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The Intergalactic Medium at High Redshifts

This presentation discusses the intricate processes of radiative feedback impacting the intergalactic medium (IGM) prior to reionization. It explores the formation and influence of the first stars and quasars, the enrichment of metals in the universe, and the complex dynamics of winds and outflows. Additionally, the role of photoheating, recombinations, and the structure of the IGM during reionization are examined. The history of the universe from the Big Bang to the emergence of the first galaxies is highlighted, providing a comprehensive overview of high redshift cosmic phenomena.

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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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