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Introduction: What is Cosmic Reionization?

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  1. Probing the neutral intergalactic medium during cosmic reionization using the 21cm line of hydrogen KIAA-PKU Summer School, Beijing, China Chris Carilli, NRAO, August 10, 2007 • Introduction: What is Cosmic Reionization? • Current constraints on the IGM neutral fraction with cosmic epoch • Neutral Intergalactic Medium (IGM) – HI 21cm signals • Low frequency telescopes and observational challenges

  2. References • Reionization and HI 21cm studies of the neutral IGM • “Observational constraints on cosmic reionization,” Fan, Carilli, Keating 2006, ARAA, 44, 415 • “Cosmology at low frequencies: the 21cm transition and the high redshift universe,” Furlanetto, Oh, Briggs 2006, Phys. Rep., 433, 181 • Early structure formation and first light • “The first sources of light and the reionization of the universe,” Barkana & Loeb 2002, Phys.Rep., 349, 125 • “The reionization of the universe by the first stars and quasars,” Loeb & Barkana 2002, ARAA, 39, 19 • “Observations of the high redshift universe,” Ellis 2007, Saas-Fe advanced course 36

  3. History of Baryons in the Universe Ionized f(HI) ~ 0 Neutral f(HI) ~ 1 Reionized f(HI) ~ 1e-5

  4. Chris Carilli (NRAO) Berlin June 29, 2005 WMAP – structure from the big bang

  5. Hubble Space Telescope Realm of the Galaxies

  6. Dark Ages Epoch of Reionization Twilight Zone • Last phase of cosmic evolution to be tested • Bench-mark in cosmic structure formation indicating the first luminous structures

  7. Dark Ages Epoch of Reionization Twilight Zone • Epoch? • Process? • Sources?

  8. Some basics Age of the Universe IGM recombination time z > 8: trecomb < tuniv At z>6: tuniv~ 0.55 [(1+z)/10]-3/2Gyr Cen 2002 • Stellar fusion produces 7e6eV/H atom. • Reionization requires 13.6eV/H atom =>Need to process only 1e-5 of baryons through stars to reionize the universe

  9. Reionization: the movie Gnedin 03 8Mpc comoving

  10. Constraint I: Gunn-Peterson Effect z Barkana and Loeb 2001

  11. Gunn-Peterson Effect toward z~6 SDSS QSOs Fan et al 2006

  12. Gunn-Peterson limits to f(HI) GP = 2.6e4 f(HI) (1+z)3/2 End of reionization? f(HI) <1e-4 at z= 5.7 f(HI) >1e-3 at z= 6.3 •  to f(HI) conversion requires ‘clumping factor’ •  >>1 for f(HI)>0.001 => low f() diagnostic • GP => Reionization occurs in ‘twilight zone’, opaque for obs <0.9 m But…

  13. Contraint II: The CMB Temperature fluctuations due to density inhomogeneities at the surface of last scattering (z ~ 1000) Baryon Acoustic Oscillations: Sound horizon at recombination Angular power spectrum ~ variance on given angular scale ~ square of visibility function Sachs-Wolfe

  14. Reionization and the CMB • Thomson scatting during reionization (z~10) • Acoustics peaks are ‘fuzzed-out’ during reionization. • Problem: degenerate with intrinsic amplitude of the anisotropies.

  15. Constraint II: CMB large scale polarization -- Thomson scattering during reionization • Scattering CMB local quadrapole => polarized • Large scale: horizon scale at reioniz ~ 10’s deg • Signal is weak: • TE ~ 10% TT • EE ~ 1% TT Hinshaw et al 2008 e = 0.084 +/- 0.016 ~ L/mfp ~Lnee(1+z)2

  16. Constraint II: CMB large scale polarization -- Thomson scattering during reionization • Rules-out high ionization fraction at z> 15 • Allows for finite (~0.2) ionization to high z • Most action occurs at z ~ 8 to 14 Dunkley et al. 2008

  17. Combined CMB + GP constraints on reionization But… e = integral measure to recombination=> allows many IGM histories

  18. Pushing into reionization: QSO 1148+52 at z=6.4 • tuniv = 0.87Gyr • Lbol = 1e14 Lo • Black hole: ~3 x 109 Mo (Willot etal.) • Gunn Peterson trough (Fan etal.)

  19. 1148+52 z=6.42: Gas detection 46.6149 GHz CO 3-2 Off channels Rms=60uJy VLA IRAM • M(H2) ~ 2e10 Mo • zhost = 6.419 +/- 0.001 (note: zly = 6.37 +/- 0.04) VLA

  20. Constrain III: Cosmic Stromgren Sphere • Accurate zhostfrom CO: z=6.419 +/- 0.001 • Proximity effect: photons leaking from 6.32<z<6.419 White et al. 2003 z=6.32 ‘time bounded’ Stromgren sphere: Rphys = 4.7 Mpc

  21. ‘time bounded’ Stromgren sphere: R = 4.7 Mpc # ionizing photons = # atoms in volume Luv • tqso = 4/3R3 • <nHI> => tqso = 1e5 R3 f(HI)~ 1e7yrs or f(HI) ~ 1 (tqso/1e7 yr) Loeb & Rybicki 2000

  22. Wyithe et al. 2005 CSS: Constraints on neutral fraction at z~6 P(>xHI) 90% probability x(HI) > curve = tqso/4e7 yrs • Nine z~6 QSOs with CO or MgII redshifts:<R> = 4.4 Mpc (Wyithe et al. 05; Fan et al. 06; Kurk et al. 07) • GP => f(HI) > 0.001 • If f(HI) ~ 0.001, then <tqso> ~ 1e4 yrs – implausibly short given QSO fiducial lifetimes (~1e7 years)? • Probability arguments: f(HI) > 0.05

  23. Difficulties for Cosmic Stromgren Spheres • (Lidz + 07, Maselli + 07) • Requires sensitive spectra in difficult near-IR band • Sensitive to R: f(HI)  R^-3 • Clumpy IGM => ragged edges • Pre-QSO reionization due to star forming galaxies, early AGN activity

  24. OI • Not ‘event’ but complex process, large variance: zreion ~ 6 to 14 • Good evidence for qualitative change in nature of IGM at z~6 ESO

  25. Integral measure? Geometry, pre-reionization? Local ionization? OI Abundance? Saturates, HI distribution function, pre-ionization? Local ioniz.? • Current probes are all fundamentally limited in diagnostic power • Need more direct probe of process of reionization = HI 21cm line

  26. Low frequency radio astronomy: Most direct probe of the neutral IGM during, and prior to, cosmic reionization, using the redshifted HI 21cm line: z>6 => 100 – 200 MHz Square Kilometer Array

  27. Advantages to the HI 21cm line Spectral line signal => full three dimensional (3D) diagnostic of structure formation. Direct probe of IGM = dominant component of baryons during reinization/dark ages Hyperfine transition = forbidden (weak) => avoid saturation (cf. Ly ),  can study the full redshift range of reionization. Diagnostics: When: direct measure of epoch of reionization Process: Inside-out vs. outside-in reionization Sources: Xrays vs. UV vs. shocks Feedback due to galaxy/AGN formation Exotic mechanisms: Very high z particle decay

  28. HI mass limits => large scale structure Reionization 1e13 Mo 1e9 Mo

  29. Brightness Temperature • Brightness temp = measure of surface brightness (Jy/SR, Jy/beam, Jy/arcsec2) • TB = temp of equivalent black body, B, with surface brightness = source surface brightness at : I = S /  = B= kTB/ 2 • TB = 2 S / 2 k  • TB = physical temperature for optically thick thermal object • TA <= TB always Source size > beam TA = TB (2nd law therm.) Source size < beam TA < TB source TB [Explains the fact that temperature in focal plane of optical telescope cannot exceed TB of a source] beam telescope

  30. HI 21cm radiative transfer: large scale structure of the IGM LSS: Neutral fraction / Cosmic density / Temperature: Spin, CMB Furlanetto et al. 2006

  31. Spin Temperature Collisions w. e- and atoms Ambient photons (predominantly CMB) Ly resonant scattering: Wouthuysen-Field effect = mixing of 1S HF levels through resonant scattering of Ly drives Ts to Tkin Ly 21cm h21/k = 0.067K Each Ly photon scatters ~ 1e5 times in IGM before redshifting out of freq window.

  32. Dark Ages HI 21cm signal • z > 200: T = TK = Ts due to collisions + Thomson scattering => No signal • z ~ 30 to 200: TK decouples from T, but collisions keep Ts ~ TK => absorption signal • z ~ 20 to 30: Density drops  Ts~ T => No signal T = 2.73(1+z) TK = 0.026(1+z)^2 Furlanetto et al. 2006

  33. TK T Enlightenment and Cosmic Reionization-- first luminous sources • z ~ 15 to 20: TScouples to TK via Lya scattering, but TK < T => absorption • z ~ 6 to 15: IGM is heated (Xrays, Lya, shocks), partially ionized => emission • z < 6: IGM is fully ionized

  34. Signal I: Global (‘all sky’) reionization signature Signal ~ 20mK < 1e-4 sky Feedback in galaxy formation No Feedback Possible higher z absorption signal via Lya coupling of Ts -- TK due to first luminous objects Furlanetto, Oh, Briggs 06

  35. Signal II: HI 21cm Tomography of IGM Zaldarriaga + 2003 z=12 9 7.6 • TB(2’) = 10’s mK • SKA rms(100hr) = 4mK • LOFAR rms (1000hr) = 80mK

  36. Signal III: 3D Power spectrum analysis only LOFAR  + f(HI) SKA McQuinn + 06

  37. PS dependence on neutral fraction z=10 ‘knee’ ~ characteristic bubble scale ~ 1 to 10Mpc (cm) xi=0.78 xi = 0.13 Furlanetto et al. 2006

  38. Inside-out vs. Outside-in z=12z=15 Inside-out Outside-in Furlanetto et al. 2004

  39. Sensitivity of MWA for PS measurements (Lidz et al. 2007) • Will measure PS variance over k ~ 0.1 to 1 Mpc-1 (cm) • Sensitivity is maximized with compact array configuration (blue) 1yr, 30MHz BW, 6MHz chan

  40. Sensitivity of MWA for PS measurements (Lidz et al. 2007) • Constrain amplitude of PS (variance) to 5 to 10 • Constrain slope of PS to similar accuracy

  41. Signal IV: Cosmic Web after reionization Ly alpha forest at z=3.6 ( < 10) Womble 96 • N(HI) = 1e13 – 1e15 cm^-2, f(HI/HII) = 1e-5 -- 1e-6 => before reionization N(HI) =1e18 – 1e21 cm^-2 • Lya ~ 1e7 21cm => neutral IGM opaque to Lya, but translucent to 21cm

  42. Signal IV: Cosmic web before reionization: HI 21Forest 19mJy z=12 z=8 130MHz 159MHz • radio G-P (=1%) • 21 Forest (10%) • mini-halos (10%) • primordial disks (100%) • Perhaps easiest to detect (use long baselines) • ONLY way to study small scale structure during reionization

  43. Radio sources beyond the EOR sifting problem (1/1400 per 20 sq.deg.) 1.4e5 at z > 6 S120 > 6mJy 2240 at z > 6

  44. Signal V: Cosmic Stromgren spheres around z > 6 QSOs • LOFAR ‘observation’: • 20xf(HI)mK, 15’,1000km/s • => 0.5 x f(HI) mJy • Pathfinders: Set first hard limits on f(HI) at end of cosmic reionization • Easily rule-out cold IGM (T_s < T_cmb): signal = 360 mK 5Mpc 0.5 mJy Wyithe et al. 2006

  45. Dark age HI 21cm signal: baryon oscillations 0.1 1.0 10 Mpc-1 Barkana & Loeb: “Richest of all cosmological data sets” • Three dimensional in linear regime • Probe to k ~ 103 Mpc-1 vs. CMB limit set by photon diffusion ~ 0.2Mpc-1 • Alcock-Pascinsky effect • Kaiser effect + peculiar velocites

  46. Challenge: sensitivity at very low frequency PS detection • 1 SKA, 1 yr, 30MHz (z=50), 0.1MHz • TBsky = 100(/200MHz)-2.7 K = 1.7e4 K At l=3000, k=0.3 Mpc-1 • Signal ~ 2 mK • Noise PS ~ 1 mK • Requires few SKAs

  47. BREAK

  48. Challenge I: Low frequency foreground – hot, confused sky Eberg 408 MHz Image (Haslam + 1982) Signal < 20mK Sky > 200 K DNR > 1e4 • Coldest regions: T ~ 100 (/200 z)-2.6 K • 90% = Galactic foreground • 10% = Egal. radio sources ~ 1 source deg-2 with S140 > 1 Jy

  49. Solution: spectral decomposition (eg. Morales, Gnedin…) • Foreground = non-thermal = featureless over ~ 100’s MHz • Signal = fine scale structure on scales ~ few MHz Signal/Sky ~ 2e-5 10’ FoV; SKA 1000hrs Cygnus A 500MHz 5000MHz Simply remove low order polynomial or other smooth function?

  50. Crosscorrelation in frequency, or 3D power spectral analysis: different symmetries in frequency space for signal and foregrounds. Freq Foreground Signal Morales 2003