Renaissance: Formation of the first light sourcesin the Universe after the Dark Ages

Justin Vandenbroucke, UC Berkeley

Physics 290H, February 12, 2008

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

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

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

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)

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

First star formation from collapsed molecular cloud

Abel et al 2002

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.

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

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

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

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

SMBH formation from dense star clustersinitial state = gas of collisionless particlesrings are spherical light flashes

Final state = BHrings trapped

Shapiro 2004 full GR simulation

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

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