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Jonathan Pritchard (Caltech)
Radiation backgrounds from the first sources and the redshifted 21 cm line Jonathan Pritchard (Caltech) Collaborators: Steve Furlanetto (Yale) Advisor: Marc Kamionkowski (Caltech)
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Overview Atomic cascades and the Wouthysen-Field Effect
Detecting the first stars through 21 cm fluctuations (Lya) Inhomogeneous X-ray heating and gas temperature fluctuations (X-ray) Observational prospects z~30 z~12
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Ionization history Gunn-Peterson Trough Becker et al. 2005
Universe ionized below z~6, some neutral HI at higher z black is black WMAP3 measurement of t~0.09 (down from t~0.17) Page et al. 2006 Integral constraint on ionization history Better TE measurements + EE observations
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?? Thermal history Lya forest Zaldarriaga, Hui, & Tegmark 2001
Hui & Haiman 2003 ?? IGM retains short term memory of reionization - suggests zR<10 Photoionization heating erases memory of thermal history before reionization CMB temperature Knowing TCMB=2.726 K and assuming thermal coupling by Compton scattering followed by adiabatic expansion allows informed guess of high z temperature evolution
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21 cm basics TS Tb Tg HI TK HI hyperfine structure
Use CMB backlight to probe 21cm transition TS Tb Tg HI TK n1 11S1/2 l=21cm 10S1/2 n0 z=0 n1/n0=3 exp(-hn21cm/kTs) z=13 fobs=100 MHz f21cm=1.4 GHz 3D mapping of HI possible - angles + frequency 21 cm brightness temperature T_s>>t_g then t_s depedence cancels out of T_b 21 cm spin temperature Coupling mechanisms: Radiative transitions (CMB) Collisions Wouthuysen-Field
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Wouthysen-Field effect
Hyperfine structure of HI 22P1/2 21P1/2 Effective for Ja>10-21erg/s/cm2/Hz/sr Ts~Ta~Tk 21P1/2 20P1/2 W-F recoils Field 1959 nFLJ Lyman a 11S1/2 Selection rules: DF= 0,1 (Not F=0F=0) 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/Pabs~10 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… Remember to smile Pritchard & Furlanetto 2006 Hirata 2006
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Lyman b gg Optically thick to Lyman series
A3p,2s=0.22108s-1 A3p,1s=1.64108s-1 gg Optically thick to Lyman series Regenerate direct transitions to ground state Two photon decay from 2S state Decoupled from Lyman a frecycle,b=0 Agg=8.2s-1
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Lyman g gg Cascade via 3S and 3D levels allows production of Lyman a
frecycle,g=0.26 Higher transitions frecycle,n~ 0.3 gg
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Lyman alpha flux Stellar contribution also a contribution from X-rays…
continuum injected also a contribution from X-rays…
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X-rays and Lya production
spiE-3 HI HII photoionization e- X-ray collisional ionization e- Lya excitation (fa0.8) HI Chen & Miralda-Escude 2006 Shull & van Steenberg 1985 heating
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Experimental efforts (f21cm=1.4 GHz) LOFAR: Netherlands
Freq: MHz Baselines: 100m km MWA: Australia Freq: MHz Baselines: 10m km PAST/21CMA: China Freq: MHz SKA: S Africa/Australia Freq: 60 MHz-35 GHz Baselines: 20m km (f21cm=1.4 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 Tskyn-2.6 Foreground removal exploits smoothness in frequency and spatial symmetries
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Global history Furlanetto 2006 + + Heating + HII regions + IGM
Adiabatic expansion + X-ray heating + Compton heating expansion Heating UV ionization + recombination HII regions X-ray ionization + recombination IGM Lya flux continuum injected Sources: Pop. II & Pop. III stars (UV+Lya) Starburst galaxies, SNR, mini-quasar (X-ray) Source luminosity tracks star formation rate Many model uncertainties
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Ionization history zR~7 t~0.07 Xi>0.1
Reionization at z=7 Tau~0.07 Models differ by factor ~10 in X-ray/Lya per ionizing photon Reionization well underway at z<12
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Thermal history Temperature outside of HII regions
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21 cm fluctuations Twilight Dark Ages Z* Za ZT ZR Z30 Density Z~150
Baryon Density W-F Coupling Velocity gradient Brightness temperature Neutral fraction Gas Temperature Cosmology Reionization X-ray sources Lya sources Cosmology Twilight Dark Ages Z* Za ZT ZR Z30 Density Z~150 Collisionally coupled regime No 21 cm signal Ly X-ray UV Comment on information the different P_mu contain Reionization TS~Tg
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Angular separation? Baryon Density W-F Coupling Velocity gradient Neutral fraction Gas Temperature In linear theory, peculiar velocities correlate with overdensities Bharadwaj & Ali 2004 Anisotropy of velocity gradient term allows angular separation Comment on information the different P_mu contain Barkana & Loeb 2005 Initial observations will average over angle to improve S/N
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21 cm fluctuations Twilight Dark Ages Z* Za ZT ZR Z30 Density Z~150
Baryon Density W-F Coupling Velocity gradient Brightness temperature Neutral fraction Gas Temperature Cosmology Reionization X-ray sources Lya sources Cosmology Twilight Dark Ages Z* Za ZT ZR Z30 Density Z~150 Collisionally coupled regime No 21 cm signal Ly X-ray UV Comment on information the different P_mu contain Reionization TS~Tg
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Reionization Z=12.1 Z=9.2 Z=8.3 Z=7.6
Neutral fraction Gas Temperature W-F Coupling Velocity gradient Density Lya coupling saturated HII regions large IGM hot TK>>Tg Z=12.1 Z=9.2 Z=8.3 Z=7.6 Usual case that people consider and first target of observations Ignore spin temperature fluctuation and consider ionization fluc Furlanetto, Sokasian, Hernquist 2003
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21 cm fluctuations: Lya Neutral fraction Gas Temperature W-F Coupling Velocity gradient Density -negligible heating of IGM -tracks density IGM still mostly neutral Lya flux varies Lya fluctuations unimportant after coupling saturates (xa>>1) Comment that opposite case to reionization Ignore ionization fluc and consider spin temperature fluctuations 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 2004 Chen & Miralda-Escude 2006
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Fluctuations from the first stars
d dV Fluctuations in flux from source clustering, 1/r2 law, optical depth,… Relate Lya fluctuations to overdensities Barkana & Loeb 2005 W(k) is a weighted average
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Determining the first sources
da dominates bias source properties density Chuzhoy, Alvarez, & Shapiro 2006 Sources Ja,* vs Ja,X Lya flux weighted average of contributions from stars and X-rays Spectral distinctions can be determined Pritchard & Furlanetto 2006 Spectra aS z=20 D=[k3P(k)/2p]1/2
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21cm fluctuations: TK Neutral fraction Gas Temperature W-F Coupling Velocity gradient Density coupling saturated IGM still mostly neutral density + x-rays In contrast to the other coefficients bT can be negative Comment on information the different P_mu contain Sign of bT constrains IGM temperature Pritchard & Furlanetto 2006
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Temperature fluctuations
TS~TK<Tg Tb<0 (absorption) Hotter region = weaker absorption bT<0 TS~TK~Tg Tb~0 21cm signal dominated by temperature fluctuations TS~TK>Tg Tb>0 (emission) Hotter region = stronger emission bT>0
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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 often assumed to be uniform as X-rays have long mean free path Simplistic, fluctuations may lead to observable 21cm signal X-ray flux -> heating rate -> temperature Mpc d dV adiabatic index -1 Barkana & Loeb 2005
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Growth of fluctuations
expansion X-rays Compton temperature fluctuations Heating fluctuations Fractional heating per Hubble time at z dT/ d=
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TK fluctuations Fluctuations in gas temperature can be substantial
Amplitude of fluctuations contains information about IGM thermal history Difference between uniform and inhomogeneous heating can be substantial
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Indications of TK dT dominates Constrain heating transition
TK<Tg TK>Tg Angle averaged power spectrum m2 part of power spectrum Dm2 <0 on large scales indicates TK<Tg (but can have Pdx<0)
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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? dx dT da
X-ray background at high z is poorly constrained Extrapolating low-z X-ray:IR correlation gives: Glover & Brand 2003 dT da dx ? 1st Experiments might see TK fluctuations if heating late
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21 cm fluctuations: z ? Exact form very model dependent
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Redshift slices: Lya z=19-20 Pure Lya fluctuations
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Redshift slices: Lya/T
z=17-18 Growing T fluctuations lead first to dip in DTb then to double peak structure Double peak requires T and Lya fluctuations to have different scale dependence
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Redshift slices: T z=15-16 T fluctuations dominate over Lya
Clear peak-trough structure visible Dm2 <0 on large scales indicates TK<Tg
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Redshift slices: T/d z=13-14 After TK>Tg , the trough disappears
As heating continues T fluctuations die out Xi fluctuations will start to become important at lower z
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Observations Need SKA to probe these brightness fluctuations
poor angular resolution foregrounds Need SKA to probe these brightness fluctuations Observe scales k= Mpc-1 Easily distinguish two models Probably won’t see trough :(
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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 Will be interesting to include spin temperature fluctuations in future simulations For more details see astro-ph/ & astro-ph/
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Extra Slides
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21 cm basics TS Tb Tg HI TK HI hyperfine structure
l=21cm n1/n0=3 exp(-hn21cm/kTs) Use CMB backlight to probe 21cm transition TS Tb Tg HI TK z=0 z=13.75 fobs=94.9 MHz f21cm=1.4 GHz (KUOW) 3D mapping of HI possible - angles + frequency
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21 cm basics 21 cm brightness temperature 21 cm spin temperature
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The first sources z=15 1000 Mpc Hard X-rays Lya 330 Mpc Soft X-rays
HII 5 Mpc 0.2 Mpc z=15
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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 often 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 photo- ionization time integral d dV
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Growth of fluctuations
expansion X-rays Compton temperature fluctuations Heating fluctuations Fractional heating per Hubble time at z
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