radiative feedback effects of the first objects in the early universe kyungjin ahn the university of...

Radiative Feedback Effects of the

First Objects in the Early Universe

Kyungjin AhnThe University of Texas at Austin

East-Asia Numerical Astrophysics MeetingKorean Astronomy and Space Science Institute

Nov. 1, 2006

Outline

No ionizing sources (Dark Ages) – 21cm radiation from minihalos and IGM

First ionizing sources – Radiative feedback from a first star on nearby minihalos

Late ionizing sources – UV Radiation background in the early universe

21cm Background from Cosmic Dark Ages (Shapiro, Ahn, Alvarez, Iliev, Martel, Ryu 2006 ApJL, 646, 681) Before Reionization - Cosmic Dark Ages

No appreciable light sources Feeble 21cm radiation from neutral hydrogen Hydrogen spin temperature decoupled from the CMB

temperature by Ly pumping and/or collisions: 21cm emission/absorption against CMB

;

With no radiation, collisional coupling inside minihalos and IGM is the only way for 21cm signal to be observed

Run cosmological N-body/hydro(TVD) simulation to quantify this signal: (500 h-1 Mpc)3 comoving, 10243 grid, 5123 dark matter particles

Summary z<~20

mean IGM in absorption (T<0) z>~20

minihalos start to emerge IGM clumping becomes significant in emission (T>0; T ~ a few mK) emission dominated by minihalos

Outline


First ionizing sources – Radiative feedback from a first Pop III star on nearby minihalos


Pop III Star Formation Gas in minihalos (104 <M/Msun <108) can cool by H2

cooling and form Pop III stars Numerical LCDM simulations predict first Pop III stars

formed in minihalos with M ~ 106 Msun @ z > ~20 (e.g. Bromm, Coppi, Larson 2002; Abel, Bryan, Norman 2002)

However, H2 abundance is strongly affected by radiation. Negative feedback: Photo-dissociation suppresses H2

formation; photo-heating evaporation Positive feedback: Ionization enhanced H2 formation

Pop III star is (believed to be) massive, energetic, short-lived. (Maybe bimodal: Nakamura & Umemura 2001)

First H II Region by a Pop III Star: I-front trapping by minihalos Alvarez, Bromm, Shapiro

(2006) cosmological SPH simulation

+ I-front tracking I-front from a Pop III star

being trapped by nearby minihalos Minihalos are NOT flash-ionized!

Further study required for the fate of the neutral core

Radiative Feedback of a Pop III star on nearby Halos Flash-Ionized minihalos?

O'shea, Abel, Whalen & Norman (2005); Mesinger, Bryan, Haiman (2006)

Assume full ionization of nearby halos of M~5*105 Msolar

Quick formation of H2

Inner core collapses; Outer region evaporates

Radiative Feedback effects of the First Stars(Ahn & Shapiro 2006, MNRAS submitted, astro-ph/0607642)

Radiative Feedback Effects of the First Stars onto Nearby Collapsed Objects Use 1-D Lagrangian, spherical, radiation-hydrodynamics

code Full treatment of primordial chemistry, radiative transfer,

cooling/heating, hydrodynamics Follow I-front propagation of the radiation from outer, Pop III

star in detail Is I-front trapped? What happens to the center? Any H2 formation/dissociation interesting? Is it positive or negative feedback effect?

Compare to Susa & Umemura (2006)

A range of distances considered between the source and the target minihalos for different minihalo masses and also the evolutionary stages of the target minihalo when radiation arrives.

• Proper distances: D = 50, 180, 360, 540, 1000 pc

• Ionizing Photon Fluxes: F0 = Nph,50 / r2kpc = 600, 46, 11, 5,

1.5 [i.e. Nph,50 (120 solar mass Pop III star) = 1.5]

• Minihalo masses: M / (105 solar mass) = 0.25, 0.5, 1, 2, 4, 5.5, 8

• Initial evolutionary stagesPhase I (no evolution) mean IGM chemical abundances

@ z = 20 H2 ~ 10-6 , e- ~ 10-4 , Tcore = Tvirial Phase II (evolved from Phase I for ~107 years < tHubble) H2 ↑ as e- ↓ in core H2 ~ 10-4 to 10-3, e- ~ 10-5 H2 radiative cooling to Tcore ~ 100 K causes core to compress by factor ~ 1 to 20, higher for higher M

120 solar mass Pop III star irradiates nearby minihalos at z=20: 1D, rad-hydro simulations of radiative feedback

•target halo @ M=2e5 Msun


•With no radiation: tcoll=31 Myrs



•With no radiation: tcoll=31 Myrs

•With radiation:

• Flux F0=46.3 (D=180 pc) • tcoll=2.3 Myrs

• expedited collapse

• second star forms before the first star dies!




•With no radiation : tcoll=89 Myrs


•With no radiation : tcoll=89 Myrs

•With radiation:

•Flux F0=46.3 (D=180 pc)

•tcoll=129 Myrs

•delayed collapse

•second star forms after the first star dies, but still within a Hubble time


Response to shock-front determines the fate of core

expedited delayed

Response to shock-front determines the fate of core

reversed unaffected

Radial Profiles at collapse

Summary Minihalos (target) nearby the first Stars (source)

I-front trapped; Ionized gas evaporates; core remains neutral Collapse, if any, doesn’t occur the way O’shea et al. have described,

since they are NOT fully ionized in the beginning.

Shock is driven to the neutral region: mixture of positive and negative feedback effects Competition between H2 cooling & shock-heating determines the fate

of neutral region. Critical minimum mass for hosting 2nd generation stars: 1-2 x 105 Msun

Susa & Umemura claim that shock delivers negative feedback only. We see positive feedback as well. Needs to be settled in higher resolution 3D simulations

Overall, halos that are destined to collapse without Pop III stellar radiation would do so with radiation. Phase I: After star dies, out of electrons that have not recombined

yet, H2 forms Phase II: Electron is low, but H2 is high such that the core is

sufficiently self-shielded + more complexity

Outline


First ionizing sources – Radiative feedback from a first star on nearby minihalos


UV background in the Early Universe (Ahn, Shapiro, Iliev, in preparation) UV background at high redshift, especially in the H2 Lyman-

Werner band (dissociating photons), may control the formation of Pop III stars in the early universe Haiman, Rees, Loeb; Haiman, Abel, Rees Ricotti, Gnedin, Shull Kitayama, Tajiri, Umemura, Susa, Ikeutshi Kitayama, Susa, Umemura, Ikeutchi Omukai, Nishii Glover, Brand

Cosmic reionization simulations Iliev, Mellema, Shapiro, … C2-ray method for hydrogen ionization front Producing consistent results with reasonably assumed fudge

factors (escape fraction, star formation efficiency, IMF, …)

Self-Regulated ReionizationIliev, Mellema, Shapiro, & Pen (2006), MNRAS, submitted; (astro-ph/0607517)•Jeans-mass filtering low-mass source halos (M < 109 Msolar) cannot form

inside H II regions ;

•35/h Mpc box, 4063 radiative transfer simulation, WMAP3,fγ = 250;

•Big enough for statistical study

•resolved all halos with M > 108 Msolar (i.e. all

atomically-cooling halos),(blue dots = source cells);

• Evolution: z=21 to zov = 7.5.

Time-slices of the reionization simulation (fγ = 2000)

z = 18.5 16.1 14.5

13.6 12.6 11.3

radiative feedback effects of the first objects in the early universe kyungjin ahn the university of...

Documents

nearby minihalos minihalos

sources radiative feedback

target minihalos

minihalos outlineno

sources dark ages

star formationgas

nearby minihaloslate

cosmic dark ages shapiro