Talk by Prof. Romeel Dave' at the SuperJEDI conference, July 2013

1. From Darkness, Light:From Darkness, Light:
Computing Cosmological ReionizationComputing Cosmological Reionization
Romeel Davé
With: Kristian Finlator, Feryal Özel, Ben Oppenheimer
Movie by T. di Matteo

2. HI

3. James Webb Space Telescope
“First Light Machine”
Planck: CMB
at high precision
Atacama Large
Millimeter Array
Square Kilometer Array
Reionization: A Multiwavelength Approach

4. When did reionization occur (start and end)?
What are the sources that reionized the Universe?
How did reionization transpire in space and time?
Fundamental Questions of Reionization

5. Observational Constraints: HI Optical Depth
Bolton & Haehnelt (2007)
Quasar spectra
show sudden
rise in τHI at
z~6.
SDSS

6. Observational Constraints: CMB Polarization
CMB T-E cross-correlation
@ low-l shows enhanced
signal from free electrons.
zreion~10±1

7. Observational constraints: z>6 galaxies
Bouwens et al. (2008)
z=4
z=5
z=6
z=7
z=6.96: Iye et al (2008)
Hundreds of (putative) z>6 galaxies now seen.
No unambiguous signature of reionization.
JWST

8. - Reionization began at z>~10 (0.5 Gyr)
- Reionization ended at z~6 (1 Gyr)
- There are plenty of galaxies (and very few quasars)
seen at z>~6.
Do galaxies alone emit enough photons to reionize?
Observations tell us…

9. QI = Volume-averaged filling factor of ionized gas
nph = # of ionizing photons per unit volume
trec = recombination time
Clumping factor CHII
= <nHII ne>/<nHII><ne>
Can (in principle) measure dnph/dt.
But to solve reionization, need CHII
, i.e.
topology.
Analytic Reionization
z=9, 1 Mpc/h, dark matter

10. Outside-in: Voids reionize first, then dense regions
Inside-out: Regions around galaxies ionize first, then voids
Competition between:
- Sources forming in overdense regions
- Galaxies are highly clustered at early epochs
vs.
- High recombination rates in dense regions
- Dense regions more self-shielded (shadowing)
Analytic results highly assumption-dependent.
Simulate!
Topology of Reionization

11. Simulating Reionization: Physics
(1) structure formation with gas (density, temperature evolution)
(2) sources of photons (normal stars, Pop III stars, AGN, exotica)
(3) non-equilibrium thermal state
(4) non-equilibrium ionization state
(5) radiation transport

12. 1 2
3

13. Code Comparison
Code method c=? shadows scaling comments
C2-ray ray-tracing ∞ yes N*
Nx
3
n-body
ART ray-tracing ∞ ? N*
Nx
3
FLASH-HC ray-tracing ∞ yes N*
Nx
3
TRAPHIC ray-tracing c yes Nc
NSPH
mass resolution limit?
CRASH Monte carlo ∞ yes N*
Nγ
time dependence?
OTVET* moments << c ? Nx
3
optically-thin fEdd
MARCH* moments c yes Nx
~4
accurate fEdd
; highly
flexible; use SPH sim's
Zel'dovich approximation
+irradiated boundary
* Have been implemented into a cosmological radiation-hydro code

14. The Moments of the Transfer Equation
- Derive fEdd
from accurate long-characteristics
→ minimize artifacts; enhance shadowing
See also Auer & Mihalas (1970); Stone, Mihalas, & Norman (1992)
Eddington tensor

15. The Future: HI 21cm maps
Redshifted 21cm (~100 MHz)
traces HI directly.
dTbright~xHI(Tspin-TCMB)
In principle, map HI
distribution
and get clumping factor.
Kinetically
coupled
to gas
Ts~(1+z)2
Returns
to CMB
Ts~(1+z)Santos+10

16. Summary
Reionization is a frontier for both observations and theory
- Planck, JWST, LOFAR, SKA, … all will play major roles
- Simulations will require next-gen techniques + technology
Our new fast & accurate cosmological rad-hydro code…
- Makes no physical approximations (outside of galaxies)
- Moment-based; doesn’t scale with # of sources
- Still limited by dynamic range
Interesting early results:
- Galaxies with normal star formation can reionize
- But… hard to match all current constraints with normal SF
- Topology of reionization: inside-outside-middle
- Clumping factor decreases rapidly with time; ~2-3 by z~6