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Renaissance: Formation of the first light sourcesin the Universe after the Dark Ages
Justin Vandenbroucke, UC Berkeley
Physics 290H, February 12, 2008
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Timeline of light and dark
CMB
z ~ 1000: Last Scattering Surface
Dark Ages
z ~ 6-30: first stars and quasars,reionization begins
Reionizaton complete
neutral, darknot observedtheory + sim’s
ionized, lightobserved
ionized, lightobserved
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Were the first light sources quasars (SMBH) or stars?
• Quasars most distant observed objects• But much brighter than stars: observational bias• Moreover, first stars would have died by now
(lifetime ~1 Myr)• Indeed, no stars observed with low enough
metallicity• WMAP: reionization began z > ~15; farthest
quasars z ~6 so stars must have come first?• Standard model:
– First stars z = 20-30– First quasars z = 6-10
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First star formation follows naturally from cosmology
z = 17
Structure formation simulation:
50 kpc field
First stars will form at intersections (bright knots)
Bromm 2004
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First stars formation
• Easy initial conditions from cosmology + structure formation– no metals: only light elements from Big Bang
Nucleosynthesis– therefore no dust– no radiation or wind from other stars– no ambient B fields
• Full 3D simulations have been done: adaptive mesh refinement (AMR) or smoothed particle hydrodynamics (SPH)
• Both methods agree! For formation, characteristic T ~ 200 K, n ~104 / cm3
• Formation in most massive of the DM halo mass distribution (~106 Msun)
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Example simulation of first star(Abel, Bryan & Norman 2002)
• Assume initial conditions from =0 CDM cosmology• Adaptive mesh refinement (AMR) over 10 orders of magnitude from
cosmological to stellar• ~100 kpc to ~1 AU• First allow cosmological structure formation (hierarchical merging) to
z~20• Gas cools, sinks into DM well until self gravitating• Dense core ~100 Msun contracts• Analytically, expected fragmentation (problem!) but in full simulation,
no fragmentation• Halos are too cold to collapse by atomic H radiation• Must cool by molecular H radiation• Need enough molecular H, formed by electron collisions when
density is sufficient
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First star formation from collapsed molecular cloud
Abel et al 2002
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First stars from collapse of molecular cloud:Simulation by another group (Bate, Bonnell & Bromm)
http://www.ukaff.ac.uk/starcluster/cluster1mre.aviMovie:
QuickTime™ and aBMP decompressor
are needed to see this picture.
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When did first supermassive black holes form?
• Quasars observed to z = 6 first ones must have formed in first Gyr
• Challenge: how can you build such massive objects so quickly?
• Hard to form unless stars already present (more later)
• Formation likely started at z ~ 10
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Possible seeds for SMBH’s
1) Direct collapse from molecular gas
2) Dense clusters of normal stars
3) Relativistic clusters of collisionless particles/stars
4) Self interacting dark matter halos
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SMBH from direct collapse of gas cloud
• Need to suppress star formation in cloud because SNe increase kinetic energy
• Need to cool via atomic H and not molecular H, due to the temperature of these clouds
• Possible if stars have already formed! (in other places, not the cloud)
• Then UV light dissociates molecular H
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SMBH from collapse of star clustersstars evolve into mass segregation
time
radi
us in
clu
ster
heavy stars migrate in
light stars migrate out
Rasio et al. 2003
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SMBH formation from dense star clustersinitial state = gas of collisionless particlesrings are spherical light flashes
Final state = BHrings trapped
Shapiro 2004 full GR simulation
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SMBH from collapse of star clustersby runaway collisions
Condition: cluster must collapse before its massive stars do
initial central density
velo
city
dis
pers
ion
tota
l mas
s
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Conclusions• First star and quasar formation limited to theory
and simulation (with some observational constraints)
• Standard model:– stars z ~ 20-30– quasars z ~ 6-10
• First star formation a natural outcome of standard cosmological structure formation
• Detailed numerical simulations using very different methods agree
• First SMBH formation a more difficult problem• Direct collapse from molecular clouds vs. collapse
of star clusters