Impact of polarised extragalactic sources on the measurement of CMBB-mode anisotropies

Abstract

One of the main goals of cosmology is to search for the imprint of primordial gravitational waves in the polarisation filed of the cosmic microwave background to probe inflation theories. One of the obstacles in detecting the primordial signal is that the cosmic microwave background B-mode polarisation must be extracted from among astrophysical contaminations. Most efforts have focus on limiting Galactic foreground residuals, but extragalactic foregrounds cannot be ignored at the large scale (ℓ ≲ 150), where the primordial B-modes are the brightest. We present a complete analysis of extragalactic foreground contamination that is due to polarised emission of radio and dusty star-forming galaxies. We update or use current models that are validated using the most recent measurements of source number counts, shot noise, and cosmic infrared background power spectra. We predict the flux limit (confusion noise) for future cosmic microwave background (CMB) space-based or balloon-borne experiments (IDS, PIPER, SPIDER, LiteBIRD, and PICO), as well as ground-based experiments (C-BASS, NEXT-BASS, QUIJOTE, AdvACTPOL, BICEP3+Keck, BICEPArray, CLASS, Simons Observatory, SPT3G, and S4). The telescope aperture size (and frequency) is the main characteristic that affects the level of confusion noise. Using the flux limits and assuming mean polarisation fractions independent of flux and frequency for radio and dusty galaxies, we computed the B-mode power spectra of the three extragalactic foregrounds (radio source shot noise, dusty galaxy shot noise, and clustering). We discuss their relative levels and compare their amplitudes to that of the primordial tensor modes parametrised by the tensor-to-scalar ratio r. At the reionisation bump (ℓ = 5), contamination by extragalactic foregrounds is negligible. While the contamination is much lower than the targeted sensitivity on r for large-aperture telescopes at the recombination peak (ℓ = 80), it is at a comparable level for some of the medium- (∼1.5 m) and small-aperture telescope (≤0.6 m) experiments. For example, the contamination is at the level of the 68% confidence level uncertainty on the primordial r for the LiteBIRD and PICO space-based experiments. These results were obtained in the absence of multi-frequency component separation (i.e. considering each frequency independently). We stress that extragalactic foreground contaminations have to be included in the input sky models of component separation methods dedicated to the recovery of the CMB primordial B-mode power spectrum. Finally, we also provide some useful unit conversion factors and give some predictions for the SPICA B-BOP experiment, which is dedicated to Galactic and extragalactic polarisation studies. We show that SPICA B-BOP will be limited at 200 and 350 μm by confusion from extragalactic sources for long integrations in polarisation, but very short integrations in intensity.

Simple halo model formalism for the cosmic infrared background and its correlation with the thermal Sunyaev-Zel’dovich effect

“…A similar trend appears for the measurements from Lenz et al (2019). A more detailed discussion of this can be found in Lagache et al (2020) and is also presented in Sect. 3.4.…”

“…The Herschel/SPIRE data points and the best-fit curves were scaled to obtain them as they would be measured using the Planck filters at 857 GHz. The procedure for this is provided in Appendix A.1 of Lagache et al (2020). A similar trend is observed for the power spectra at 545 GHz.…”

“…This discrepancy cannot be reconciled considering the difference in amplitude of the one-halo term. This discrepancy of the Herschel and Planck shot-noise measurements, and with the shot-noise values derived from models of galaxy number counts, has previously been pointed out by Lagache et al (2020). Inconsistencies also exist inside a single experiment.…”

“…These colour corrections are 1.119, 1.097, 1.068, and 0.995 at 217, 353, 545, and 857 GHz, respectively, for Planck (Planck Collaboration XXX 2014). They are equal to 0.974, 0.989, and 0.988 for 600, 857, and 1200 GHz channels of Herschel/SPIRE, respectively, using the extended relative spectral response function (Lagache et al 2020). The CIB power spectra were then corrected as…”

“…Appendix C: Comparison of the Planck and Herschel CIB power spectra Planck and Herschel data are fit separately. The SPIRE data have been rescaled to Planck data using the cross-calibration factor derived in Bertincourt et al (2016) and the factor that allows converting the measurement through the SPIRE bandpass into a measurement as it would be obtained through the Planck bandpass from Lagache et al (2020). Overall, C Planck,857 GHz = 1.008 × C SPIRE,857 GHz .…”