Radiation backgrounds from the first sources and the redshifted 21 cm line

1
Radiation backgrounds from the first sources and
the redshifted 21 cm line

• Jonathan Pritchard
• (Caltech)

Collaborators Steve Furlanetto (Yale) Marc
Kamionkowski (Caltech) Work based on
astro-ph/0607234 astro-ph/0508381
2
Overview

• 21 cm physics
• Atomic cascades and the Wouthysen-Field Effect
• Detecting the first stars through 21 cm
  fluctuations (Lya)
• Inhomogeneous X-rayheating and gas temperature
  fluctuations (X-ray)

3
Ionization history

• Gunn-Peterson Trough

Becker et al. 2005

• Universe ionized below z6, approaching neutral
  at higher z

• WMAP3 measurement of t0.09 (down from t0.17)

Page et al. 2006

• Integral constraint on ionization history
• Better TE measurements EE observations

4
Thermal history

• Lya forest

Hui Haiman 2003
??

• IGM retains short term memory of reionization -
  suggests zRlt10
• Photoionization heating erases memory of thermal
  history beforereionization

• CMB temperature

• Knowing TCMB2.726 K and assuming thermal
  coupling byCompton scattering followed by
  adiabatic expansion allows informed guess of
  high z temperature evolution

5
21 cm basics

• HI hyperfine structure

• Use CMB backlight to probe 21cm transition

n1
11S1/2
l21cm
10S1/2
n0
z0
z13.75
n1/n03 exp(-hn21cm/kTs)
fobs94.9 MHz
f21cm1.4 GHz
(KUOW)

• 3D tomography possible - angles frequency

• 21 cm brightness temperature

• 21 cm spin temperature

6
Wouthysen-Field effect
Hyperfine structure of HI
22P1/2
21P1/2
Effective for Jagt10-21erg/s/cm2/Hz/sr
TsTaTk
21P1/2
20P1/2
W-F
recoils
Field 1959
nFLJ
Lyman a
11S1/2
Selection rules DF 0,1 (Not F0?F0)
10S1/2
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Higher Lyman series

• Two possible contributions
• Direct pumping Analogy of the W-F effect
• Cascade Excited state decays through cascade to
  generate Lya
• Direct pumping is suppressed by the possibility
  of conversion into lower energy photons
• Ly a scatters 106 times before redshifting
  through resonance
• Ly n scatters 1/Pabs10 times before converting
• Direct pumping is not significant
• Cascades end through generation of Ly a or
  through a two photon decay
• Use basic atomic physics to calculate fraction
  recycled into Ly a
• Discuss this process in the next few slides

Hirata 2006
Pritchard Furlanetto 2006
8
Lyman b
A3p,1s1.64?108s-1
A3p,2s0.22?108s-1
gg

• Optically thick to Lyman series
• Regenerate direct transitions to ground
  state
• Two photon decay from 2S state
• Decoupled from Lyman a
• frecycle,b0

Agg8.2s-1
9
Lyman g

• Cascade via 3S and 3D levelsallows production of
  Lyman a
• frecycle,g0.26
• Higher transitions frecycle,n 0.3

gg
10
Lyman alpha flux

• Stellar contribution

continuum
injected

• also a contribution from any X-rays

11
X-rays and Lya production
spi?E-3
HI
HII
photoionization
e-
X-ray
collisionalionization
e-
Lya
excitation
(fa?0.8)
HI
Chen Miralda-Escude 2006
Shull van Steenberg 1985
heating
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Experimental efforts
MWA Australia Freq 80-300 MHz Baselines 10m-
1.5km
PAST China Freq 70-200 MHz
LOFAR Netherlands Freq 120-240 MHz Baselines
100m- 100km
SKA ??? Freq 60 MHz-35 GHz Baselines 20m-
3000km
(f21cm1.4 GHz)
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Foregrounds

• Many foregrounds
• Galactic synchrotron (especially polarized
  component)
• Radio Frequency Interference (RFI) e.g. radio,
  cell phones, digital radio
• Radio recombination lines
• Radio point sources
• Foregrounds dwarf signal foregrounds 1000s K
  vs 10s mK signal
• Strong frequency dependence Tsky?n-2.6
• Foreground removal exploits smoothness in
  frequency and spatial symmetries

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The first sources
1000 Mpc
Hard X-rays
Lya
330 Mpc
Soft X-rays
HII
5 Mpc
0.2 Mpc
z15
15
Cosmological context
ZR
ZT
Za
Z
Z?30
CMB
Lya
X-ray
UV

• Three main regimes for 21 cm signal
• Each probes different radiation field

16
Global history
Furlanetto 2006
Adiabaticexpansion
X-rayheating
Comptonheating


Heating
expansion
UV ionization

HII regions
recombination
X-ray ionization

IGM
recombination
Lya flux
continuum
injected

• Sources Pop. II Pop. III stars (UVLya)
  Starburst galaxies, SNR, mini-quasar
  (X-ray)
• Source luminosity tracks star formation rate

17
Thermal history
18
Ionization history
Xigt0.1

• Ionization fluctuations relevant for zlt12, not so
  important above that redshift.
• Well restrict to fluctuations at zgt13

Furlanetto, Zaldarriaga, Hernquist 2004
19
21 cm fluctuations
W-FCoupling
Velocitygradient
BaryonDensity
Gas Temperature
Neutralfraction
Radiation backgroundprobed
UV
X-ray
Lya

• In linear theory, peculiar velocities correlate
  with overdensities

Bharadwaj Ali 2004

• Anisotropy of velocity gradient term allows
  angular separation

Barkana Loeb 2005

• Initial observations will average over angle to
  improve S/N

20
21 cm fluctuations z
?

• Exact form very model dependent

21
21 cm fluctuations Lya
Gas Temperature
W-FCoupling
Neutralfraction
Velocitygradient
Density
negligible heating of IGM
IGM still mostlyneutral
Lya flux varies

• Lya fluctuations unimportant after coupling
  saturates (xagtgt1)

• Three contributions to Lya flux
• Stellar photons redshifting into Lya resonance
• Stellar photons redshifting into higher Lyman
  resonances
• X-ray photoelectron excitation of HI

Chen Miralda-Escude 2006
Chen Miralda-Escude 2004
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Fluctuations from the first stars
Density

• Overdense region modifies observed flux from
  region dV
• Relate Lya fluctuations to overdensities

• W(k) is a weighted average

Barkana Loeb 2005
W_K plot - stars/ X-rays
23
Determining the first sources
Chuzhoy Shapiro 2006
Sources
Ja, vs Ja,X
Spectra
aS
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Summary Lya

• Including correct atomic physics is important for
  extracting astrophysical information from 21cm
  fluctuations
• Lya fluctuations dominate 21 cm signal at high z
• Can be used to determine major source of Lya
  photons
• Intermediate scales give information on X-ray
  spectrum
• Constrain bias of sources at high z
• Probe early star formation
• Poisson fluctuations may also be interesting

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21cm fluctuations TK
Gas Temperature
W-FCoupling
Neutralfraction
Velocitygradient
Density
couplingsaturated
density x-rays
IGM still mostlyneutral

• In contrast to the other coefficients bT can be
  negative

• Sign of bT constrains IGM temperature

Pritchard Furlanetto 2006
26
Temperature fluctuations
TSTKltTg Tblt0 (absorption)Hotter region
weaker absorption bTlt0
TSTKTg Tb021cm signal dominated by
temperature fluctuations
TSTKgtTg Tbgt0 (emission) Hotter region
stronger emission bTgt0
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X-ray heating

• X-rays provide dominant heating source in early
  universe(shocks possibly important very early
  on)
• X-ray heating usually assumed to be uniform as
  X-rays have long mean free path
• Simplistic, fluctuations may lead to observable
  21cm signal
• Fluctuations in JX arise in same way as Ja

Mpc
time integral
photo-ionization
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Growth of fluctuations
expansion
X-rays
Compton
Heating fluctuations
Fractional heating per Hubble time at z
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TK fluctuations

• Fluctuations in gas temperature can be
  substantial
• Uniform heating washes out fluctuation on small
  scales
• Inhomogeneous heating amplifies fluctuation on
  large scales
• Amplitude of fluctuations contains information
  about IGM thermal history

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Indications of TK

• When TKltTg very different formfrom Lya
• Dm2 can be negative which is clear indication
  of bT lt0 (trough)
• Existence of featureswill help constrain
  astrophysical parameters

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X-ray source spectra

• Sensitivity to aS through peak amplitude and
  shape
• Also through position of trough
• Effect comes from fraction of soft X-rays

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X-ray background?

• X-ray background at high z is poorly constrained
• Decreasing fX helps separates different
  fluctuations
• Also changes shape of Lya power spectrum
• If heating is late might see temperature
  fluctuations with first 21 cm experiments

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Summary TK

• Inhomogeneous X-ray heating leads to significant
  fluctuations in gas temperature
• Temperature fluctuations track heating rate
  fluctuations, but lag somewhat behind
• Gas temperature fluctuations contain information
  about the thermal evolution of the IGM before
  reionization
• bTlt0 leads to interesting peak-trough structure
• Structure will assist astrophysical parameter
  estimation
• 21cm observations at high-z may constrain
  spectrum and luminosity of X-ray sources

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Redshift slices Lya
z19-20

• Pure Lya fluctuations

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Redshift slices Lya/T
z17-18

• Growing T fluctuationslead first to dip in DTb
  then to double peak structure
• Double peak requiresT and Lya fluctuationsto
  have different scaledependence

36
Redshift slices T
z15-16

• T fluctuations dominate over Lya
• Clear peak-trough structure visible
• Dm2 lt0 on largescales indicates TKltTg

37
Redshift slices T/d
z13-14

• After TKgtTg , thetrough disappears
• As heating continuesT fluctuations die out
• Xi fluctuations willstart to become important
  at lower z

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Observations
poor angular resolution
foregrounds

• Need SKA to probe these brightnessfluctuations
• Observe scalesk0.025-3 Mpc-1
• Easily distinguishtwo models
• Probably wont seetrough (

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Conclusions

• 21 cm fluctuations potentially contain much
  information about the first sources
• Bias
• X-ray background
• X-ray source spectrum
• IGM temperature evolution
• Star formation rate
• Lya and X-ray backgrounds may be probed by future
  21 cm observations
• Foregrounds pose a challenging problem at high z
• SKA needed to observe the fluctuations described
  here

For more details see astro-ph/0607234
astro-ph/0508381
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The end