Point Source Contamination in CMB Non-Gaussianity Analyses

TL;DR

This work demonstrates that point-source contamination—especially cross-correlations between point-source power and CMB temperature via ISW and matter density—can induce bispectra that resemble the local $f_{NL}$ signal. By deriving the estimator bias and computing contributions from radio sources, thermal SZ, and their lensing/modulation effects, the authors quantify biases for WMAP3 and Planck across frequencies. They find the biases are small for current WMAP data but non-negligible for Planck, with exact numbers sensitive to source redshift distributions and flux cuts. The study emphasizes that precise knowledge of point-source properties is essential for robust constraints on primordial non-Gaussianity with future CMB experiments.

Abstract

In this paper we analyze the biasing effect of point sources, either thermal Sunyaev-Zeldovich clusters or standard radio sources, on the estimated strength of the non-Gaussianity in the Cosmic Microwave Background (CMB). We show that the biggest contribution comes from the cross--correlation of the CMB with the matter density rather than from the poisson term which is conventionally assumed in these calculations. For the three year WMAP data, we estimate that point sources could produce a non--Gaussian signature equivalent to a bias in $f_{NL}$ of $0.35, 0.24, -0.097, -0.13$ in the Ka, Q, V and W bands respectively. The level of bias we find is largely insufficient to explain the very high $f_{NL}$ values recently detected by Yadav and Wandelt. For Planck, we estimate the point source bispectra to contaminate the $f_{NL}$ estimator with a bias of $1.3, 0.34, -0.25, -0.48$ at $30, 44, 70, 100 {\rm GHz}$ respectively. These results depend on the assumed redshift distribution of the point sources. However, given the projected Planck sensitivity of $Δf_{NL} \simeq 5$ (95 % C.L.), a good estimate of point sources' properties including their number density and redshift distribution is essential before deriving strong conclusions on primordial non--Gaussianity.

Figures (7)

• Figure 1: Redshift weight functions for the ISW effect (green, dot-dashed); radio point sources -- model 1 (red, dashed) and model 2 (blue, long-dashed); and thermal SZ effect (solid, black).
• Figure 2: Density-ISW cross-correlation spectrum for thermal SZ (solid, black); radio point sources -- model 1 (red, dashed) and model 2 (blue, long-dashed).
• Figure 3: Number density modulation bispectrum for thermal SZ (solid, black); radio point sources -- model 1 (red, dashed) and model 2 (blue, long-dashed). The equilateral shape $(\ell_1 = \ell_2 = \ell_3 = \ell)$ of the local model ($f_{NL} = 1$) (green, dot-dashed) is also shown for reference.
• Figure 4: Convergence-ISW cross-correlation spectrum for thermal SZ (solid, black); radio point sources -- model 1 (red, dashed) and model 2 (blue, long dashed)
• Figure 5: Magnification modulation bispectrum for thermal SZ effect (solid, black); radio point sources -- model 1 (red, dashed) and model 2 (blue, long-dashed). The local model bispectrum ($f_{NL} = 1$) -- equilateral shape $(\ell_1 = \ell_2 = \ell_3 = \ell)$ (green, dot-dashed) is also shown for reference.