Prospects for independent measurement of $\boldsymbol{\ell}$=1,2,3 CMB anisotropy multipoles using the anisotropic Sunyaev-Zel'dovich effect

TL;DR

The paper addresses the problem of independently measuring the CMB anisotropy multipoles $\ell=1,2,3$ by exploiting the anisotropic Sunyaev-Zel'dovich (aSZ) effect in galaxy clusters. It develops and tests an iterative, modified Least Response Method for multi-frequency component separation to extract the aSZ signal from foregrounds and instrumental noise. In simulations, it shows that dipole, quadrupole, and octupole contributions can be retrieved with sensitivities down to a few Jy sr$^{-1}$, with the dipole being the strongest and the octupole requiring substantially higher sensitivity. The work suggests that aSZ observations can provide an ISW-free, near-local probe of low-$\ell$ CMB anisotropy, potentially mitigating cosmic variance, and outlines future extensions to include kSZ and polarization for richer constraints.

Abstract

We investigate the prospects for observing a specific spectral distortion of the cosmic microwave background, which occurs due to the anisotropy of the radiation when it is scattered by hot plasma of galaxy clusters. Detection of this "anisotropic Sunyaev-Zel'dovich effect" will allow us to independently measure the anisotropy multipoles with $\ell=1,2,3$, separate the Sachs-Wolf effect from the integrated Sachs-Wolf effect (Rees-Sciama effect) and, to a certain extent, circumvent the 'cosmic variance' problem for low multipoles. We propose a modified Least Response Method for the components separation in the data processing and estimate the required sensitivity of the experiment for such observations. We test our approach on a simulated signal that is contaminated by various foregrounds with poorly defined spectral shapes, along with distortions of the relic blackbody spectrum caused by the Sunyaev-Zel'dovich effect and its relativistic corrections.

Figures (2)

• Figure 1: Left panel. Spectra of the aSZ signal and main foregrounds. Emissions from dust, the CIB and the synchrotron are poorly defined spectra with floating parameters. Shaded areas show possible variations of these spectra. The remaining spectra have a strictly defined and well known shapes. Right panel. Probability distribution function of the $T$ and $b$ parameters for dust and CIB from the 'cleanest' regions of the sky. Isocontour lines limit the $\tilde{\Omega}_1$ and $\tilde{\Omega}_2$ regions of the parameter variations for dust and CIB respectively. The results are obtained from Planck data.
• Figure 2: Application of the LRM method and its iterative modification to extract the aSZ signal from the observed spectrum. Left panel. The solid stepped lines show the average response to all foregrounds and noise $\sqrt{\langle(R_1^n)^2\rangle}$ after each iteration $n$ for different values of photon noise. Iteration with number n=1 corresponds to the usual unmodified LRM method. The straight dashed lines correspond to the response to poorly defined foregrounds (dust+CIB+synchrotron) and noise. The red straight solid lines represent the response to the aSZ signal caused by the dipol ($\ell=1$) quadrupol ($\ell=2$) and octupol $\ell=3$ temperature anisotropy. Right panel. The solid blue line corresponds to the response to all foregrounds and noise after 10 iterations as a function of the photon noise (sensitivity). The upper dashed-dotted line shows the result of the conventional LRM method. The lower dashed line corresponds to the response to poorly defined foregrounds + noise. Red lines are the responces to the aSZ signal caused by multipoles with $\ell=1,2,3$.