Observing the First Galaxies and the Reionization Epoch
Steve Furlanetto
UCLA
February 5, 2008

Outline




Introduction: Observing Reionization
Galaxy Surveys


Current observations of LAEs
The Clustering Signature
The 21 cm Transition as a Cosmological Probe





Basic Physics
The Mean 21 cm Background
Measurements and Challenges
The Pre-reionization IGM
Reionization
Conclusion

A Brief History of the Universe
Big Bang
Last
Scattering
 Last scattering: z=1089, t=379,000 yr
Dark Ages
 Today: z=0, t=13.7
Gyr
First
Galaxies
Reionization
 Reionization: z=6-20, t=0.2-1 Gyr
Galaxies,
Clusters, etc.
 First galaxies: ?
G. Djorgovski

Reionization




First stars and galaxies produce ionizing photons
Ionized bubbles grow and merge
Affects all baryons in the universe
Phase transition
Mesinger & Furlanetto

Reionization




First stars and galaxies produce ionizing photons
Ionized bubbles grow and merge
Affects all baryons in the universe
Phase transition
Mesinger & Furlanetto

Reionization




First stars and galaxies produce ionizing photons
Ionized bubbles grow and merge
Affects all baryons in the universe
Phase transition
Mesinger & Furlanetto

Reionization




First stars and galaxies produce ionizing photons
Ionized bubbles grow and merge
Affects all baryons in the universe
Phase transition
Mesinger & Furlanetto

Reionization:
Observational Constraints
 Quasars/GRBs


CMB optical depth
Ly

-selected galaxies
Furlanetto, Oh, & Briggs (2006)


Ly

Emitters and HII Regions


Total optical depth in
Ly
 transition:
GP

3x10
5 x
HI 
1
 z
7
 3 / 2

 Damping wings are strong
IGM HI

x
H
=0
LAEs During Reionization x
H
=0.26
x
H
=0.51
x
H
=0.77
Mesinger & Furlanetto (2007)

 z=9, R=125 observation, with M>1.7x10
10 Msun
Galaxies in small bubbles (underdense regions) masked out by absorption

A Declining Number Density?
 Largest survey to date with Subaru
 Apparent decline at bright end
 Disputed by Dawson et al. (2007)
Kashikawa et al. (2006)

A Declining Number Density?
 Similar behavior to z=7
 One (!) detection
 L>10 43 erg/s detection threshold
Iye et al. (2006)

An Increasing Number Density?
 Stark et al. (2007) found 6 candidate LAEs behind massive clusters




Search along lensing caustics z=9-10
L~10 41 -10 42 erg/s
Most obvious interlopers ruled out
Stark et al. (2007), z=9
Kashikawa et al. (2006)

An Increasing Number Density?



Solid curves show mass functions with absorption
Four scenarios for luminosities
(right to left):




Same as z=6 LAEs
Same as z=6 LAEs, but Pop III
All baryons form Pop II stars, simultaneously
All baryons form Pop III stars, simultaneously
Reasonable scenarios require fully ionized!
Mesinger & Furlanetto (2008)

LAE Clustering
During Reionization



Nearly randomly distributed galaxy population
Small bubble: too much extinction, disappears
Large bubble: galaxies visible to survey

LAE Clustering
During Reionization



Small bubble: too much extinction, disappears
Large bubble: galaxies visible to survey
Absorption selects large bubbles, which tend to surround clumps of galaxies

LAE Clustering
During Reionization



Small bubble: too much extinction, disappears
Large bubble: galaxies visible to survey
Absorption selects large bubbles, which tend to surround clumps of galaxies

Enhanced Clustering
During Reionization
 Shows enhanced probability to have
N>1 galaxies in an occupied cell
 Measuring requires deep survey over
~10 6 -10 7 Mpc 3
Mesinger & Furlanetto (2008)

The Future of LAE Surveys
 Advantages:
 Familiar strategies
 Study galaxies as well
 Disadvantages:
 Uncertainties about galaxy formation
 Need large volume, deep surveys
 Indirect information about IGM

The Spin-Flip Transition



Proton and electron both have spin  magnetic fields
Produces 21 cm radiation
( n
=1420 MHz)
Extremely weak transition
 Mean lifetime ~10 7 yr
 Optical depth ~1% in fully neutral IGM


The 21 cm Transition
 Map emission (or absorption) from IGM gas




Requires no background sources
Spectral line: measure entire history
Direct measurement of
IGM properties
No saturation!

T b

23 x
HI
(1
 
)

1
 z
10
 1/ 2



T
S

T bkgd
T
S



H ( z ) /(1
 z )
 v r
/
 r


mK
SF, AS, LH (2004)

The Spin Temperature


CMB photons drive toward invisibility:
T
S
=T
CMB
Collisions couple T
S
 to T
K
Dominated by electron exchange in H-H collisions in neutral medium (Zygelman 2005)
 Dominated by H-e collisions in partially ionized medium (Furlanetto & Furlanetto
2006), with some contribution from H-p collisions (Furlanetto & Furlanetto 2007)

The Global Signal:
The Dark Ages
 Straightforward physics
 Expanding gas
 Recombination
 Compton scattering
SF, PO, FB (2006)

1
S
1/2
0
S
1/2
The Wouthuysen-Field
Mechanism I
Selection Rules:
D F=0,1 (except F=0  F=0)
2
P
3/2
1
P
3/2
1
P
1/2
0
P
1/2
Mechanism is effective with
~0.1 Ly
 photon/baryon

The Wouthuysen-Field
Mechanism II
Ly
 Ly

Ly

Ly

 Relevant photons are continuum photons that redshift into the Ly
 resonance
…

The Global Signal:
First Light
 First stars (quasars?) flood Universe with photons
 W-F effect
 Trigger absorption in cold IGM feedback
Pop III Stars
Pop II Stars
SF (2006)

The First Sources of Light:
X-ray Heating


X-rays are highly penetrating in IGM

Mean free path >Mpc
 Deposit energy as heat, ionization
Produced by…




Supernovae
Stellar mass black holes
Quasars
Very massive stars

The Global Signal:
First Light
 First stars (quasars?) flood Universe with photons
 W-F effect
 Heating
 Ionization
 Timing depends on f
*
, f esc
, f
X
, stellar population feedback
Pop III Stars
Pop II Stars
SF (2006)

21 cm Observations
 Experiments
 Global Signal: CoRE-
ATNF, EDGES
 Fluctuations: 21CMA,
LOFAR, MWA, GMRT,
PAPER, SKA
 Imaging: SKA
MWA

Terrestrial Interference



Mileura spectrum, 15 sec integrations
Two types:


Fixed site (low frequency filling factor)
Aircraft/meteor trail reflections (low duty cycle)
Basic strategy: excise contaminated channels
Bowman et al. (2007)

Ionospheric Distortions




Refraction in ionosphere distorts wavefronts
Analog of optical seeing layer
Solved on software level with calibration sources
Challenge: wide-field imaging
W. Cotton

Astronomical Foregrounds

Map at 150 MHz
 Contours are in Kelvin
Landecker et al. (1969)

The Synchrotron Foregrounds
 A single synchrotron electron produces a broad but smooth spectrum
B
Intensity
Frequency e path

The Synchrotron Foregrounds
 A single synchrotron electron produces a broad but smooth spectrum
Intensity
 Electron velocity scales the spectrum
Frequency

The Synchrotron Foregrounds
 Synchrotron spectrum mirrors distribution of
Intensity fast electrons
 Typically near powerlaw, with ~K/MHz gradient
Frequency

Measuring the Global Signal?



Signal gradient is few mK/MHz
Foregrounds vary as
(near) power law


Synchrotron, free-free
Gradient is few K/MHz
CoRE-ATNF, EDGES experiments are trying

Distinctive shape may help
SF (2006)

Foregrounds on Small Scales
0.5 MHz

Foreground Removal
Total Signal ~ 400 K
 Removal algorithms fairly well-developed
Zaldarriaga et al.
(2004), Morales &
Hewitt (2004), Santos et al. (2005),
McQuinn et al. (2007)
T b
Cleaned Signal ~ 10 mK
Frequency

Foreground Noise
 Thermal noise is NOT smooth: varies between each channel
 For first generation instruments, 1000 hr observations still have S/N<1 per pixel
 Imaging is not possible until SKA!

The Murchison Widefield Array
 Low Frequency
Demonstrator under construction (fully funded, first light
~2008)
Bowman et al. (2007)
 Located on sheep ranch in Western
Australia

The Murchison Widefield Array
Bowman et al. (2007)
 Bowtie antennae grouped in tiles of 16


Broad frequency response
Large field of view

Murchison Widefield Array:
Low Frequency Demonstrator
 Instrument characteristics







Radio-quiet site
500 16-element antennae in
1.5 km distribution
7000 m 2 total collecting area
Full cross-correlation of all 500 antennae
80-300 MHz
32 MHz instantaneous bandwidth at 8 kHz resolution
20-30 degree field of view Bowman et al. (2007)

Error Estimates: z=8
MWA
Foreground limit
(Mpc -1 )
SKA




Survey parameters




 z=8
T sys
=440 K t int
=1000 hr
B=6 MHz
No systematics!
MWA (solid black)


A eff
=7000 m 2
1.5 km core
SKA (dotted blue)


A eff
=1 km 2
5 km core
LOFAR very close to MWA

Error Estimates: z=12
Foreground limit
MWA
SKA



Survey parameters




 z=12
T sys
=1000 K t int
=1000 hr
B=6 MHz
No systematics!
MWA (solid black)


A eff
=9000 m 2
1.5 km core
SKA (dotted blue)


A eff
=1 km 2
5 km core
(Mpc -1 )

That’s a whole lotta trouble…
So what good is it, really?

The Global Signal
 Four Phases
 Dark Ages
 First Stars
 First Black Holes
 Reionization
Reionization BHs Stars Dark
Ages
SF (2006)

Ly

Fluctuations
 Ly
 photons decrease T
S near sources (Barkana &
Loeb 2004)
 Clustering
 1/r 2 flux
 Strong absorption near dense gas, weak absorption in voids
Cold, Absorbing
Cold, invisible

Ly

Fluctuations



Ly
 photons decrease T
S sources near


Clustering
1/r 2 flux
Strong absorption near dense gas, weak absorption in voids
Eventually saturates when IGM coupled everywhere
Cold, Absorbing

The Pre-Reionization Era
Net
X-ray
Ly

Pritchard & Furlanetto (2007)





Thick lines: Pop II model, z r
=7
Thin lines: Pop III model, z r
=7
Dashed: Ly
 fluctuations
Dotted: Heating fluctuations
Solid: Net signal

X-ray Fluctuations
 X-ray photons increase T
K near sources (Pritchard &
Furlanetto 2007)
 Clustering
 1/r 2 flux
 Hot IGM near dense gas, cool IGM near voids
Hot
Cool

+
X-ray and Ly

Fluctuations
=
Hot, emitting
Invisible

The Pre-Reionization Era
Net
X-ray
Ly

Pritchard & Furlanetto (2007)





Thick lines: Pop II model, z r
=7
Thin lines: Pop III model, z r
=7
Dashed: Ly
 fluctuations
Dotted: Heating fluctuations
Solid: Net signal

+
X-ray Fluctuations
=
Hot, emitting
Cold, absorbing

The Pre-Reionization Era
Net
X-ray
Ly

Pritchard & Furlanetto (2007)





Thick lines: Pop II model, z r
=7
Thin lines: Pop III model, z r
=7
Dashed: Ly
 fluctuations
Dotted: Heating fluctuations
Solid: Net signal

+
X-ray Fluctuations
=
Hot, emitting

The Pre-Reionization Era
Net
X-ray
Ly

Pritchard & Furlanetto (2007)





Thick lines: Pop II model, z r
=7
Thin lines: Pop III model, z r
=7
Dashed: Ly
 fluctuations
Dotted: Heating fluctuations
Solid: Net signal

21 cm Observations:
Reionization
100 Mpc comoving
Mesinger & Furlanetto

Reionization “Simulations”
100 Mpc comoving
 Implement in numerical simulation boxes
 Step 1: Generate initial conditions
 Step 2: Identify galaxies
Mesinger & Furlanetto

Biased Galaxy Formation:
Peaks and Patches
 Galaxies form at peaks in the density field
 Threshold decreases with time
 More galaxies
 Bigger galaxies

Identifying Galaxies




Filter density field to find peaks
Use excursion-set barrier to find masses
Adjust locations using
Zeldovich approximation
Similar to “peak-patch” method (Bond & Myers
1996), PINOCCHIO,
PTHALOS
Mesinger & Furlanetto (2007)

Identifying Galaxies
 Excellent statistical agreement
 Large-scale structure
 Poisson noise
 Accurate one-to-one for large galaxies
(Bond & Myers 1996)
Mesinger & Furlanetto (2007)

Reionization “Simulations”
100 Mpc comoving
 Implement in numerical simulation boxes
 Step 1: Generate initial conditions
 Step 2: Identify galaxies
Mesinger & Furlanetto

Photon Counting
 Assume galaxies have fixed ionizing efficiency
 Isolated galaxies generate HII regions
 Clustered galaxies work together
Galaxy
Ionized IGM
Neutral IGM

Photon Counting
 Assume galaxies have fixed ionizing efficiency
 Isolated galaxies generate HII regions
 Clustered galaxies work together

Reionization “Simulations” z=9.75, x i
=0.2




Implement in numerical simulation boxes
Step 1: Generate initial conditions
Step 2: Identify galaxies
Step 3: Paint on bubbles, working from outside in
100 Mpc comoving
Mesinger & Furlanetto

Reionization “Simulations” z=8.75, x i
=0.4




Implement in numerical simulation boxes
Step 1: Generate initial conditions
Step 2: Identify galaxies
Step 3: Paint on bubbles, working from outside in
100 Mpc comoving
Mesinger & Furlanetto

Reionization “Simulations” z=8, x i
=0.6




Implement in numerical simulation boxes
Step 1: Generate initial conditions
Step 2: Identify galaxies
Step 3: Paint on bubbles, working from outside in
100 Mpc comoving
Mesinger & Furlanetto

Reionization “Simulations” z=7.25, x i
=0.8





Implement in numerical simulation boxes
Step 1: Generate initial conditions
Step 2: Identify galaxies
Step 3: Paint on bubbles, working from outside in
Five hours on desktop!
100 Mpc comoving
Mesinger & Furlanetto

Success!
Filter A on halos Filter B on halos from N-body simulation from N-body simulation
Radiative
Transfer
Simulation


Excellent match for large scale features
Map details depend on filtering algorithm
Zahn et al. (2007), Mesinger & Furlanetto (2007)

The 21 cm Power Spectrum
MWA
SKA
MWA
SKA
(Mpc -1 )
Mesinger & Furlanetto (2007)

21 cm-Galaxy
Cross-Correlation z=8, x i
=0.6
Mesinger & Furlanetto

21 cm-Galaxy
Cross-Correlation
 Key advantages
 Unambiguous confirmation of cosmological signal
 Vastly reduces difficulty of foreground cleaning
 Only emission from sources in survey slice contributes
 Increase sensitivity and dynamic range

Helps with angular structure
 Science!

The 21 cm-Galaxy
Cross-Correlation



Can be done with LAEs or
LBGs
Significant advantages in 21 cm data analysis (SF & AL
2007)
Challenge: wide-field near-IR surveys



JWST?
JDEM?
Ground-based cameras?
Lidz, Zahn, & Furlanetto (in prep)

Conclusions



LAE searches beginning to pay off


Strange star formation?
Robust signatures will be in clustering
The 21 cm transition offers great possibilities



Pre-reionization: properties of first sources, cosmology
Reionization: morphology and growth of bubbles
Experimental challenges still large
Good synergy with other probes of high-z universe!
 Cross-correlation, Ly
 searches, quasars…
See our Physics Reports review (Furlanetto,
Oh, & Briggs 2006, astro-ph/0608032) for more information on 21 cm possibilities!