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Register a new userPredictions on high frequency polarization properties of extragalactic radio sources and implications for CMB polarization measurements
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We present a method to simulate the polarization properties of extragalactic radio sources at microwave frequencies. Polarization measurements of nearly 2x10^6 sources at 1.4 GHz are provided by the NVSS survey. Using this catalogue and the GB6 survey, we study the distribution of the polarization degree of both steep- and flat-spectrum sources. We find that the polarization degree is anti-correlated with the flux density for the former population, while no correlation is detected for the latter. The available high-frequency data are exploited to determine the frequency dependence of the distribution of polarization degrees. Using such information and the evolutionary model by Toffolatti et al. (1998), we estimate the polarization power spectrum of extragalactic radio sources at geq 30 GHz and their contamination of CMB polarization maps. Two distinct methods to compute point-source polarization spectra are presented, extending and improving the one generally used in previous analyses. While extragalactic radio sources can significantly contaminate the CMB E-mode power spectrum only at low frequencies (<30 GHz), they can severely constrain the detectability of the CMB B-mode up to ~100 GHz.
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We have studied the implications of high sensitivity polarization measurements of objects from the WMAP point source catalogue made using the VLA at 8.4, 22 and 43 GHz. The fractional polarization of sources is almost independent of frequency with a median of ~2 per cent and an average, for detected sources, of ~3.5 per cent. These values are also independent of the total intensity over the narrow range of intensity we sample. Using a contemporaneous sample of 105 sources detected at all 3 VLA frequencies, we have investigated the spectral behaviour as a function of frequency by means of a 2-colour diagram. Most sources have power-law spectra in total intensity, as expected. On the other hand they appear to be almost randomly distributed in the polarized intensity 2-colour diagram. This is compatible with the polarized spectra being much less smooth than those in intensity and we speculate on the physical origins of this. We have performed an analysis of the correlations between the fractional polarization and spectral indices including computation of the principal components. We find that there is little correlation between the fractional polarization and the intensity spectral indices. This is also the case when we include polarization measurements at 1.4 GHz from the NVSS. In addition we compute 45 rotation measures from polarization position angles which are compatible with a lambda^2 law. We use our results to predict the level of point source confusion noise that contaminates CMB polarization measurements aimed at detecting primordial gravitational waves from inflation. We conclude that some level of source subtraction will be necessary to detect r~0.1 below 100 GHz and at all frequencies to detect r~0.01. We present estimates of the level of contamination expected and the number of sources which need to be subtracted as a function of the imposed cut flux density and frequency.
We present polarization measurements at 8.4, 22, and 43 GHz made with the VLA of a complete sample of extragalactic sources stronger than 1 Jy in the 5-year WMAP catalogue and with declinations north of -34 degrees. The observations were motivated by the need to know the polarization properties of radio sources at frequencies of tens of GHz in order to subtract polarized foregrounds for future sensitive Cosmic Microwave Background (CMB) experiments. The total intensity and polarization measurements are generally consistent with comparable VLA calibration measurements for less-variable sources, and within a similar range to WMAP fluxes for unresolved sources. A further paper will present correlations between measured parameters and derive implications for CMB measurements.
Recent polarimetric surveys of extragalactic radio sources (ERS) at frequencies u>1GHz are reviewed. By exploiting all the most relevant data on the polarized emission of ERS we study the frequency dependence of polarization properties of ERS between 1.4 and 86GHz. For flat-spectrum sources the median (mean) fractional polarization increases from 1.5% (2-2.5%) at 1.4GHz to 2.5-3% (3-3.5%) at u>10GHz. Steep-spectrum sources are typically more polarized, especially at high frequencies where Faraday depolarization is less relevant. As a general result, we do not find that the fractional polarization of ERS depends on the total flux density at high radio frequencies, i.e >20GHz. Moreover, in this frequency range, current data suggest a moderate increase of the fractional polarization of ERS with frequency. A formalism to estimate ERS number counts in polarization and the contribution of unresolved polarized ERS to angular power spectra at Cosmic Microwave Background (CMB) frequencies is also developed and discussed. As a first application, we present original predictions for the Planck satellite mission. Our current results show that only a dozen polarized ERS will be detected by the Planck Low Frequency Instrument (LFI), and a few tens by the High Frequency Instrument (HFI). As for CMB power spectra, ERS should not be a strong contaminant to the CMB E-mode polarization at frequencies u>70GHz. On the contrary, they can become a relevant constraint for the detection of the cosmological B--mode polarization if the tensor-to-scalar ratio is <0.01.
We show that the delay of structure formation from WMAP3 can not fully account for the reduction of electron optical depth from WMAP1 to WMAP3 when the radiative transfer effects and feedback mechanisms are took into account in computing the reionization history of the Universe. As the ultimate limit in constraining the reionizatin history of the Universe with Planck will be placed by the our understanding of systematic effects and foregrounds removal, we discuss also these aspects.
Confusion noise due to extragalactic sources is a fundamental astrophysical limitation for experiments aimed at accurately determining the power spectrum of the Cosmic Microwave Background (CMB) down to arcmin angular scales and with a sensitivity $Delta T/T simeq 10^{-6}$. At frequencies $lsim 200-300$ GHz, the most relevant extragalactic foreground hampering the detection of intrinsic CMB anisotropies is constituted by radio loud Active Galactic Nuclei (AGN), including ``flat--spectrum radiogalaxies, quasars, BL-LACs and blazars. We review our present understanding of astrophysical properties, spectra, and number counts of the above classes of sources. We also study the angular power spectrum of fluctuations due both to Poisson distributed and clustered radio sources and give preliminary predictions on the power spectrum of their polarized components. Furthermore, we discuss the capabilities of future space missions (NASAs MAP, Bennett et al. 1995; ESAs Planck Surveyor, Bersanelli et al. 1996) in studying bright radio sources over an almost unexplored frequency interval where spectral signatures, essential for the understanding of the physical processes, show up.
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