Feedback Effects of the First Stars on Nearby Halos Kyungjin Ahn The University of Texas at Austin The End of the Dark Ages STSCI March 13, 2006

Outline Introduction Code Description Initial Setup Result Summary/Conclusion

Dark Ages and Reionization End of dark ages – reionization – is only observed indirectly  WMAP 1st year result : Need for high-redshift reionization sources  Gunn-Peterson Effect  Ly Forest Temperature First Stars  Prime candidates for early reionization sources  Forms by H 2 cooling  Feedback effects may be self-regulating (e.g. Haiman, Abel, Rees 2000)

Feedback effects of the First Stars Feedback Effects (positive vs. negative for further star formation)  Negative H 2 is fragile: dissociation by Lyman-Werner bands (Haiman, Abel, Rees 2000; Machacek, Bryan, Abel 2001)  Positive Hard photons partially ionize IGM to create H 2 (Haiman, Rees, Loeb 1996; Ricotti, Gnedin, Shull 2002)

Feedback effects of the First Stars Feedback Effects of the First Stars onto Nearby Collapsed Objects (study by 3-D simulations)  O'shea et al. (2005) Assume full ionization of nearby halos of M~5*10 5 M solar Quick formation of H 2 after source dies Inner core collapses; Outer region evaporates  Alvarez, Bromm, Shapiro (2005) Track I-front propagation through nearby halos of M~5*10 5 M solar I-front slows down, forms D-type front I-front fails to reach the center: Center remains neutral Neutral center: no further formation of H 2 after source dies at the center Negative feedback then??

Feedback effects of the First Stars Alvarez, Bromm, Shapiro (2005) Fig. 8.— Volume visualization at z = 20 of neutral density field (blue – low density, red – high density) and I- front (translucent white surface). Top row panels show a cubic volume ∼ 13.6 kpc (proper) across, middle row ∼ 6.8 kpc, and bottom row ∼ 3.4 kpc. Left column is at the initial time, middle column shows simulation at t ∗ = 3 Myr for the run with stellar mass M ∗ = 80M ⊙, and the right column shows simulation at t ∗ = 2.2 Myr for the run with stellar mass M ∗ = 200M ⊙. The empty black region in the lower panels of middle and right columns indicates fully ionized gas around the source, and is fully revealed as the volume visualized shrinks to exclude the I-front that obscures this region in the larger volumes above.

Feedback effects of the First Stars Feedback Effects of the First Stars onto Nearby Collapsed Objects (study by 1-D simulation)  Use 1-D radiation, hydrodynamics code Full treatment of primordial chemistry, radiative transfer, cooling/heating, hydrodynamics 1-D spherical geometry  Ultra high resolution possible  Wide dynamic range covered  Follow I-front propagation of the radiation from outer source in detail Is radiation trapped? Does ionized gas evaporate? What happens to the center? Any H 2 formation/dissociation interesting? Is it positive or negative feedback effect?

1-D Spherical, Radiation-Hydro Code Radiative transfer  inner point souce: radial only, easy. (ex) one star at the center of halo  outer isotropic source: 3-D type ray tracing. Practical. (ex) background radiation field  outer plane-parallel source (with care). (ex) source far from halo.  N of frequency bins: >~200 bins are OK.  Optical depth calculated for each frequency bin.  Radiative rates calculated for each frequency bin.  H 2 self-shielding: special and simple treatment by Draine & Bertoldi (optical depth ~ 1 at n HI ~10 14 cm -2 )

1-D Spherical, Radiation-Hydro Code Radiative transfer for outer isotropic source  7 discrete ordinates for angle, using Gaussian quadrature (with N Gaussian, 2N Simpson achieved)

1-D Spherical, Radiation-Hydro Code Gravity  Dark matter: use fluid approximation. Better than radial shells. (NFW-type density structure reproduced). Ocasionally frozen gravity is not a bad approximation.  Baryon: Gravity involved hydrodynamics. Chemistry  Solve primordial chemistry, neglecting HD and HLi. H, H -, H +, H 2, He, He +, He ++, e  ionization, dissociation, recombination Cooling/Heating  excitation, recombination, free-free, H 2  photoheating  adiabatic compression/rarefaction

Initial Setup Experiment 1 (Artificial)  Test O'shea et al. result Fully ionize the target halo without disturbing the structure Let it evolve without source Experiment 2 (Realistic)  Start out with a halo profile (TIS profile)  Abundance of electron and H 2 molecule with equilibrium value of a given halo: Departs from primodial values x e =10 - 4, x H2 =2*10 -6  Send plane-parallel, black-body radiation from outside 120M solar, 10 5 K, L solar, tstar=2.5 Myr  Place target at different locations: 180, 360, 540, 1000 pc

Initial Setup Experiment 2 (Realistic)

Result: Experiment 1 Fully-ionized gas quickly forms H 2 Core region cools quickly Outer region gains momentum from initial high temperature

Result: Experiment 2 (Before t * )

Result: Experiment 2 (After t * )

Result: Experiment 2 I-front trapped D-type front Different linetypes for different distances (with M target =2e5 M sun ). Snapshot at the end of the lifetime of the source.

Result: Experiment 2 Temperature  ionized, T~10 4 K  compressed, T~10 3 K  unperturbed, T~ 300 K H 2 shell formation  partial ionization ahead of I-front  T~ 100 K

H 2 Shell Forms through electrons in the partially ionized region Gains substantial column density: Self- shields against dissociating photons! (Threshold N H2 ~10 14 cm -2 ) Neutral region sees weekned dissociating photons. Helps cool the gas against shock-heating

H 2 Shell Structure Ricotti, Gnedin, Shull 2001 Our result

Fate of neutral region Size is determined by the ionization trapping radius. Competition between shock-heating and H 2 cooling determines collapse / disruption Higher the halo mass, more cooling.

Fate of ionized region After source dies, H 2 is formed through abundant electrons remaining Keeps evaporating because escape velocity has been achieved. Exodus.

Conclusion 2 nd Generation, Pop III star formation  Minihalos (target) nearby the first Stars (source)  I-front trapped; Ionized gas evaporates  H 2 formation in evaporating gas doesn’t help, just evaporates  H 2 shell forms ahead of I-front (Robust): interior neutral region shielded; active cooling.  Shock is driven to the neutral region: active heating  Competition between H 2 cooling & shock-heating determines the fate of neutral region. Higher the mass, more efficient the cooling -> critical mass for 2 nd generation stars.  Future work More parameter space search. Subsequent star formation? Ionization photon budget? Higher Pop III star formation rate?