The impact of He II reionisation on the H I

Ly- forest

Jamie Bolton

Reionisation@Ringberg

Peng Oh (UCSB), Steve Furlanetto (UCLA)

Outline

1) The IGM temperature during He II reionisation (How much? How quickly?)

2) Implications for observations of the H I Ly- forest opacity

Photo-heatingPhotons not only ionise – if they have E>54.4eV then they also heat the IGM.

154 eV photon

100 eV electron

eHeHe 2He II

Electrons share their energy with the baryons via Coulomb scattering.

How much heating?

KTh

E th 2004,2

3

KTh

E th 00030,1

0

Optically thin

Optically thick approximation

J

The mean free path

)2(

)(

th

thh

d

hd

E

th

th

The amount of filtering depends on how clumpy the IGM is:

Key parameter is the mean free path of He II ionising photons: for a uniform optically thick IGM, while optically thin has (frequency independent mean free path)

will be intermediate between these two cases – hard photons are preferentially absorbed in dense regions, and the abundance of He II LLS is therefore crucial.

Bolton, Oh & Furlanetto (2009a)

He II Lyman limit systems

KT 00015,5.2

KT 0007,5.1

The He II column density distribution is not observationally constrained. Models must be used to predict the relationship between physical gas density and the observed HI column density distribution.

McQuinn et al. (2009)

For

HI

HeII

N

N

Haardt & Madau (1996)

How quickly?Filtering of ionising radiation not only hardens the spectrum – it also weakens the intensity.

ion

HeIIHeII

B

H

tdt

dnE

dt

dn

k

m

dt

dT 1,

3

2

If tion>> ts, no significant photo-heating will occur, even if <E> is very large. This limits the maximum temperature boost achievable in the IGM over a given timescale.

Bottom line: the entire IGM cannot be rapidly heated (z~0.2) by 104K unless the average quasar spectrum is much harder than =1.5 (see also McQuinn et al. 2009)

Depends on incident spectrum onlyDepends on radiation intensity

Bolton, Oh & Furlanetto (2009a)

Outline

1) The IGM temperature during He II reionisation (How much? How quickly?)

2) Implications for observations of the H I Ly- forest opacity

The mysterious Ly- opacity “dip”

Feature is seen in 3 independent data sets (although see McDonald et al. 2006)

Faucher-Giguere et al. (2008), see also Bernardi et al. (2003), Dall’Aglio et al. (2008)

27.0

226

0 )()(

)()1(

zzHT

hz b

Evidence for photo-heating during HeII reionisation?

Previous explanation from hydro simulations: sudden heating followed rapid recovery due to the hydrodynamical response of the IGM – much quicker than one would expect due to adiabatic cooling following reheating alone.

Theuns et al. (2002)

Monotonic evolution in all models

Bolton, Oh & Furlanetto (2009b)

T~104K at z=3.4 No extra heatingT~104K over z~2

Simulating the “dip” with GADGET-2Five high resolution hydrodynamical simulations

(15h-1 cMpc, 2x4003 gas+DM particles) follow non-equilibrium gas chemistry

Bolton, Oh & Furlanetto (2009b)

Peculiar velocity gradients following reheating

The peculiar velocities due to expanding gas are small compared to typical line widths in the Ly- forest (20 km s-1).

Could it be the HI PI rate?

Bolton, Oh & Furlanetto (2009b)

27.0

226

0 )()(

)()1(

zzHT

hz b

Some possibilities • A modulation in the mean free path for HI ionising photons

as HI Lyman limit system expand/contract as the pressure gradient changes following heating (but requires cosmic variance to be small).

• Recombination radiation - the reprocessing of He II ionising photons into lower frequency radiation could boost the HI photo-ionisation rate towards the tail-end of He II reionisation.

L

dhHI

Hot IGM

Cooler LLS

Haardt & Madau (1996), but see also Faucher-Giguere et al.

(2009)

Conclusions

• Photo-heating during He II reionisation is model dependent, but is unlikely to result in large (T>104K) rapid (z<0.2) temperature boosts for typical quasar spectra.

• The dip in the effective optical depth cannot be explained by the thermal evolution during He II reionisation – the timescales for heating and cooling are too long.

• Even for rapid reheating, hydrodynamical effects do not aid the recovery seen in the feature: changes in the peculiar velocity field following reheating are small in comparison to typical line widths

• Other RT effects associated with He II reionisation could beresponsible – but the exact explanation (if the feature is real!) remains an open question…