1. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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

2. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 Lya is produced following recombination in HII regions surrounding O + B stars. L lya scales in proportion with SFR*(1-f esc ) (L lya =7-25%L bol !) Ionization state of the IGM may leave imprint on observed Lya flux. Introduction 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). HI

3. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 Observed number density of LAEs drops suddenly beyond z=6? Are existing known LAEs probing the EoR? Log (Lya Luminosity) Cum. Number density 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. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 Observed number density of LAEs drops suddenly beyond z=6? Log (Lya Luminosity) Cum. Number density 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! Are existing known LAEs probing the EoR?

5. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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? Log (Lya Luminosity) Cum. Number density Are existing known LAEs probing the EoR?

6. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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 ; n HI ~(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 Are existing known LAEs probing the EoR?

7. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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 ; n HI ~(1+z) 6 (Re)Ionized gas can be significantly more opaque at z=6.5 than at z=5.7. HI Residual HI gas inside a reionized IGM may be comprise a (large) evolving source of opacity for LAEs Are existing known LAEs probing the EoR?

8. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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 (~v circ of host DM halo) Photons emitted blueward of Lya resonance eventually redshift into Lya resonance where IGM is opaque: transmission is ~T IGM (1) blueward (redward) of Lya resonance, I.e T>0.5. -ln T IGM Faucher-Giguere+ 08 Blue Red The Opacity of the Ionized IGM to LAEs

9. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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) The Opacity of the Ionized IGM to LAEs

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11. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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 The Opacity of the Ionized IGM to LAEs

12. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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. Lya source RED BLUE Verhamme+06 ~1-10 kpc The Impact of Winds on the Lya Line Shape

13. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 Backscattering mechanism can nicely reproduce some observed Lya line shapes (Verhamme+08,Schaerer & Verhamme+08). Blue Red ‘Backscattering’ transforms originally Gaussian emission line into a redshifted (few hundred km/s) Lya emission line. The Impact of Winds on the Lya Line Shape

14. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 Can we constrain ‘wind properties’ (NHI and vexp) from the Lya line shape? Blue Red 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. Do We Understand the Winds + Their Impact on the Lya Line?

15. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 Furthermore, outflows may be ‘clumpy’, which allows a larger fraction of Lya to escape at the systemic velocity (Hansen & Oh 06). VS Do We Understand the Winds + Their Impact on the Lya Line?

16. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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. Which Model Aspects Need to be Improved?

17. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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. He Reionization with LAEs Faucher-Giguere+08, Bernardi+03 ‘Feature’ in average IGM opacity --> He + reionization? ~ 1 million LAEs at z=2-4 by ~2011.

18. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 Appendix

19. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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[sin 2  /(1+cos 2  )] % polarization. Electric vector of photon Propagation direction of photon

21. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 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=|I l -I r |/(I l +I r ). Single photon contributes cos 2  to I l and sin 2  to I r (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 N HI and v exp.

22. ‘LAEs as a Probe of the High-z Universe’, Ringberg, March, 2009 III: Polarization of Scattered Lya Lya can reach high levels of polarization (~40%, D & Loeb ‘08) Polarization depends on N HI and v sh, and therefore provides additional constraints on scattering medium (frequency dependence of polarization also constrains sign of v sh, see D & Loeb ‘08). Polarization Impact parameter 45% 18% Impact parameter