protostellar core at very high densities
dN
/dlo
gM
log M
Pop III stars: ~ 100 Mo
• High-redshift universe: ~ 200 K
~ 1 Mo
Pop I/II
Mchar
~ 100 Mo
Pop III
IMF?
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Review: First StarsReview: First StarsReview: First Stars
Caveats:
• On main sequence: pulsations, winds
• Magnetic fields
• Protostellar accretion phase:
• Radiative feedback on parent cloud
• Disk formation
• Fragmentation?
• Low-mass primordial stars?
Still many open questions!
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Review: First StarsReview: First StarsReview: First Stars
Simulation setup:
• Box Size: 150 kpc (comoving)
• Collapse of first star-forming minihalo at z ~ 25 (~ 106 Mo)
• Fully cosmological initial conditions at z = 100
Result:
• Further evolution?
Questions:
• Stellar feedback?
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Radiative FeedbackRadiativeRadiativeFeedbackFeedback
• Ionizing flux ~ 100 times higher than for normal Pop I/II stars
• HII regions extend out to a few kpc
• Pop III stars have surface temperatures around 105 K
80 Mo
200 Mo
~ 10 kpc (physical)
Alvarez, Bromm & Shapiro 06
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Radiative FeedbackRadiativeRadiativeFeedbackFeedback
• Multiple HII regions
• Large Box: 500 kpc (comoving)
Johnson, Greif & Bromm 07
Density Temperature
e- fractionH2 fraction
Recent work:
• Inclusion of Lyman-Werner radiation
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Radiative FeedbackRadiativeRadiativeFeedbackFeedback
• But: rapid reformation of molecules in relic HII regions
Johnson, Greif & Bromm 07
Crucial:
H2 fraction
• Initially radiation destroys molecules
• Cooling to the CMB
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Radiative FeedbackRadiativeRadiativeFeedbackFeedback
Johnson, Greif & Bromm 07
The Formation of the First Stars and Galaxies ITA Colloquium April 2008
In 3D:
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Radiative FeedbackRadiativeRadiativeFeedbackFeedback
• Relic HII region gas likely forms less massive (~ 10 Mo) primordial stars
Implications:
• Two populations of metal-free stars:
• Truly primordial: Pop III.1 with ~ 100 Mo
• Previous ionization: Pop III.2 with ~ 10 Mo
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Fate of Pop III StarsFate of Pop III StarsFate of Pop III Stars
Heger et al. 03
Pop III star
Pop I/II star
The Formation of the First Stars and Galaxies HGSFP April 2008
PISN10 Mo 100 Mo MassBH BH
Metallicity
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PISNPISNPISN
• Metal yields of order 50%
• Extremely violent explosion: up to 1053 ergs of kinetic energy
Pair-instability supernova:
• Profound impact in terms of dynamics and chemical enrichment
Goal:
• Simulate PISN in cosmological context!
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Supernova ExplosionSupernova ExplosionSupernova Explosion
• Inject 1052 ergs of kinetic energy
PISN Simulation Setup:
• Form Pop III star
• Create HII region
• Cosmological initial conditions
• Box size: 200 kpc (comoving)
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Supernova ExplosionSupernova ExplosionSupernova Explosion
The Simulation:
The Formation of the First Stars and Galaxies ITA Colloquium April 2008
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Supernova ExplosionSupernova ExplosionSupernova Explosion
The Simulation:
The Formation of the First Stars and Galaxies ITA Colloquium April 2008
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Supernova ExplosionSupernova ExplosionSupernova Explosion
In 3D:
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Supernova ExplosionSupernova ExplosionSupernova Explosion
Shock radius: Free expansionSedov-Taylor blastwavePressure-driven snowplowMomentum-conserving snowplow
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Supernova ExplosionSupernova ExplosionSupernova Explosion
Swept-up mass: Total swept-up mass: 2.5×105 Mo
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Supernova ExplosionSupernova ExplosionSupernova Explosion
Distribution of metals:
• Metals are generally expelled into the voids
• Larger haloes must assemble to recollect the expelled metals
• Nest step: simulate the formation of a first galaxy!
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First GalaxiesFirst GalaxiesFirst Galaxies
Definition of a first galaxy:
• Virial temperature exceeds ~ 104 K or virial mass exceeds ~ 5×107 Mo
• Photoheated gas is retained
• Self-regulated star formation
Remember: onset of atomic hydrogen cooling at ~ 104 K!
Why does this make sense?
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First GalaxiesFirst GalaxiesFirst Galaxies
Goal:
• Push simulations to Mvir ~ 5×107 Mo (i.e. to z ~ 10)
• Neglect radiative and supernova-driven feedback
• Concentrate on consequences of halo collapse on baryonic physics
First step:
• Use primordial chemistry
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First GalaxiesFirst GalaxiesFirst Galaxies
Simulation setup:
• Cosmological initial conditions
• Box Size: 700 kpc (comoving)
• Create ~ 5×107 Mo halo
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First GalaxiesFirst GalaxiesFirst Galaxies
Star formation:
• Individual Pop III stars form, denoted by sink particles (i.e. black holes)
• Of order 10 Pop III stars form prior to the assembly of the galaxy
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First GalaxiesFirst GalaxiesFirst Galaxies
Virial heating:
• Virial temperature gradually increases to ~ 104 K
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Chemistry and CoolingChemistry and CoolingChemistry and Cooling
Temperature:
• Adiabatic heating to virial temperature followed by onset of H2 cooling
• Isothermality at ~ 104 K due to atomic hydrogen cooling
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Chemistry and CoolingChemistry and CoolingChemistry and Cooling
Electron fraction:
• Elevated electron fraction once virial temperature approaches ~ 104 K
• Residual ionization
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Chemistry and CoolingChemistry and CoolingChemistry and Cooling
Molecule fractions:
• Elevated H2 and HD abundances
• Second cooling channel
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Chemistry and CoolingChemistry and CoolingChemistry and Cooling
Evolution in phase space:
• Cooling to the CMB?
• Formation of Pop III.2 stars?
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TurbulenceTurbulenceTurbulence
Hot Accretion:
• Accretion of hot gas through the virial shock
• Accretion of cold gas via filaments
Cold Accretion:
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TurbulenceTurbulenceTurbulence
Consequences:
• Cold accretion generates turbulence
• Transitory density perturbations arise that may become Jeans-unstable
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TurbulenceTurbulenceTurbulence
In 3D:
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TurbulenceTurbulenceTurbulence
Energy content:
• Dominated by kinetic energy
The Formation of the First Stars and Galaxies ITA Colloquium April 2008
• Dominated by thermal energy
In minihalo:
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TurbulenceTurbulenceTurbulence
High mach numbers:
• Of order 10 in filaments
• Of order 3 at the center
• Formation of shocks!
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TurbulenceTurbulenceTurbulence
Shocks:
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TurbulenceTurbulenceTurbulence
Implications:
• First turbulence in first galaxies!
• Fundamental difference to minihaloes
• Signs of transition to present-day star formation?
Future Work:
• Add metal advection and cooling (in progress)
• Redo supernova simulation in the context of a first galaxy
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SummarySummarySummary
• They exert strong radiative and supernova-driven feedback
• Possible existence of two physically distinct populations of metal-free stars:
• Generation of turbulence in the first galaxies by cold accretion
• Truly primordial: Pop III.1 with ~ 100 Mo
• Previous ionization: Pop III.2 with ~ 10 Mo
The Formation of the First Stars and Galaxies ITA Colloquium April 2008
• Pop III stars form in dark matter minihaloes with ~ 105 - 106 Mo
• Transition to present-day star formation?