[astro-ph/0503166] The influence of non-uniform reionization on the CMB

Authors:  Oliver Zahn, Matias Zaldarriaga, Lars Hernquist, Matthew McQuinn
Abstract:  We investigate the impact of spatial variations in the ionized fraction during reionization on temperature anisotropies in the CMB. We combine simulations of large scale structure to describe the underlying density field with an analytic model based on extended Press-Schechter theory to track the reionization process. We find that the power spectrum of the induced CMB anisotropies depends sensitively on the character of the reionization epoch. Models that differ in the extent of the "patchy phase" could be distinguished by future experiments such as the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT). In our models, the patchy signal peaks at $l\simeq 2000$, where it can be four times larger than the kinetic Sunyaev-Zel'dovich (kSZ)/Ostriker-Vishniac (OV) signal ($\Delta T_\text{tot} \simeq 2.6 \mu K$). On scales beyond $l \simeq 4000$ the total Doppler signal is dominated by kSZ/OV, but the patchy signal can contribute up to 35% to the power spectrum. The effect of patchy reionization is largest on scales where the primordial CMB anisotropies dominate. Ignoring this contribution could lead to significant biases in the determination of cosmological parameters derived from CMB temperature measurements. Improvements in the theoretical modeling of the reionization epoch will become increasingly important to interpret the results of upcoming experiments.
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Antony Lewis
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[astro-ph/0503166] The influence of non-uniform reionization

Post by Antony Lewis » March 09 2005

This seems to be a nice paper looking at the effect of various inhomogeneous reionization scenarios on the CMB. In particular they claim a potentially very significant contribution to the [tex]C_l[/tex] at [tex]l < 2000[/tex], enough to significantly bias parameters with Planck at [tex]l>1000[/tex] if not included in the model. This is of course well known (e.g. astro-ph/9805012, astro-ph/0305471), but their estimates are rather higher than previous ones.

I note that they are using [tex]n_s=1, \,\sigma_8=0.9[/tex] for their simulations, both choices giving small scale power at the high end of what is currently allowed. How sensitive are the results? (e.g. would [tex]n_s=0.96, \,\sigma_8=0.8[/tex] give a much smaller signal due to a high-power scaling with [tex]\sigma_8[/tex]?)

I would also be interested to see Table 2 for a more sensible cut at [tex]l\sim 2000[/tex] rather than at [tex]l=4000[/tex] - ie. how wrong are people (like me) who usually cut at [tex]l\sim 2000[/tex] in the hope that by doing that problems with (non-lensing) non-linear physics and non-Gaussianity are mostly avoided. Also presumably the lensed CMB power spectra should be used for Table 2, since lensing increases the small scale power at [tex]l>2000[/tex] and hence decreases the fractional effect of kSZ.

Finally, if, as they suggest, the [tex]l>1000[/tex] temperature is not used (using only polarization at [tex]l<2000[/tex]), I'd have thought parameter uncertainties are likely to be more biased by the uncertainty in the recombination history (c.f. http://cosmocoffee.info/viewtopic.php?t=174) which has a larger effect on the polarization.

Oliver Zahn
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Affiliation: Berkeley Center for Cosmological Physics

[astro-ph/0503166] The influence of non-uniform reionization

Post by Oliver Zahn » March 10 2005

Thanks for those comments,

the reason why our parameter biases are larger than the results of 9805012 is that their patchy power spectrum peaks at around l=4000 as opposed to at around l=2000 in our model. In 0305471 on the other hand, the signal peaks at a similar angular scale, and it has a larger amplitude. The reason why their biases are slightly smaller than ours should be that they use parameters in their Fisher analysis that are degenerate with the patchiness, such as running of the spectral index, the primordial Helium fraction, massive neutrinos, and the equation of state parameter.

Concerning the second point, that sigma8=0.9 and n_s=1 in the lss simulations we had are on the high side of what is currently favoured by experiments (at least sigma8=0.9 is fine with current CMB experiments). The dependence of Doppler anisotropies induced by patchy reionization will not be the same as the dependence of kSZ/OV induced by density fluctuations (e.g. sigma8^4-6), and it would be interesting to look into that. Most changes can probably be compensated by varying the (unknown) ionization efficency zeta (compare equation 4 of the paper).

About your third point, if I do the analysis of Table 2 by using Cls only out to l=2000, the parameter errors stay almost the same, while the biases go down 20-25%. The bias from patchiness at l=2000 is already significant because this is roughly where our signal peaks. Plancks constraints will anyway not be affected much by what is going on on smaller scales.

With respect to the fourth point, it may well be that the best strategy is to incorporate patchy reionisation as an extra parameter (as we did at the end of section 5.2). This would be most accurate if the epoch is understood well from simulations as well as from experiments, for example by measuring on the smallest scales with ACT, SPT, or APEX, which should happen at around the same time as Planck. Furthermore our understanding of recombination should improve with intermediate scale polarization data coming out in the next couple of years.

All the best, Oliver

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