1. A New Constraint on the Intergalactic HeII Fraction at z~3 Matt McQuinn Einstein Fellows Symposium

2. HeII Reionization Best Guess for reionization history of IGM – stars ionized HI (13.6 eV) and HeI (24.6 eV) at z > 6 – quasars ionized HeII (54.4 eV) at z ~ 3. Tentative indications for HeII reionization – Temperature of IGM as inferred from HI Lyα forest is too high at z~3 – Evolution seen in the ionization state of certain metals at z~3 interpreted as evolution in hardness of ionizing background around 54eV – Gunn-Peterson troughs appear in HeII Lyα forest at z >2.8

3. HeII Reionization in 430 Mpc (4 ο ) Box White > 25,000 K Black < 10,000 K

4. The HI and HeII Lyα forest IGM Absorption QSO Telescope Keck HiRes observation, Rauch 1216 (1+z) A 304 (1+z) A

5. Implications of Gunn-Peterson troughs z > 6, see GP troughs in HI Lyman-α forest. – May not indicate that reionization is happening because τ GP = 5x10 5 x HI Δ b. – Best constraint (utilizing Lyβ and Lyγ absorption) is x HI > 3x10 -4 at z>6.3 (Fan et al 2006) z >3, see GP troughs in HeII Lyman-α forest. – τ GP = 3x10 3 x HeII Δ b, and so thought that this saturates too easily to study HeII reionization. – Previous constraint on x HeII was x HeII >10 -3. – What follows is a simple argument that shows that it is likely that a 30x stronger constraint holds.

6. We know what the density distribution of gas is in the IGM Miralda-Escude, Haehnelt, & Rees (1998)PDF fairly robust to assumed cosmology. Minimum density in IGM about 0.1 ρ crit Ω b (z) or Δ b = 0.1

7. The HeII Lya Forest HI Lya (dotted) and HeII (solid) of Q0302-003 In the past, people said a Lyα optical depth of say 4 in a region implied xHeII > 10 -3 (i.e. τ GP evaluated at Δ b = 1) Theoretical literature obsessed with τ eff – add up transmission from large region

8. Constraints on ionization State of Gas Parcels Exposed to same Γ HeII During ReionizationAfter reionization When you volume average blue dashed curve, yields constraint x HeII > 0.03.

9. Mean free path of photons QSOs ~30 cMpc apart If QSOs source Γ HeII, it shouldn’t correlate with density of Lyα absorber Mean free path (mfp) of HeII Lyman-limit photons is ~ x HeII -1 cMpc for homogeneous IGM (and several times larger when spectrally averaged) inhomogeneities decrease mfp, but not by enough 30 cMpc

10. Correlation length also >10 Mpc in simulations ---200 cMpc--- McQuinn et al. (2009)

11. Constraints on x HeII within ~ 10 comoving Mpc of saturated underdense region Only Δ b < 10 included in averages McQuinn (2009) Lower limit in saturated voids Predicted future constraint from Lyβ absorption

12. Conclusions Largely model-independent constraint x HeII, V > 0.03 from current data in >~ 10 Mpc saturated regions around underdensities. – 30 times tighter than previous analyses – 100 times larger than best constraint of volume- averaged HI fraction from the z ~ 6.5 HI Lyman forest – QSO Luminosity Function falling rapidly towards higher-z Future data should constrain x HeII > 0.1 Probably GP troughs indicate HeII reionization is happening!

13. A More Direct Method to Probe HeII Reionization: 3 He+ 8.7 GHz Absorption For hydrogen, Lyα probably saturates too easily to study reionization, but have 21cm line with τ 21 ~ 10 -5 Δ b (1000 K/T g ). For HeII, 4 He+ doesn’t have hyperfine transition, but 3 He+ does at 8.7 GHz. Can study HeII reionization and BBN Amazingly, τ 8.7 ~ 10 -6 Δ b (and this line is probably easier to observe than high-z 21cm emission from IGM). Brightest AGN at 2GHz are 1 Jy, such that we are looking for 1 μJy signal at the cosmic mean density Lyα forest absorption also tells you where most dense regions lie and can be used as template McQuinn & Switzer (2009)

14. THANK YOU

15. A MORE DIRECT METHOD TO PROBE HEII REIONIZATION: 3 HE+ 8.7 GHZ ABSORPTION

16. A More Direct Method to Probe HeII Reionization: 3 He+ 8.7 GHz Absorption Townes (1957) and Sunyaev (1966) pointed out may be an observable galactic emission line This is a hyperfine transition like the 21cm line Abundance relative to H down by 10 5 Transition rate 680 times larger than HI (A ~ Z 9 for hydrogen-like hyperfine lines) Backgrounds are small [T sky = 4 K versus T sky = 1000 (1+z/13) 2.6 K for 21cm]

17. The Spin Temperature Collisional CouplingRadiative Coupling HeII Ion Unlike for HI 21cm, neither of these mechanisms is strong enough to decouple T s from T cmb (McQuinn & Switzer 2009). Therefore, it is most promising to look for this transition in absorption (τ ~ T s -1 ).

18. 8.7 GHz 3 He+ Absorption (cont.) In Hubble flow: τ 3He+ =0.5x10 -6 (1+δ) ([1+z]/5) 1/2 The brightest sources at z~4 at 2 GHz is S~2 Jy The signal size: S abs = S τ 3He+ = 1 μJy (and smaller at higher z) The sensitivity of a radiometer to a point source is McQuinn and Switzer 2009

19. The Signal! 200 Mpc We predict that an RMS noise of 30 x HeII μJy in.1MHz over 100 MHz results on average in a 3σ detection.

20. Using Lyα Forest as a Template 3 σ 5 σ 10 σ

21. Conclusions Gunn-Peterson troughs in HeII Lya forest suggest significant HeII fraction. 3 He+ absorption promising for next generation of interferometers to definitively detect HeII reionizationa (and constrain BBN 3 He abundance)

22. Arugment The least dense regions in the IGM have Δ=0.1 HeII Lya is sensitive to HeII fraction of 0.01 at Δ = 0.1 (rather than 0.001 at Δ = 1) In photoionization equilibrium x HeII ~ Δ, so x HeII = 0.1 for a neighboring absorber of Δ = 1 if x HeII = 0.01 at Δ =0.1 [for isothermal IGM and assuming neighboring absorbers exposed to the same Γ HeII (t)] Photoionization equilibrium is established on a timescale Γ HeII -1 << H(z) -1 *generally* (and is only evaded such that x HeII at Δ = 1 is overestimated if the HeII was recently ionized) HeII Lya sensitive to x HeII = 0.1 at Δ = 0.1

23. If gas is in photoionization equilibrium, implies large x HeII Ignoring temperature dependence on Δ b, which is included in analysis

24. Density scaling for ionization equilibrium a good assumption for macroscopic region exposed to same Γ HeII The constraint x HeII = 0.01 @ Δ b =0.1 implies x HeII = 0.1 @ Δ b =1 (for isothermal IGM)

25. Can you avoid this constraint? Can only evade if the HeII were recently ionized in region!

26. HI Lyα gorest can be used as a template to detect signal 3 σ 5 σ 10 σ McQuinn & Switzer (2009)