a new constraint on the intergalactic heii fraction at z~3
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/1.jpg)
A New Constraint on the Intergalactic HeII Fraction at z~3
Matt McQuinn
Einstein Fellows Symposium
![Page 2: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/2.jpg)
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
![Page 3: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/3.jpg)
HeII Reionization in 430 Mpc (4ο) Box
White > 25,000 KBlack < 10,000 K
![Page 4: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/4.jpg)
The HI and HeII Lyα forest
IGM Absorption QSOTelescope
Keck HiRes observation, Rauch
1216 (1+z) A 304 (1+z) A
![Page 5: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/5.jpg)
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.
![Page 6: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/6.jpg)
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
![Page 7: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/7.jpg)
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
![Page 8: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/8.jpg)
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.
![Page 9: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/9.jpg)
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
![Page 10: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/10.jpg)
Correlation length also >10 Mpc in simulations
---200 cMpc--- McQuinn et al. (2009)
![Page 11: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/11.jpg)
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
![Page 12: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/12.jpg)
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!
![Page 13: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/13.jpg)
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)
![Page 14: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/14.jpg)
THANK YOU
![Page 15: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/15.jpg)
A MORE DIRECT METHOD TO PROBE HEII REIONIZATION: 3HE+ 8.7 GHZ ABSORPTION
![Page 16: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/16.jpg)
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]
![Page 17: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/17.jpg)
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).
![Page 18: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/18.jpg)
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
![Page 19: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/19.jpg)
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.
![Page 20: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/20.jpg)
Using Lyα Forest as a Template
3 σ
5 σ
10 σ
![Page 21: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/21.jpg)
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)
![Page 22: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/22.jpg)
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
![Page 23: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/23.jpg)
If gas is in photoionization equilibrium, implies large xHeII
Ignoring temperature dependence on Δb, which is included in analysis
![Page 24: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/24.jpg)
• 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)
![Page 25: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/25.jpg)
Can you avoid this constraint?
Can only evade if the HeII were recently ionized in region!
![Page 26: A New Constraint on the Intergalactic HeII Fraction at z~3](https://reader036.vdocuments.us/reader036/viewer/2022062808/56815470550346895dc287a0/html5/thumbnails/26.jpg)
HI Lyα gorest can be used as a template to detect signal
3 σ
5 σ
10 σ
McQuinn & Switzer (2009)