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Image credit: Kim Nillson

Lyman Alpha Emitters (LAEs) as a Probe of the High-Redshift Universe Mark Dijkstra (CfA) Collaborators: Stuart Wyithe (Melbourne), Zoltan Haiman, (Columbia) Avi Loeb, Adam Lidz (CfA), Andrei Z Mesinger (Prinzeton), Marco Spaans (Groningen). Image credit: Kim Nillson. Introduction.

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Image credit: Kim Nillson

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  1. Lyman Alpha Emitters (LAEs) as a Probe of the High-Redshift UniverseMark Dijkstra (CfA)Collaborators: Stuart Wyithe (Melbourne), Zoltan Haiman, (Columbia) Avi Loeb, Adam Lidz (CfA),Andrei Z Mesinger (Prinzeton), Marco Spaans (Groningen) Image credit: Kim Nillson

  2. Introduction • Lya is produced following recombination in HII regions surrounding O + B stars. Llya scales in proportion with SFR*(1-fesc) (Llya=7-25%Lbol!) • Ionization state of the IGM may leave imprint on observed Lya flux. HI • A neutral IGM may suppress the number of observed Lya emitters beyond the redshift corresponding to the end of the EoR (e.g. Haiman & Spaans 99).

  3. Are existing known LAEs probing the EoR? • Observed number density of LAEs drops suddenly beyond z=6? Cum. Number density Log (Lya Luminosity) • 89 LAEs observed at z=5.7 (Shimasaku+06, blue squares), • 57 LAEs observed at z=6.5 (Kashikawa+06, red circles) (also see Ota+08, and talks by Ota & M. Ouchi)

  4. Are existing known LAEs probing the EoR? • Observed number density of LAEs drops suddenly beyond z=6? Cum. Number density Log (Lya Luminosity) • 89 LAEs observed at z=5.7 (Shimasaku+06, blue squares), • 57 LAEs observed at z=6.5 (Kashikawa+06, red circles) Restframe UV LF remains constant!

  5. Are existing known LAEs probing the EoR? • Observed number density of LAEs drops suddenly beyond z=6? • For galaxies of a given restframe UV flux density, their corresponding measured Lya flux from galaxies at z=6.5 is lower than at z=5.7 (Kashikawa+06). How much lower? Cum. Number density Log (Lya Luminosity)

  6. Are existing known LAEs probing the EoR? • Observations imply we receive ~ 10-80% (~95% CL) Lya photons per restframe UV continuum photon from z=6.5 compared to z=5.7 (D, Wyithe & Haiman+07). ~ 30% see M. Ouchi’s talk. • Why? • Evolution in f_esc? (Because Llya ~ [1-fesc]). See Yajima’s talk. • Dust? • These effects involve a detailed understanding of galaxies at z>5.5 • Less Lya is transmitted through IGM? • Gas densities evolve as (1+z)3; nHI~(1+z)6 • (Re)Ionized gas can be significantly more opaque at z=6.5 than at z=5.7. 1.0 1.2 1.4 1.6 1.8 Opacity Ratio

  7. Are existing known LAEs probing the EoR? • Observations imply we receive ~ 10-80% (~95% CL) Lya photons per restframe UV continuum photon from z=6.5 compared to z=5.7 (D, Wyithe & Haiman+07) • Why? • Evolution in f_esc? (Because Llya ~ [1-fesc]). • Dust? • These effects involve a detailed understanding of galaxies at z>5.5 • Less Lya is transmitted through IGM? • Gas densities evolve as (1+z)3; nHI~(1+z)6 • (Re)Ionized gas can be significantly more opaque at z=6.5 than at z=5.7. HI HI HI Residual HI gas inside a reionized IGM may be comprise a (large) evolving source of opacity for LAEs

  8. The Opacity of the Ionized IGM to LAEs • What is the opacity of residual HI gas in a reionized patch of the IGM to LAEs? • ‘First-order’ treatment the IGM: Lya line before IGM processing assumed to be a Gaussian with FWHM set by bulk motions of HII regions within galaxy (~vcirc of host DM halo) • Photons emitted blueward of Lya resonance eventually redshift into Lya resonance where IGM is opaque: transmission is ~TIGM (1) blueward (redward) of Lya resonance, I.e T>0.5. Faucher-Giguere+ 08 BlueRed -ln TIGM

  9. The Opacity of the Ionized IGM to LAEs • This prescription for the IGM is only valid when galaxies are randomly distributed throughout the Universe. • However, galaxies preferentially form in overdense regions of the Universe and are highly clustered. • When quantifying the opacity of the IGM around Lya emitting galaxies one must account for (e.g. D. Lidz & Wyithe 07, Iliev+08): • local overdensity of IGM gas around galaxies • Infall of IGM gas near galaxies (gas is *not* comoving with Hubble flow ) • Enhancement of local ionizing background (due to source clustering)

  10. Chicken Chicken Chicken Chicken • Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken. • Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken. • Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken Chicken (Chicken+07,Chicken+08): • Chicken Chicken Chicken Chicken Chicken • Chicken Chicken Chicken Chicken (Chicken Chicken Chicken Chicken ) • Chicken Chicken Chicken (Chicken Chicken Chicken )

  11. The Opacity of the Ionized IGM to LAEs • Impact IGM in more realistic model. • Account for ‘average’ overdensity + peculiar velocities of gas around virialized halo (Barkana 04) (in red, schematically) • IGM transmits 10-30% of emitted flux (D, Lidz & Wyithe +07) • IGM at z=6.5 can be up to 30% times more opaque (10-80% was observed)-> cannot conclude that the observed evolution ‘proof’ of probing EoR. Absorption redward of systemic, by infalling gas, which is denser

  12. The Impact of Winds on the Lya Line Shape • However, the impact of the IGM is depends on prominence ‘back-scattering’ mechanism. • Depending on velocity + HI column density, majority of Lya photons can escape from galaxy with large enough redshift for the IGM to become irrelevant. REDBLUE ~1-10 kpc Lya source Verhamme+06

  13. The Impact of Winds on the Lya Line Shape • Backscattering mechanism can nicely reproduce some observed Lya line shapes (Verhamme+08,Schaerer & Verhamme+08). BlueRed ‘Backscattering’ transforms originally Gaussian emission line into a redshifted (few hundred km/s) Lya emission line.

  14. Do We Understand the Winds + Their Impact on the Lya Line? • Can we constrain ‘wind properties’ (NHI and vexp) from the Lya line shape? BlueRed Spectrum associated with specific IGM model (completely different model..) D, Haiman & Spaans 06 Degeneracies likely exist when modeling the Lya line shape. Furthermore, outflows may be ‘clumpy’, which allows a larger fraction of Lya to escape at the systemic velocity.

  15. Do We Understand the Winds + Their Impact on the Lya Line? Furthermore, outflows may be ‘clumpy’, which allows a larger fraction of Lya to escape at the systemic velocity (Hansen & Oh 06). VS

  16. Which Model Aspects Need to be Improved? • Opacity + redshift its evolution of (re)ionized IGM are not well constrained. • Main source of opacity is gas at 1-5 rvir. Variation from sightline-to-sightline is expected. How much? To be investigated with high resolution hydro simulations (Mesinger+ in prep) . • How important are HI outflows in redshifting the Lya line emerging from LAEs? • If very important: I.e. all Lya emerges with a systemic redshift of  few hundred km/s, then the ionized IGM may not provide an important source of opacity to LAEs. • However, if only a small fraction (~10 %) of Lya still emerges at systemic velocity, then studying the resonant opacity of the IGM is important (which is likely the case). • Interesting that existing single-shell outflow models (e.g. Verhamme+08) for LAEs predict high level of linear polarization (D & Loeb 08) > testable. • Note that a significantly neutral ‘interbubble’ IGM suppresses the flux regardless of winds etc.

  17. He Reionization with LAEs • Lyman Alpha Emitting Galaxies/ Lyman Alpha Emitters (LAEs) can probe the ionization state of H in the intergalactic medium (IGM). • The H ionization state may be affected by the process of Helium reionization. ‘Feature’ in average IGM opacity --> He+ reionization? ~ 1 million LAEs at z=2-4 by ~2011. Faucher-Giguere+08, Bernardi+03

  18. Appendix

  19. III: Polarization of Scattered Lya • Scattered photons can appear polarized to an observer (electric vectors of photons have some preferred directions). • Consider photon whose path is indicated with

  20. III: Polarization of Scattered Lya • Scattered photons can appear polarized to an observer (electric vectors of photons have some preferred directions). • Lya scattering can in practise be described accurately by Rayleigh scattering, for which scattering by  deg, results in 100[sin2 /(1+cos2 )] % polarization. Electric vector of photon Propagation direction of photon

  21. III: Polarization of Scattered Lya • Compute polarization of backscattered Lya radiation using a Monte-Carlo radiative transfer code (D & Loeb ‘08, also see Lee & Ahn ‘98). In this code: • the trajectories of individual photons are simulated as they scatter off H atoms (microphysics of scattering is accurate) • can attach a polarization vector to each photon, and • compute observed quantities such as the Lya spectrum, surface brightness profile, and the polarization • Polarization quantified as P=|Il-Ir|/(Il+Ir). Single photon contributes cos2 to Il and sin2 to Ir (Rybicki & Loeb 99). • Apply Monte-Carlo code to a central Lya emitting source, completely surrounded by a thin, single, expanding shell of HI gas (as in Verhamme+06,08). Free parameters are NHI and vexp.

  22. III: Polarization of Scattered Lya • Lya can reach high levels of polarization (~40%, D & Loeb ‘08) • Polarization depends on NHI and vsh, and therefore provides additional constraints on scattering medium (frequency dependence of polarization also constrains sign of vsh , see D & Loeb ‘08). 45% Polarization 18% Impact parameter Impact parameter

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