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Register a new userThe effect of reionization on the CMB-density correlation
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Tommaso Giannantonio
Publication date
2007
fields
Physics
and research's language is
English
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In this paper we show how the rescattering of CMB photons after cosmic reionization can give a significant linear contribution to the temperature-matter cross-correlation measurements. These anisotropies, which arise via a late time Doppler effect, are on scales much larger than the typical scale of non-linear effects at reionization; they can contribute to degree scale cross-correlations and could affect the interpretation of similar correlations resulting from the integrated Sachs-Wolfe effect. While expected to be small at low redshifts, these correlations can be large given a probe of the density at high redshift, and so could be a useful probe of the cosmic reionization history.
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One of the most debated issues in the theoretical modeling of cosmic reionization is the impact of small-mass gravitationally-bound structures. We carry out the first numerical investigation of the role of such sterile `minihaloes, which serve as self-shielding screens of ionizing photons. Minihaloes are too small to be properly resolved in current large-scale cosmological simulations, and thus we estimate their effects using a sub-grid model, considering two cases that bracket their effect within this framework. In the `extreme suppression case in which minihalo formation ceases once a region is partially ionized, their effect on cosmic reionization is modest, reducing the volume-averaged ionization fraction by an overall factor of less than 15%. In the other extreme, in which minihalo formation is never suppressed, they delay complete reionization as much as Delta z~2, in rough agreement with the results from a previous semi-analytical study by the authors. Thus, depending on the details of the minihalo formation process, their effect on the overall progress of reionization can range from modest to significant, but the minihalo photon consumption is by itself insufficient to force an extended reionization epoch.
We study constraints on allowed reionization histories by comparing predictions of a physical semi-numerical model with secondary temperature and polarization anisotropies of the cosmic microwave background (CMB). Our model has four free parameters characterizing the evolution of ionizing efficiency $zeta$ and the minimum mass $M_{mathrm{min}}$ of haloes that can produce ionizing radiation. Comparing the model predictions with the presently available data of the optical depth $tau$ and kinematic Sunyaev-Zeldovich signal, we find that we can already rule out a significant region of the parameter space. We limit the duration of reionization $Delta z=1.30^{+0.19}_{-0.60}$ ($Delta z < 2.9$ at $99%$ C.L.), one of the tightest constraints on the parameter. The constraints mildly favour $M_{mathrm{min}} gtrsim 10^9 mathrm{M}_{odot}$ (at $68%$ C.L.) at $z sim 8$, thus indicating the presence of reionization feedback. Our analysis provides an upper bound on the secondary $B$-mode amplitude $D_{l=200}^{BB}<18$ nK$^2$ at $99%$ C.L. We also study how the constraints can be further tightened with upcoming space and ground-based CMB missions. Our study, which relies solely on CMB data, has implications not only for upcoming CMB surveys for detecting primordial gravitational waves but also redshifted 21 cm studies.
We propose a new reionization probe that uses cosmic microwave background (CMB) observations; the cross-correlation between fluctuations in the CMB optical depth which probes the integrated electron density, $deltatau$, and the Compton $y$-map which probes the integrated electron pressure. This cross-correlation is much less contaminated than the $y$-map power spectrum by late-time cluster contributions. In addition, this cross-correlation can constrain the temperature of ionized bubbles while the optical-depth fluctuations and kinetic SZ effect can not. We measure this new observable using a Planck $y$-map as well as a map of optical-depth fluctuations that we reconstruct from Planck CMB temperature data. We use our measurements to derive a first CMB-only upper limit on the temperature inside ionized bubbles, $T_{rm b}lesssim 7.0times10^5,$K ($2,sigma$). We also present future forecasts, assuming a fiducial model with characteristic reionization bubble size $R_{rm b}=5,$Mpc and $T_{rm b}=5times10^4,$K. The signal-to-noise ratio of the fiducial cross-correlation using a signal dominated PICO-like $y$-map becomes $simeq7$ with CMB-S4 $deltatau$ and $simeq13$ with CMB-HD $deltatau$. For the fiducial model, we predict that the CMB-HD $-$ PICO cross-correlation should achieve an accurate measurement of the reionization parameters; $T_{rm b}simeq 49800^{+4500}_{-5100},$K and $R_{rm b}simeq 5.09^{+0.66}_{-0.79},$Mpc. Since the power spectrum of the electron density fluctuations is constrained by the $deltatau$ auto spectrum, the temperature constraints should be only weakly model-dependent on the details of the electron distributions and should be statistically representative of the temperature in ionized bubbles during reionization. This cross-correlation could, therefore, become an important observable for future CMB experiments.
We first compare the CMB lensing model of Seljak (1996) with the empirical model of Lieu & Mittaz (2005) to determine if the latter approach implies a larger effect on the CMB power-spectrum. We find that the empirical model gives significantly higher results for the magnification dispersion, assuming standard cosmological parameters. However, when the empirical foreground model is modelled via correlation functions and used in the Seljak formalism, the agreement is considerably improved. Thus we conclude that the main difference may be in the assumed foregrounds. We then discuss a foreground mass distribution which gives a lensing dispersion which is constant with angle. In Seljaks formalism, we show this can lead to a smoothing of the CMB power-spectrum which may be able to move the first acoustic peak to smaller l, if the mass clustering amplitude is high enough. Evidence for such a high amplitude comes from the QSO magnification results of Myers et al (2003, 2005) who suggest that foreground galaxy groups may be more massive than expected, implying that Omega_m ~ 1 and strong galaxy anti-bias. We then show that an inflationary model with neither CDM nor a cosmological constant and that predicts a primordial first peak at l=330 may fit the first acoustic peak of the WMAP data. This fit may be regarded as somewhat contrived since it also requires high redshift reionisation at the upper limit of what is allowed by the WMAP results. But given the finely-tuned nature of the standard LCDM model, the contrivance may be small in comparison and certainly the effect of lensing and other foregrounds may still have a considerable influence on the cosmological interpretation of the CMB.
We present a study of the effect of component separation on the recovered cosmic microwave background (CMB) temperature distribution, considering Gaussian and non-Gaussian input CMB maps. First, we extract the CMB component from simulated Planck data (in small patches of the sky) using the maximum entropy method (MEM), Wiener filter (WF) and a method based on the subtraction of foreground templates plus a linear combination of frequency channels (LCFC). We then apply a wavelet-based method to study the Gaussianity of the recovered CMB and compare it with the same analysis for the input map. When the original CMB map is Gaussian (and assuming that point sources have been removed), we find that none of the methods introduce non-Gaussianity (NG) in the CMB reconstruction. On the contrary, if the input CMB map is non-Gaussian, all the studied methods produce a reconstructed CMB with lower detections of NG than the original map. This effect is mainly due to the presence of instrumental noise. In this case, MEM tends to produce slightly higher non-Gaussian detections in the reconstructed map than WF whereas the detections are lower for the LCFC. We have also studied the effect of point sources in the MEM reconstruction. If no attempt to remove point sources is performed, they clearly contaminate the CMB reconstruction, introducing spurious NG. When the brightest point sources are removed from the data using the Mexican Hat Wavelet, the Gaussian character of the CMB is preserved. However, when analysing larger regions of the sky, the variance of our estimators will be appreciably reduced and, in this case, we expect the point source residuals to introduce spurious NG in the CMB. Thus, a careful subtraction (or masking) of point source emission is crucial when studying the Gaussianity of the CMB.
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