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Atomic physics and the cosmological 21 cm signal

This presentation provides an overview of the 21 cm signal in atomic physics and cosmology, including topics such as the Wouthysen-Field effect, heating of the IGM, and 21 cm fluctuations from spin temperature variation.

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Atomic physics and the cosmological 21 cm signal

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  1. Atomic physics and the cosmological 21 cm signal Jonathan Pritchard (CfA) Collaborators: Steve Furlanetto (UCLA) Avi Loeb (CfA)

  2. Overview • 21 cm basics • Wouthysen-Field Effect • Heating of the IGM • 21 cm fluctuations from spin temperature variation • Redshift evolution of the 21 cm signal z~30 z~12

  3. TS Tb Tg HI TK 21 cm basics • Use CMB backlight to probe 21cm transition • HI hyperfine structure n1 11S1/2 l=21cm 10S1/2 n0 z=0 z=13 n1/n0=3 exp(-hn21cm/kTs) fobs=100 MHz f21cm=1.4 GHz • 3D mapping of HI possible - angles + frequency • 21 cm brightness temperature • 21 cm spin temperature Coupling mechanisms: Radiative transitions (CMB) Collisions Wouthuysen-Field

  4. 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 Lymana 11S1/2 Selection rules: DF= 0,1 (Not F=0F=0) 10S1/2

  5. Spectral Distortion of Lya • Hubble flow, scatteringrecoils • Recoil sources featureas photons diffuse in frequency • Colour temperature numberflux • Coupling ≤1 frequency Chen & Miralde-Escude 2004

  6. Lya heating • Recoils lead to heating Madau, Meiksin, Rees 1997 • Spectral distortion + diffusion leads to counterterm Chen & Miralda-Escude 2004Rybicki 2008 • Lya heating much reduced over initial MMR estimates since T~TR -> possibility of WF coupled 21 cm absorption regime • Injected photons can cool (T>10K) as well as heat (T<10K) • X-ray heating dominates over Lya heating for sensible parameters Chen & Miralda-Escude 2004 Furlanetto & Pritchard 2006 • In absence of other heat sources continuum and injection processes -> T~100K Chuzhoy & Shapiro 2006

  7. Detailed atomic corrections • Fine structure of levels • Spin diffusivity • Mild heating of gas so Teff≠TK • Corrections most important at low temperatures, but typically small • Detailed calculations at z~20 will need to take these into account Hirata 2006

  8. 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… Pritchard & Furlanetto 2006 Hirata 2006

  9. Lyman b A3p,2s=0.22108s-1 A3p,1s=1.64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,b=0 Agg=8.2s-1

  10. Lyman g gg • Cascade via 3S and 3D levelsallows production of Lyman a

  11. Lyman alpha flux • Stellar contribution continuum injected • Cascades modify estimate that all Lyn are equal • Spatial distribution of Lya photons important for 21 cm signal

  12. Scattering in IGM source line center • Optical depth so large photons scatter in wings before reaching line center • Steepens Lya flux profile about sources • Increases inhomogeneity of Lya background Chuzhoy & Zheng 2007Smelin, Combes & Beck 2007 Naoz & Barkana 2007

  13. 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, inhomogeneities may lead to observable 21cm signal • X-ray flux -> heating rate -> temperature • Weak constraints from diffuse soft x-ray background Mpc Dijikstra, Haiman, Loeb 2004

  14. HI HII photoionization X-ray e- collisionalionization e- excitation Lya HI heating Energy deposition • X-ray energy partitioned heating ionization Ly other  Shull & van Steenberg 1985Valdes & Ferrara 2008 • X-rays ionize HeI mostly, but secondary electrons ionize HI Chen & Miralda-Escude 2006 • Lya from X-ray can dominate over stellar Lya near into sources • Makes X-ray pre-heating without WF coupling unlikely • Two phase medium: ionized HII regions and partially ionized IGM outside xe<0.1

  15. Thermal history Furlanetto 2006

  16. 21 cm fluctuations W-FCoupling Velocitygradient BaryonDensity Neutralfraction Gas Temperature Brightnesstemperature b Cosmology Reionization X-raysources Lyasources Cosmology Amount ofneutral hydrogen Spin temperature Velocity(Mass)

  17. Temporal separation? W-FCoupling Velocitygradient BaryonDensity Neutralfraction Gas Temperature Brightnesstemperature b Cosmology Reionization X-raysources Lyasources Cosmology Twilight Dark Ages

  18. d dV Fluctuations from the first stars • 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

  19. Determining the first sources da dominates source properties density bias Chuzhoy,Alvarez, & Shapiro 2006 Sources Ja,* vs Ja,X Pritchard & Furlanetto 2006 Spectra aS z=20 D=[k3P(k)/2p]1/2

  20. Growth of fluctuations adiabatic index -1 expansion X-rays Compton temperature fluctuations Heating fluctuations Fractional heating per Hubble time at z dT/ d=

  21. TK<Tg TK>Tg Indications of TK • Learn about source bias and spectrum in same way as Lya • Constrain heating transition dT dominates

  22. dx dT da ? X-ray background? • X-ray background at high z is poorly constrained Extrapolating low-z X-ray:IR correlation gives: Glover & Brand 2003 • 1st Experiments might see TK fluctuations if heating late

  23. Experimental efforts MWA: Australia Freq: 80-300 MHz Na= 500 Atot= 0.007 km2 21CMA: China Freq: 70-200 MHz Na= 20 Atot= 0.008 km2 LOFAR: Netherlands Freq: 120-240 MHz Na= 64 Atot= 0.042 km2 SKA: S Africa/Australia Freq: 60 MHz-35 GHz Na= 5000 Atot= 0.6 km2 (f21cm=1.4 GHz)

  24. x  T SKA ? MWA Temporal separation Pritchard& Loeb 2008

  25. High-z Observations poor angular resolution foregrounds • Need SKA to probe these brightnessfluctuations • Observe scalesk=0.025-2 Mpc-1 • Easily distinguishtwo models • Optimistic foregroundremoval • B~12MHz ->Dz~1.5

  26. Conclusions • Significant recent progress in understanding details of Wouthysen-Field effect • Lya heating is weak allowing WF coupled 21 cm absorption signal • X-rays probably most significant heating source • 21 cm fluctuations contain information about early luminous sources and thermal history • Need detailed knowledge of atomic physics to extract precise information from upcoming 21 cm observations • Lots of work to be done!

  27. X-ray heating • To heat gas above TCMB estimate the required fraction of baryons in stars as: • Compare with Lya coupling and reionization Lya coupling Reionization • Expect Lya coupling > X-ray heating > Reionization

  28. Lya production • For stars to produce critical Lya flux photons/baryon required fraction ofbaryon in stars

  29. Profile around the first stars Chen & Miralda-Escude 2006

  30. X-ray heating about a source Zaroubi et al. 2006

  31. Lya heating? • Lya can heat • Lyn can cool • Deutrium important • Effect from stellar photons usually small T_K<100K, so X-rayheating dominates. • Scattering blue ward of resonance important here - makes • Heating more efficient as all scatterings heat gas. • Figure?

  32. 21cm fluctuations: TK Neutralfraction Gas Temperature W-FCoupling Velocitygradient Density b couplingsaturated IGM still mostlyneutral density + x-rays • In contrast to the other coefficients bT can be negative • Sign of bT constrains IGM temperature Pritchard & Furlanetto 2006

  33. 21 cm fluctuations: Lya Neutralfraction Gas Temperature W-FCoupling Velocitygradient Density b -negligible heating of IGM-tracks density IGM still mostlyneutral Lya flux varies • Lya fluctuations unimportant after coupling saturates (xa>>1) • 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

  34. 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 • Cross-correlation with galaxies also important

  35. Angular separation? W-FCoupling Velocitygradient BaryonDensity Neutralfraction Gas Temperature b • 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

  36. Global history Furlanetto 2006 Adiabaticexpansion X-rayheating + + Comptonheating Heating expansion 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

  37. Ionization history zR~7t~0.07 Xi>0.1 • Models differ by factor ~10 in X-ray/Lya per ionizing photon • Reionization well underway at z<12 (Pop II=later generation stars, Pop. III = massive metal free stars)

  38. TK fluctuations • Fluctuations in gas temperature can be substantial • Amplitude of fluctuations contains information about IGM thermal history

  39. Temperature fluctuations TS~TK<Tg Tb<0 (absorption)Hotter region = weaker absorption bT<0 TS~TK~Tg Tb~021cm signal dominated by temperature fluctuations TS~TK>Tg Tb>0 (emission) Hotter region = stronger emission bT>0

  40. Cosmology? Neutralfraction Gas Temperature W-FCoupling Velocitygradient Density b couplingsaturated IGM still mostlyneutral IGM hotTK>>Tg • Probe smaller scales than CMB • Unless pristine very difficult toimprove on Planck level Kleban, Sigurdson & Swanson 2007

  41. Density power spectrum McQuinn et al. 2006 • Moderate improvements - cross-check • Improve ns and Wnmost • See baryonic oscillation at few sigma

  42. Reionization Neutralfraction Gas Temperature W-FCoupling Velocitygradient Density b Lya coupling saturated IGM hotTK>>Tg HII regions large Z=12.1 Z=9.2 Z=8.3 Z=7.6 Furlanetto, Sokasian, Hernquist 2003

  43. Growth of HII Regions Trac & Cen 2006 • 100 Mpc cube N-body + UV photon ray tracing • Details of heating and coupling are neglected

  44. The first sources 1000 Mpc Hard X-rays Lya 330 Mpc Soft X-rays HII 5 Mpc 0.2 Mpc z=15

  45. 21 cm fluctuations: z ? • Exact form very model dependent

  46. Redshift slices: Lya z=19-20 • Pure Lya fluctuations

  47. Redshift slices: Lya/T z=17-18 • Growing T fluctuationslead first to dip inDTb then to double peak structure • Double peak requiresT and Lya fluctuationsto have different scaledependence

  48. Redshift slices: T z=15-16 • T fluctuations dominate over Lya • Clear peak-trough structure visible • Dm2 <0 on largescales indicates TK<Tg

  49. Redshift slices: T/d z=13-14 • After TK>Tg , thetrough disappears • As heating continuesT fluctuations die out

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