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The Intergalactic Medium at High Redshifts

The Intergalactic Medium at High Redshifts. Steve Furlanetto Yale University September 25, 2007. Outline. Radiative Feedback on the IGM Before Reionization Physics: first stars, first quasars Metal Enrichment Physics: winds/outflows Reionization and the IGM

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The Intergalactic Medium at High Redshifts

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  1. The Intergalactic Medium at High Redshifts Steve Furlanetto Yale University September 25, 2007

  2. Outline • Radiative Feedback on the IGM Before Reionization • Physics: first stars, first quasars • Metal Enrichment • Physics: winds/outflows • Reionization and the IGM • Physics: photoheating, recombinations, IGM structure • Conclusion

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

  4. Part I: Radiative Feedback on the IGM

  5. The First Sources of Light • First sources produce… • Small HII regions • Lyman-series photons: interact with IGM hydrogen, H2 • X-rays

  6. The First Sources of Light: Ultraviolet Feedback • H2 Cooling • Most important coolant for Pop III stars • Photo-dissociated by Lyman-Werner photons (11.26-13.6 eV)

  7. X-rays are highly penetrating in IGM Mean free path >Mpc Deposit energy as heat, ionization Free electrons catalyze H2 formation! Produced by… Supernovae Stellar mass black holes Quasars Very massive stars The First Sources of Light:X-ray Heating

  8. The First Sources of Light • First sources produce… • Small HII regions • Lyman-series photons: interact with IGM hydrogen, H2 • X-rays • How can we observe these backgrounds?

  9. The X-Ray Background • Hard X-rays can redshift to present day • Limited by unresolved soft X-ray background to ~10 X-rays/H atom • 1 keV/X-ray ~107 K: lots of heat! Number of X-rays/H atom Mean QSO spectrum Miniquasars? Dijkstra et al. (2004)

  10. 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! SF, AS, LH (2004)

  11. The Spin Temperature • CMB photons drive toward invisibility: TS=TCMB • Collisions couple TS to TK • At mean density, assuming TK and xi from recombination, efficient until z~50 • 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)

  12. The Wouthuysen-Field Mechanism I Selection Rules: DF=0,1 (except F=0  F=0) 2P3/2 1P3/2 1P1/2 0P1/2 1S1/2 Mechanism is effective with ~0.1 Ly photon/baryon 0S1/2

  13. The Wouthuysen-FieldMechanism II • Relevant photons are continuum photons that redshift into the Ly resonance • Same photons that dissociate H2! … Ly Ly Ly Ly

  14. The Global Signal:First Light • First stars flood Universe with soft-UV photons • W-F effect • Photodissociation • X-rays follow later • Heating • Low ionization Pop III Stars Pop II Stars SF (2006)

  15. Ly Fluctuations • Ly photons decrease TS near sources (Barkana & Loeb 2004) • Clustering • 1/r2 flux • Strong absorption near dense gas, weak absorption in voids Cold, Absorbing Cold, invisible

  16. Ly Fluctuations Cold, Absorbing • Ly photons decrease TS near sources • Clustering • 1/r2 flux • Strong absorption near dense gas, weak absorption in voids • Eventually saturates when IGM coupled everywhere

  17. X-ray Fluctuations • X-ray photons increase TK near sources (Pritchard & Furlanetto 2007) • Clustering • 1/r2 flux • Hot IGM near dense gas, cool IGM near voids Hot Cool

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

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

  20. X-ray Fluctuations = + Hot, emitting

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

  22. Part II: Metal Enrichment

  23. Metal Enrichment • How does the transition from Pop III to Pop II occur? • How do metals reach the Ly forest? • How do galaxy’s metals build up?

  24. Small galaxies have small potential wells Supernova winds easily escape into IGM Parameterized as “filling factor” of IGM Supernova Winds

  25. Wind Characteristics • Simple analytic model • Mechanical Luminosity provided by SN rate (and hence SFR) • Use thin-shell approximation (Tegmark et al. 1993) • All mass confined to spherical thin shell (no fragmentation) • Sweeps up all IGM mass • Driving force is hot bubble interior • MANY uncertainties SF, AL (2003)

  26. Metal Enrichment in Simulations Oppenheimer & Davé (2006) • Expect ~1-10% of IGM enriched by z=6; galaxies surrounded by ~10-100 kpc “wind bubbles” • Typically Z~0.01 Zsun in these regions • Expect significant absorption, e.g. CII: GP~0.16 (Z/10-2.5 Zsun) (1+z/7)3/2

  27. Metal Absorption Lines SDSS collaboration (1+zs)lLya (1+zs)lmetal  Can probe lLya/lmetal< (1+z)/(1+zs) < 1

  28. Metal Absorption Lines • Important lines: • Most abundant elements produced by Type II SNe: C (YCSN=0.1 Msun), O (0.5 Msun), Si (0.06 Msun), Fe (0.07 Msun) • Most abundant elements produced by VMS SNe: C (YCSN=4.1 Msun), O (44 Msun), Si (16 Msun), Fe (6.4 Msun) • Ionization states determined by radiation background and nearby galaxy  CII, OI, SiII, FeII for neutral medium  CIV, SiIV for ionized medium • Identifying lines may be difficult • Doublets straightforward (high-ionization) • Low-ionization probably require several lines

  29. What Can We Learn? • z=8,f*=0.1, Q~0.07 • Net absorption similar for low, high-ionization states • Strong absorbers surround large, young galaxies • Distribution of strong/weak absorbers depends on filling factor, galaxy distribution SF, AL (2003)

  30. Metal Lines and Reionization • OI/HI in tight charge-exchange equilibrium • ~0.14 (Z/10-2.5 Zsun) for equivalent GP trough • Dense regions enriched first  “forest” of (unsaturated) OI lines near reionization, if they remain neutral Oh (2002)

  31. The Real OI “Forest” • Becker et al. (2006) detected six OI systems at z>5 • Four along one (highly-ionized) line of sight! • CIV also detected (Ryan-Weber et al. 2006) • Comparable total metal abundance to lower redshifts

  32. Other Ways to Observe Metal Enrichment • Metal lines in the CMB (Basu et al. 2004, Hernandez-Monteagudo et al. 2007) • Direct observations of cooling lines • “Fossil” enrichment at z<6 • “Near-field cosmology” and old stars • Ongoing Pop III star formation? • Inefficient micro-mixing? (Jimenez & Haiman 2007) • New galaxies in voids? (Scannapieco et al. 2006, Tornatore et al. 2007)

  33. Part III: Reionization and the IGM

  34. Some Unsolved IGM Questions in Reionization… • What is the Ly forest actually telling us?

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

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

  37. Lyman-series Optical Depths • When integrating over large path length, must include cosmic web • Transmission samples unusually underdense voids • Requires model for density distribution! • Extremely difficult to measure xHI! • Different lines sample different densities Oh & Furlanetto (2005)

  38. Some Unsolved IGM Questions in Reionization… • What is the Ly forest actually telling us? • Need precise model of the IGM • What role does photoheating play?

  39. Photoheating Feedback • Effectiveness is controversial (Dijkstra et al. 2004) • “Bias” of photoheating has similar effects to those for metal enrichment

  40. Some Unsolved IGM Questions in Reionization… • What is the Ly forest actually telling us? • Need precise model of the IGM • What role does photoheating play? • Need to observe the process in detail • What role do IGM recombinations play?

  41. Recombinations and Reionization • Diffuse IGM • “Clumping factor” uncertain by factor~30! • Minihalos • Marginally important • Difficult to observe • Lyman Limit systems • Dramatically affect topology of reionization and transition to “cosmic web” domination

  42. Some Unsolved IGM Questions in Reionization… • What is the Ly forest actually telling us? • Need precise model of the IGM • What role does photoheating play? • Need to observe the process in detail • What role do IGM recombinations play? • Need good models for interaction of sources and IGM structures

  43. Helium Reionization • HeII has ionization potential of 54 eV • Ionized by quasars • Recombination rate ~5.5 times faster • Appears to occur at z~3 • Direct evidence from quasar spectra • Wide range of indirect evidence Heap et al. (2000) Shull et al. (2004)

  44. Modeling Helium Reionization • Apply models of hydrogen reionization to helium! • Key differences: • Recombinations much faster • Double reionization? • Sources rare and bright (more stochasticity) • Source population is known • IGM properties are known

  45. Evidence for Helium Reionization: Equation of State Schaye et al. (2000) • Minimum observed temperature experiences jump at z~3.2 (though others disagree) • Accompanied by flattening of equation of state (see also Ricotti et al. 2000)

  46. Models for Helium Reionization: Equation of State • Similar temperature jump to observed value • Requires slightly higher temperatures than expected • Mean temperature lacks sudden jump (may resolve controversy!) Furlanetto & Oh (in prep)

  47. Models for Helium Reionization: Equation of State • Equation of state is highly structured! • Amount of structure depends on density-ionization correlation Furlanetto & Oh (in prep)

  48. Evidence for Helium Reionization: eff • Lyforest optical depth depends on temperature through recombination coefficient • Expect drop in eff at z~3 • See also Bernardi et al. (2003) Faucher-Giguère et al. (2007)

  49. Evidence for Helium Reionization: eff • Top panel: without helium reionization • Bottom panel: with helium reionization • Similar magnitude to observed value, but much different shape Furlanetto & Oh (in prep)

  50. Conclusions • Radiative Feedback on the IGM Before Reionization • Physics: first galaxies, first X-ray sources • Key observations: the 21 cm line, X-ray background • Metal Enrichment • Physics: metallicity threshold, winds/outflows • Key observations: quasar/GRB spectra, cooling lines • Reionization and the IGM • Physics: photoheating, density distribution, recombinations • Key observations: helium reionization (actually tells you a lot more)! • Conclusion

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