Cosmic Variance of Small-Scale Structure

Formation: Large-Scale Density and CDM-Baryon

Drift Velocity Environment

Based on collaboration with:

Paul Shapiro (Texas), Ilian Iliev (Sussex), Garrelt Mellema

(Stockholm), Jun Koda (Studi Roma Tre), Yi Mao (Tsinghua), Ue-Li

Pen (CITA);

Britton Smith (UC San Diego), Mike Norman (UC San Diego)

Kyungjin Ahn (Chosun U, Korea)

Aspen Winter Conference

Feb 2018

Outline- Motivation

- Previous work: cosmic reionizaiton with minihalo

stars and massive-halo stars

- Recent work: CDM-baryon streaming velocity +

Density environment affecting halo & star formation

- In progress, along this line

- BCCOMICS (initial condition generator)

- BCCOMICS+enzo cosmic variance of halo

formation, star formation, etc.

- Summary/prospect

Reionization by minihalo stars and

massive halo stars(KA, Iliev, Shapiro, Mellema, Koda, Mao 2012)

• Large-box simulation preferred– The larger, the better in statistics (Barkana & Loeb )

– >~100 Mpc to see bubbles with ~20 Mpc or larger

• Resolution limited– Cannot resolve minihalos

– Cannot use minihalo stars

– Losing photon budget

• Need subgrid treatment– Then can cover all stars

Motivation

What’s new?• Populating grid with

minihalos (first stars!)– small-box (6.3/h Mpc)

simulation resolving minihalos– correlation between density &

minihalo population (nonlinear bias: KA+ in preparation)

– put one Pop III star per minihalo

• Considering photo-dissociation of coolant– calculate transfer of Lyman-

Werner Background (KA, Shapiro, Iliev, Mellema, Pen 2009)

– remove first star from minihalos, if LW intensity over-critical

• Retarded-time Green’s function calculation (KA 2015, JKAS)

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• Self-regulated first star formation epoch as expected by us? (Miranda, Lidz, Heinrich, Hu 2017)

Some hint of early first star formation from Planck

CDM-baryon streaming velocity

& density environment affecting

halo and star formation(KA 2016)

Power spectrum @ recombination (from CAMB)

• baryon moving against dark matter – velocity offset @ recombination – ~ km/s velocity offset @ z~20

(Tseliakhovich & Hirata 2010)

• baryon formation offset– velocity offset formation offset

(O’Leary & McQuinn 2012)– Jeans mass up Delayed star

formation (Maio, Koopmans, Ciardi 2011; Greif, White, Klessen, Springel2011; Stacy, Bromm, Loeb 2011)

– heating 21cm signal of streaming-velocity (McQuinn & O’Leary 2012)

– Variation of baryon/CDM fraction (Fialkov, Barkana, Tseliakhovich, Hirata 2012)

Baryon bulk motion w.r.t. CDM

Equations that are integrated←Tseliakhovich & Hirata 2010 (streaming velocity but without density environment)

KA 2016 (streaming velocity with density environment)

↓

Equations that are integrated←Tseliakhovich & Hirata

KA

↓

Equations that are integrated←Tseliakhovich & Hirata

KA

↓

What various fields look like (at z=1000)

What CDM overdensity and Vcb look like (at z=1000)

BCCOMICS: Baryon-Cold dark

matter COsMological Initial

Condition generator for Small-scale

structures(by KA, soon to be released)

Flow Chart

run CAMB:TF output @ z=zi=1000

Radom seed and FFT

Δc Θc

Δb Θb

ΔT Vc-Vb

δc θc

δb θb

δT

which patch?

Δc Θc

Δb Θb

ΔT Vc-Vb

Time-integrate to zf

TF output @ zf

(e.g. zf =200)

Radom seed and FFT

δb θb δT

Lagrangianperturbation

xc vc

(xb vb)

Small-scale fields (Lbox=1.3 Mpc at z=200): CDM overdensity

Small-scale fields (Lbox=1.3 Mpc at z=200): baryon overdensity

• CICsASS (O’Leary & McQuinn 2012)– Cosmological Initial Conditions for AMR and SPH

Simulations

– Based on Tseliakhovich & Hirata 2010 (streaming velocity variation)

• BCCOMICS (for overdense patch)– Baryon-Cold dark matter COsMological Initial Condition

generator for Small-scale structures

– Based on KA 2016 (streaming velocity, density, velocity divergence, temperature)

• Bad practice – not using these IC generators– Sudden turn-on of streaming velocity very common, but this

is starting from wrong IC (e.g. O’Leary & McQuinn)

– Zero streaming velocity is too rare!

CICsASS vs BCCOMICS

BCCOMICS + Enzo (by KA, Smith, Norman, in preparation)

• No need to change cosmological parameters– May use CICsASS

– For more variants (e.g. non-zero temperature overdensity), should use BCCOMICS

Mean-density cases

• So far, BCCOMICS is the only tool– Non-zero overdensity, non-zero temperature overdensity,

non-zero streaming velocity

• In practice, take the overdense patch as a separate universe with non-zero curvature– “local”: what’s observed in the patch

– Local H different: 𝐻𝑙𝑜𝑐𝑎𝑙 = 𝐻 − ሶ∆/3(1 + ∆)

– Local ρcrit different, local Ω different

– Local scale factor different

– Simulating structure formation inside a peak or a void (e.g. Goldberg & Vogeley 2004)

Non-zero overdensity cases

• Warning: overdense cases are wrong, they should have more halos than the mean-density patches.– Will find bugs and correct.

Halo mass function ((too) preliminary)

• Star formation rate

• Baryon/DM ratio inside halos

• Spatial modulation of IGM heating– relevant to high-z 21cm cosmology (McQuinn & O’Leary

2012)

• Finding rare, bright objects– Can help pencil-beam observations to find targets

– Can help push source-finding observations to go to higher-than-usual redshifts

Other useful analysis

• Minihalo stars (Pop III mostly) in reionization– Density bias

• Minihalos in general (high-z astrophysics)– Streaming velocity bias (Tseliakhovich & Hirata 2010)

– Streaming velocity bias, density bias, temperature bias (KA 2016)

– Use BCCOMICS; I am

• High-z 21cm cosmology– Streaming velocity bias, density bias, temperature bias

• Local patch simulation scheme– Useful for larger-scale structure too

– Useful for rare-object hunters

Summary (cosmic variance of structure formation)