A New Constraint on the Intergalactic HeII Fraction at z~3

Matt McQuinn

Einstein Fellows Symposium

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

HeII Reionization in 430 Mpc (4ο) Box

White > 25,000 KBlack < 10,000 K

The HI and HeII Lyα forest

IGM Absorption QSOTelescope

Keck HiRes observation, Rauch

1216 (1+z) A 304 (1+z) A

Implications of Gunn-Peterson troughs

• z > 6, see GP troughs in HI Lyman-α forest. – May not indicate that reionization is happening

because τGP = 5x105 xHI Δb. – Best constraint (utilizing Lyβ and Lyγ absorption) is xHI >

3x10-4 at z>6.3 (Fan et al 2006)

• z >3, see GP troughs in HeII Lyman-α forest. – τGP = 3x103 xHeII Δb, and so thought that this saturates too

easily to study HeII reionization. – Previous constraint on xHeII was xHeII >10-3. – What follows is a simple argument that shows that it is

likely that a 30x stronger constraint holds.

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 ρcrit Ωb(z) orΔb= 0.1

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. τGP evaluated at Δb = 1)

Theoretical literature obsessed with τeff – add up transmission from large region

Constraints on ionization State of Gas Parcels Exposed to same ΓHeII

During Reionization After reionization

When you volume average blue dashed curve, yields constraint xHeII > 0.03.

Mean free path of photons• QSOs ~30 cMpc apart• If QSOs source ΓHeII, it

shouldn’t correlate with density of Lyα absorber

• Mean free path (mfp) of HeII Lyman-limit photons is ~ xHeII

-1 cMpc for homogeneous IGM (and several times larger when spectrally averaged)

• inhomogeneities decrease mfp, but not by enough

30 cMpc

Correlation length also >10 Mpc in simulations

---200 cMpc--- McQuinn et al. (2009)

Constraints on xHeII within ~ 10 comoving Mpc of saturated underdense region

Only Δb < 10 included in averagesMcQuinn (2009)

Lower limit in saturated voids

Predicted future constraint from Lyβ absorption

Conclusions

• Largely model-independent constraint xHeII, 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 xHeII > 0.1

• Probably GP troughs indicate HeII reionization is happening!

A More Direct Method to Probe HeII Reionization: 3He+ 8.7 GHz Absorption

• For hydrogen, Lyα probably saturates too easily to study reionization, but have 21cm line with τ21 ~ 10-5Δb(1000 K/Tg).

• For HeII, 4He+ doesn’t have hyperfine transition, but 3He+ does at 8.7 GHz. Can study HeII reionization and BBN

• Amazingly, τ8.7 ~ 10-6Δb (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)

THANK YOU

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

A More Direct Method to Probe HeII Reionization: 3He+ 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 105 • Transition rate 680 times larger than HI

(A ~ Z9 for hydrogen-like hyperfine lines)• Backgrounds are small [Tsky = 4 K versus Tsky =

1000 (1+z/13)2.6 K for 21cm]

The Spin Temperature

Collisional Coupling Radiative Coupling

HeII Ion HeII Ion

Unlike for HI 21cm, neither of these mechanisms is strong enough to decouple Ts from Tcmb (McQuinn & Switzer 2009). Therefore, it is most promising to look for this transition in absorption (τ ~ Ts

-1).

8.7 GHz 3He+ Absorption (cont.)

• In Hubble flow: τ3He+=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: Sabs = S τ3He+ = 1 μJy (and smaller at

higher z)• The sensitivity of a radiometer to a point source is

McQuinn and Switzer 2009

The Signal!200 Mpc

200 MpcWe predict that an RMS noise of 30 xHeII μJy in .1MHz over 100 MHz results on average in a 3σ detection.

Using Lyα Forest as a Template

3 σ

5 σ

10 σ

Conclusions

• Gunn-Peterson troughs in HeII Lya forest suggest significant HeII fraction.

• 3He+ absorption promising for next generation of interferometers to definitively detect HeII reionizationa (and constrain BBN 3He abundance)

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 xHeII ~ Δ, so xHeII = 0.1 for a neighboring absorber of Δ = 1 if xHeII = 0.01 at Δ =0.1 [for isothermal IGM and assuming neighboring absorbers exposed to the same ΓHeII(t)]

• Photoionization equilibrium is established on a timescale ΓHeII

-1 << H(z)-1 *generally* (and is only evaded such that xHeII at Δ = 1 is overestimated if the HeII was recently ionized)

• HeII Lya sensitive to xHeII = 0.1 at Δ = 0.1

If gas is in photoionization equilibrium, implies large xHeII

Ignoring temperature dependence on Δb, which is included in analysis

• Density scaling for ionization equilibrium a good assumption for macroscopic region exposed to same ΓHeII

• The constraint xHeII = 0.01 @ Δb=0.1 implies xHeII = 0.1 @ Δb=1 (for isothermal IGM)

Can you avoid this constraint?

Can only evade if the HeII were recently ionized in region!

HI Lyα gorest can be used as a template to detect signal

3 σ

5 σ

10 σ

McQuinn & Switzer (2009)