Mathieu Langer

Detecting the integrated Sachs-Wolfe effect with stacked voids

The stacking of cosmic microwave background (CMB) patches has been recently used to detect the integrated Sachs-Wolfe effect (iSW). When focusing on the locations of superstructures identified in the Sloan Digital Sky Survey (SDSS), Granett et al. (2008a, Gr08) found a signal with strong significance and an amplitude reportedly higher than expected within the CDM paradigm. We revisit the analysis using our own robust protocol, and extend the study to the two most recent and largest catalogues of voids publicly available. We quantify and subtract the level of foreground contamination in the stacked images and determine the contribution on the largest angular scales from the first multipoles of the CMB. We obtain the radial temperature and photometry profiles from the stacked images. Using a Monte Carlo approach, we computed the statistical significance of the profiles for each catalogue and identified the angular scale at which the signal-to-noise ratio (S/N) is maximum. We essentially confirm the signal detection reported by Gr08, but for the other two catalogues, a rescaling of the voids to the same size on the stacked image is needed to find any significant signal (with a maximum at 2:4 ). This procedure reveals that the photometry peaks at unexpectedly large angles in the case of the Gr08 voids, in contrast to voids from other catalogues. Conversely, the photometry profiles derived from the stacked voids of these other catalogues contain small central hot spots of uncertain origin. We also stress the importance of a posteriori selection effects that might arise when intending to increase the S/N, and we discuss the possible impact of void overlap and alignment effects. We argue that the interpretation in terms of an iSW effect of any detected signal via the stacking method is far from obvious.

On the first generation of stars

We argue that the first stars may have spanned the conventional mass range rather than be identified with the very massive objects (∼100-10 3 M ⊙ ) favoured by numerical simulations. Specifically, we find that magnetic field generation processes acting in the first protostellar systems suffice to produce fields that exceed the threshold for magneto-rotational instability (MRI) to operate, and thereby allow the MRI dynamo to generate equipartition-amplitude magnetic fields on protostellar mass scales below ∼50 M ⊙ . Such fields allow primordial star formation to occur at essentially any metallicity by regulating angular momentum transfer, fragmentation, accretion and feedback in much the same way as occurs in conventional molecular clouds.

Biases on the cosmological parameters and thermal Sunyaev–Zel'dovich residuals

We examine the biases induced on cosmological parameters when the presence of secondary anisotropies is not taken into account in Cosmic Microwave Background analyses. We first develop an exact analytical expression for computing the biases on parameters when any additive signal is neglected in the analysis. We then apply i t in the context of the forthcoming Planck experiment. For illustration, we investigate the effect of the sole residual thermal Sunyaev‐Zel’dovich signal that remains after cluster extr action. We find in particular that analyses neglecting the presence of this contribution intr oduce on the cosmological parameters ns andτ biases, at least∼ 6.5 and 2.9 times their oneσ confidence intervals. The b parameter is also biased to a lesser extent.

Magnetic fields from reionisation

We present a complementary study to a new model for generating magnetic fields of cosmological interest. The driving mechanism is the photoionisation process by photons provided by the first luminous sources. Investigating the transient regime at the onset of inhomogeneous reionisation, we show that magnetic field amplitudes as high as

Cosmological magnetogenesis driven by radiation pressure

2times 10^{-16}

A cross-correlation study between the cosmological 21 cm signal and the kinetic Sunyaev–Zel'dovich effect

Gauss can be obtained within a source lifetime. Photons with energies above the ionisation threshold accelerate electrons, inducing magnetic fields outside the Stromgren spheres which surround the ionising sources. Thanks to their mean free path, photons with higher energies propagate further and lead to magnetic field generation deeper in the neutral medium. We find that soft X -ray photons could contribute to a significant premagnetisation of the intergalactic medium at a redshift of

Small scale contributions to the cosmic microwave background: a coherent analysis

z=15

Large Scale Magnetogenesis Through Radiation Pressure

.

A new parameterization of the reionisation history

The origin of large scale cosmological magnetic fields remains a mystery, despite the continuous efforts devoted to that problem. We present a new model of magnetic field generation, based on local charge separation provided by an anisotropic and inhomogeneous radiation pressure. In the cosmological context, the processes we explore take place at the epoch of the reionization of the Universe. Under simple assumptions, we obtain results (i) in terms of the order of magnitude of the field generated at large scales and (ii) in terms of its power spectrum. The amplitudes obtained

The cross-correlation of the CMB polarization and the 21-cm line fluctuations from cosmic reionization

(Bensuremath{sim}8ifmmodetimeselsetexttimesfi{}{10}^{ensuremath{-}6}ensuremath{mu}mathrm{G})