Talk by Dr Andrei Mesinger at the SuperJEDI conference, July 2013

1. Tuning your radio to
reionization and the cosmic dawn
Andrei Mesinger
Scuola Normale Superiore, Pisa

2. Cosmic History
z
~
6
tage
~
1
Gyr
z~1100
tage
~
0.4
Myr
Reioniza5on
Dark
Ages
Recombina5on
HII
z
~
20
tage
~
150
Myr
z
=
0
tage ~ 14 Gyr
HI

3. Cosmic Dawn and Reionization
z
~
6
tage
~
1
Gyr
z~1100
tage
~
0.4
Myr
Reioniza5on
Dark
Ages
Recombina5on
HII
z
~
20
tage
~
150
Myr
z
=
0
tage ~ 14 Gyr
HI
Bulk of our light cone: observational future!

4. Sources Sinks
Understanding reionization means understanding sources and sinks
of ionizing photons.
simple analytic model for global evolution (e.g. Barkana Loeb 2004):
e ∼ 1 – 3.
SNe Rates
och, and hence
re unknown at
SOs offer sug-
onization was
quite extended
SFR and SNR
However, even
elow) scenario
n feature is as
not be smooth,
uch transitions
fferent sources
er 2003).
h and shape of
e filling factor
small, vulner-
mal state of the
SNRs, in each
es
nizing sources
formation with
es
are still forming at such late stages are probably not going to be
very near the large overdensities which were likely to be ion-
ized during earlier stages (Furlanetto Oh 2005; Ricotti et al.
2002). We also require (4) to be reasonably high (i.e. that the
dominant ionizing sources appear around the same time, with-
out too much cosmic scatter). Below, we further quantify such
a scenario.
One can get a sense of the possible shapes of the reionization
feature through an estimate of the evolution of the filling factor
of ionized regions, FHII(z), (c.f. Barkana Loeb 2001; Haiman
Holder 2003):
dFHII(z)
dt
= ∗ fesc
Nph/b
0.76
dFcol( Mmin(z),z)
dt
αBC n0
H (1!z)3
FHII .
(7)
Here fesc is the escape fraction of ionizing photons, Nph/b is
the number of ionizing photons per baryon emitted by a typical
source, Fcol( M,z) is the fraction of baryons that reside in col-
lapsed halos with a total mass greater than M at redshift z, αB is
the hydrogen case B recombination coefficient, C ≡ n2
H / nH
2
is the clumping factor, and n0
H is the current hydrogen number
density. The first term on the right hand side accounts for “new”
ionizations contributing to the growth of the HII regions and the
last term on the right hand side accounts for “old” reionizations
due to recombinations inside the HII region. This equation is
a very rough approximation, as it does not include feedback
effects, light travel time, and it does not accurately model the
period when bubbles start overlapping (i.e. FHII(z) ∼ 1). How-
ever, it can suffice for the crude estimates we are making here.
In Figure 9, we plot FHII(z) for several values of Mmin(z) cor-
sources
sinks
Even such an overly-simplified model has several unknown,
redshift and spatial dependent parameters:
nant ionizing sources appear around the same time,
oo much cosmic scatter). Below, we further quantify
nario.
ne can get a sense of the possible shapes of the reioniz
re through an estimate of the evolution of the filling f
nized regions, FHII(z), (c.f. Barkana Loeb 2001; Ha
older 2003):
I(z)
= ∗ fesc
Nph/b
0.76
dFcol( Mmin(z),z)
dt
αBC n0
H (1!z)
fesc is the escape fraction of ionizing photons, Nph
umber of ionizing photons per baryon emitted by a ty
ce, Fcol( M,z) is the fraction of baryons that reside in
d halos with a total mass greater than M at redshift z,
2
ill forming at such late stages are probably not going to be
near the large overdensities which were likely to be ion-
during earlier stages (Furlanetto Oh 2005; Ricotti et al.
). We also require (4) to be reasonably high (i.e. that the
nant ionizing sources appear around the same time, with-
oo much cosmic scatter). Below, we further quantify such
nario.
e can get a sense of the possible shapes of the reionization
re through an estimate of the evolution of the filling factor
nized regions, FHII(z), (c.f. Barkana Loeb 2001; Haiman
lder 2003):
(z)
= ∗ fesc
Nph/b
0.76
dFcol( Mmin(z),z)
dt
αBC n0
H (1!z)3
FHII .
(7)
fesc is the escape fraction of ionizing photons, Nph/b is
umber of ionizing photons per baryon emitted by a typical
e, Fcol( M,z) is the fraction of baryons that reside in col-
d halos with a total mass greater than M at redshift z, αB is
ydrogen case B recombination coefficient, C ≡ n2
H / nH
2
0
ough small halos at that epoch to act as signposts for
tion. From Figure 6, we see that this a reasonable as-
n, especially given the fact that most small halos which
forming at such late stages are probably not going to be
ar the large overdensities which were likely to be ion-
ing earlier stages (Furlanetto Oh 2005; Ricotti et al.
We also require (4) to be reasonably high (i.e. that the
nt ionizing sources appear around the same time, with-
much cosmic scatter). Below, we further quantify such
io.
an get a sense of the possible shapes of the reionization
hrough an estimate of the evolution of the filling factor
ed regions, FHII(z), (c.f. Barkana Loeb 2001; Haiman
er 2003):
= ∗ fesc
Nph/b
0.76
dFcol( Mmin(z),z)
dt
αBC n0
H (1!z)3
FHII .
(7)
c is the escape fraction of ionizing photons, Nph/b is
ber of ionizing photons per baryon emitted by a typical
Fcol( M,z) is the fraction of baryons that reside in col-
alos with a total mass greater than M at redshift z, αB is
2 2
ure 6, we see that this a reasonable as-
ven the fact that most small halos which
late stages are probably not going to be
rdensities which were likely to be ion-
es (Furlanetto Oh 2005; Ricotti et al.
(4) to be reasonably high (i.e. that the
ces appear around the same time, with-
catter). Below, we further quantify such
of the possible shapes of the reionization
mate of the evolution of the filling factor
(z), (c.f. Barkana Loeb 2001; Haiman
dFcol( Mmin(z),z)
dt
αBC n0
H (1!z)3
FHII .
(7)
fraction of ionizing photons, Nph/b is
photons per baryon emitted by a typical
most small halos which
robably not going to be
were likely to be ion-
Oh 2005; Ricotti et al.
ably high (i.e. that the
d the same time, with-
e further quantify such
apes of the reionization
tion of the filling factor
Loeb 2001; Haiman
)
αBC n0
H (1!z)3
FHII .
(7)
zing photons, Nph/b is
- escape fraction of ionizing photons
- mass efficiency of conversion of gas to stars
- mean # of ionizing photons per stellar baryon
- minimum halo mass to host ionizing sources
- clumping factor (measurement of the average recombination rate)
Many groups are working on modeling such parameters!

5. Challenges
~ FoV of 21cm
interferometers
• Dynamic range required is enormous: single star -- Universe
• We know next to nothing about high-z -- ENORMOUS parameter space to explore
S1 S3 S4S2
z=7.7z=7.3z=8.7
Figure 3. Comparison of four radiative transfer simulations post-processed on the same density field, but using different source prescriptions parametrized by
˙N(m) = α(m) m. The white regions are ionized and the black are neutral. The left-hand panel, left centre panel, right centre panel and right-hand panels are,
respectively, cuts through Simulations S2 (α ∝ m−2/3), S1 (α ∝ m0), S3 (α ∝ m2/3) and S4 (α ∝ m0, but only haloes with m 4 × 1010 M host sources). For
the top panels, the volume-ionized fraction is ¯xi,V ≈ 0.2 (the mass-ionized fraction is ¯xi,M ≈ 0.3) and z = 8.7. For the middle panels, ¯xi,V ≈ 0.5(xi,M ≈ 0.6)
and z = 7.7, and for the bottom panels, ¯xi,V ≈ 0.7(¯xi,M ≈ 0.8) and z = 7.3. Note that the S4 simulation outputs have the same ¯xi,M , but ¯xi,V that are typically
0.1 smaller than that of other runs. In S4, the source fluctuations are nearly Poissonian, resulting in the bubbles being uncorrelated with the density field
(¯xi,V ≈ ¯xi,M ). Each panel is 94 Mpc wide and would subtend 0.6 degrees on the sky.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1 10
RdP/dR
0.01
0.1
0.1 1 10
∆xx
2
k (h Mpc
-1
)
z = 7.3
0.01
0.1
∆xx
2
z = 7.7
0.01
0.1
∆xx
2
z = 8.7
!#$$$% !#$$%
x
HI
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
z = 5.00
xHI v = 0.10
1Mpc
94Mpc
2Gpc
Wise+ (2010)
McQuinn+ (2007)
Mesinger (2010)

6. Philosophy… how to approach the problem
scale
Hydrodynamical Numerical Simulations (+RT)
Seminumerical Simulations or
lower resolution large-scale numerical simulations
Seminumerical Simulations
or Analytic Estimates
Strategy #1:

7. Philosophy… how to approach the problem
Strategy #2:
Large scales/analytic models to generate
general, robust claims (true for large swaths
of parameter space)
make predictions match observations
(caution: interpretation is difficult; watch
out for degeneracies..)

8. DexM 21cmFAST
• Combines excursion-set approach with perturbation theory for efficient generation
of large-scale density, velocity, halo, ionization, radiation, 21cm brightness fields
• Portable and FAST! (if it’s in the name, it must be true…)
– A realization can be obtained in ~ minutes on a single CPU
– New parallelized version, optimized for parameter studies
• Run on arbitrarily large scales
• Vary many independent free parameters; cover wide swaths of parameter space
• Tested against state-of-the-art hydrodynamic cosmological simulations (Trac Cen
2007; Trac+ 2008)
• Publically available!
semi-numerical simulation (Mesinger Furlanetto 2007;
Mesinger, Furlanetto, Cen 2011)
Tools for modeling large-scale signal:

9. Density Fields
z=7
0.19 Mpc cells
143 Mpc

10. Halo Finder (DexM)
without adjusting halo locations
with adjusting halo locations

11. Halo Finder (DexM)
z=8.7 N-body halo field from
McQuinn et al. (2007)

12. Ionization fields
Trac Cen (2007)
21cmFAST (Mesinger+ 2011)
Zahn+ (2010)
DexM (with halos;
Mesinger Furlanetto; 2007)
6
McQuinnetal.TracCenFFRT
X=0.25 X=0.51 X=0.72
z=8.49 z=7.56 z=7.11
MesingerFurlanetto
Fig. 1.— Comparison of ionization fields generated from four schemes: McQuinn et al., Trac Cen, MF07, and FFRT. The maps are
from the same slice (100 Mpc/h by 100 Mpc/h with depth of 0.4 Mpc/h) through the simulation box.

13. Redshift space distortions (sorry no pics)
nonlinear structure formation creates an asymmetric velocity gradient distribution

14. 21cm comparison (stay tuned…)
hydro+DM+RT
DexM (with halos)
21cmFAST (no halos)
~ 1 week on 1536 cores
~ few min on 1 core
100 Mpc/h

15. Get on board!
http://homepage.sns.it/mesinger/Sim
In just over 2 years, 21cmFAST is being used by researchers in 12 countries,
and most of the 1st gen. 21cm experiments: LOFAR, MWA, 21CMA

16. Example:
cosmic 21cm signal

17. 21 cm line from neutral hydrogen
Hyperfine transition in the ground
state of neutral hydrogen produces
21cm line.
Predicted by van den Hulst when
Oort told him to find unknown
radio lines to study our galaxy

18. Now widely used to map the HI content of
nearby galaxies
Circinus Galaxy
ATCA HI image by B. Koribalski (ATNF, CSIRO), K. Jones, M. Elmouttie (University
of Queensland) and R. Haynes (ATNF, CSIRO).

19. Once upon a time, HI was much more abundant
z
~
6
z~1100
Recombina5on
HII
z
~
20
CMB backlight
z
=
0
HI
υ21~
70
MHz
υ21~
200
MHz
Redshifted 21cm signal.
tune radio to:

20. Once upon a time, HI was much more abundant
z
~
6
z~1100
Recombina5on
HII
CMB backlight
z
~
20
HI
υ21~
70
MHz
υ21~
200
MHz
Redshifted 21cm signal.
tune radio to:
LOFAR,
MWA,
PAPER,
21CMA,
GMRT
2nd gen: SKA
interferometer

21. What we learn: Cosmological 21cm Signal
neutral fraction
gas density
LOS velocity gradient
spin temperature

22. Cosmological 21cm Signal
Powerful probe:
Astrophysics
Has something everyone can enjoy!
The trick is to disentangle the components:
• separation of epochs and/or
• accurate, efficient modeling (21cmFAST)
Cosmology

23. 21cm evolution
decoupling
(cosmic web)
Lyα pumping
(first stars)
X-ray heating
(first BHs)
Reionization

24. http://homepage.sns.it/mesinger/21cm_fiducial.mov

25. Power of the pre-reionization thermal
evolution to constrain astro and cosmo
spin temperature:
fields, using excursion set formalism to estimate the mean num-
ber of sources inside spherical shells corresponding to some higher
redshift. As discussed above, bypassing the halo field allows the
code to be faster, with modest memory requirements. Below we go
through our formalism in detail.
The spin temperature can be written as (e.g. Furlanetto et al.
2006):
T−1
S =
T−1
γ + xαT−1
α + xcT−1
K
1 + xα + xc
(5)
where TK is the kinetic temperature of the gas, and Tα is the color
temperature, which is closely coupled to the kinetic gas tempera-
ture, Tα ≈ TK (Field 1959). There are two coupling coefficients
in the above equation. The collisional coupling coefficient can be
written as:
xc =
0.0628 K
A10Tγ
h
nHIκHH
1−0(TK) + neκeH
1−0(TK) + npκpH
1−0(TK)
i
,
(6)
Tγ – temperature of the CMB
TK – gas kinetic temperature
Tα – color temperature ~ TK
the spin temperature interpolates between Tγ and TK
Any source of heat could leave an imprint:
-X-rays, shocks, DM annihilation, cosmic strings…

26. Fiducial heating: X-rays (HMXBs)
Mesinger Ewall-Wice, in prep

27. Fiducial heating: X-rays (HMXBs)
Mesinger Ewall-Wice, in prep

28. But 21cm also probes cosmology
1) “clean” epochs where cosmo signal dominates à Dark
Ages z 40
!#$%'($)*+,-.$
/0,.$1,12-3$
45,$

29. But 21cm also probes cosmology
2) Models which suppress small-scale power, like WDM
result in a dearth of low mass galaxies

30. But 21cm also probes cosmology
3) Heat input (e.g. DM annihilations)
FIG. 3: Evolution of the 21cm power at k = 0.1 h Mpc−1
.
Evoli+, in prep

31. Conclusions
• Cosmological 21cm signal is very rich in information, containing both
cosmological and astrophysical components.
• The range of scales and unknown parameter space is enormous! We need (i)
bottom-up modeling; (ii) parameter space explorations
• SKA is great!
!#$%'($)*+,-.$
/0,.$1,12-3$
45,$