1. The impact of He II reionisation on the H I Ly-  forest Jamie Bolton Reionisation@Ringberg Peng Oh (UCSB), Steve Furlanetto (UCLA)

2. Outline 1) The IGM temperature during He II reionisation (How much? How quickly?) 2) Implications for observations of the H I Ly-  forest opacity

3. Photo-heating Photons not only ionise – if they have E>54.4eV then they also heat the IGM. 154 eV photon 100 eV electron He II Electrons share their energy with the baryons via Coulomb scattering.

4. How much heating? Optically thin Optically thick approximation

5. The mean free path 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)

6. He II Lyman limit systems 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 Haardt & Madau (1996)

7. How quickly? Filtering of ionising radiation not only hardens the spectrum – it also weakens the intensity. If t ion >> t s, no significant photo-heating will occur, even if 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 10 4 K unless the average quasar spectrum is much harder than  =1.5 (see also McQuinn et al. 2009 ) Depends on incident spectrum only Depends on radiation intensity Bolton, Oh & Furlanetto (2009a)

8. Outline 1) The IGM temperature during He II reionisation (How much? How quickly?) 2) Implications for observations of the H I Ly-  forest opacity

9. 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)

10. 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)

11. Monotonic evolution in all models Bolton, Oh & Furlanetto (2009b)  T~10 4 K at z=3.4 No extra heating  T~10 4 K over  z~2 Simulating the “dip” with GADGET-2 Five high resolution hydrodynamical simulations (15h -1 cMpc, 2x400 3 gas+DM particles) follow non-equilibrium gas chemistry

12. 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 ).

13. Could it be the HI PI rate? Bolton, Oh & Furlanetto (2009b)

14. 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. Hot IGM Cooler LLS Haardt & Madau (1996), but see also Faucher-Giguere et al. (2009)

15. Conclusions Photo-heating during He II reionisation is model dependent, but is unlikely to result in large (  T>10 4 K) 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 be responsible – but the exact explanation (if the feature is real!) remains an open question…