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 backgroundB-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 primordialB-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 theB-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 ratior. At the reionisation bump (ℓ= 5), contamination by extragalactic foregrounds is negligible. While the contamination is much lower than the targeted sensitivity onrfor 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 primordialrfor 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 primordialB-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 350μm by confusion from extragalactic sources for long integrations in polarisation, but very short integrations in intensity.
Highlights
The ΛCDM model is the standard model of cosmology
We constrained the main parameters of our halo model using six measurements of CIB angular auto- and cross-power spectra at 250, 350, and 500 μm from Herschel/SPIRE
To further constrain the model, we computed the star formation rate density in the range 0 < z < 6, and we fit to the compilation of star formation rate density measurements from Madau & Dickinson (2014)
Summary
The ΛCDM model is the standard model of cosmology. It is the simplest parametrisation of the Hot Big Bang model, with two principal ingredients: Λ refers to a cosmological constant (i.e. the energy density of the vacuum), and CDM stands for cold dark matter, that is, dark matter particles that move slowly. We compute the expected level of polarised fluctuations from the shot noise of radio galaxies and DSFG and from the CIB onehalo using current or updated models for a large set of future 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, SO, SPT3G, and S4). We are the first to use our radio and DSFG models in combination with the CIB and CMB contamination and instrument noise to iteratively predict the confusion noise that is due to extragalactic sources for all experiments and derive the level of polarised fluctuations. We use these models to compute the flux limit (caused by the fluctuations of the background sky brightness below which sources cannot be detected individually, i.e. the confusion noise) for a large number of future CMB experiments and for SPICA B-POP Lagache et al.: Impact of polarised extragalactic sources on the measurement of CMB B-mode anisotropies
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