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Register a new userForecasting Polarized Radio Sources for CMB observations
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Added by
Giuseppe Puglisi Ph.D.
Publication date
2017
fields
Physics
and research's language is
English
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We combine the latest datasets obtained with different surveys to study the frequency dependence of polarized emission coming from Extragalactic Radio Sources (ERS). We consider data over a very wide frequency range starting from $1.4$ GHz up to $217$ GHz. This range is particularly interesting since it overlaps the frequencies of the current and forthcoming Cosmic Microwave Background (cmb) experiments. Current data suggest that at high radio frequencies, ($ u geq 20$ GHz) the fractional polarization of ERS does not depend on the total flux density. Conversely, recent datasets indicate a moderate increase of polarization fraction as a function of frequency, physically motivated by the fact that Faraday depolarization is expected to be less relevant at high radio-frequencies. We compute ERS number counts using updated models based on recent data, and we forecast the contribution of unresolved ERS in CMB polarization spectra. Given the expected sensitivities and the observational patch sizes of forthcoming cmb experiments about $sim 200 $ ( up to $sim 2000 $ ) polarized ERS are expected to be detected. Finally, we assess that polarized ERS can contaminate the cosmological B-mode polarization if the tensor-to-scalar ratio is $r< 0.05$ and they have to be robustly controlled to de-lens cmb B-modes at the arcminute angular scales.
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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 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.
Aims. We selected two radio quasars (J1036+1326 and J1353+5725) based on their 1.4-GHz radio structure, which is dominated by a bright central core and a pair of weaker and nearly symmetric lobes at ~10 angular separation. They are optically identified in the Sloan Digital Sky Survey (SDSS) at spectroscopic redshifts z>3. We investigate the possibility that their core-dominated triple morphology can be a sign of restarted radio activity in these quasars, involving a significant repositioning of the radio jet axis. Methods. We present the results of high-resolution radio imaging observations of J1036+1326 and J1353+5725, performed with the European Very Long Baseline Interferometry (VLBI) Network (EVN) at 1.6 GHz. These data are supplemented by archive observations from the Very Large Array (VLA).We study the large- and small-scale radio structures and the brightness temperatures, then estimate relativistic beaming parameters. Results. We show that the central emission region of these two high-redshift, core-dominated triple sources is compact but resolved at ~10 milli-arcsecond resolution. We find that it is not necessary to invoke large misalignment between the VLBI jet and the large-scale radio structure to explain the observed properties of the sources.
We present the results of Karl G. Jansky Very Large Array (VLA) observations to study the properties of FR0 radio galaxies, the compact radio sources associated with early-type galaxies which represent the bulk of the local radio-loud AGN population. We obtained A-array observations at 1.5, 4.5, and 7.5 GHz for 18 FR0s from the FR0CAT sample: these are sources at $z<0.05$, unresolved in the FIRST images and spectroscopically classified as low excitation galaxies (LEG). Although we reach an angular resolution of $sim$0.3 arcsec, the majority of the 18 FR0s is still unresolved. Only four objects show extended emission. Six have steep radio spectra, 11 are flat cores, while one shows an inverted spectrum. We find that 1) the ratio between core and total emission in FR0s is $sim$30 times higher than in FRI and 2) FR0s share the same properties with FRIs from the nuclear and host point of view. FR0s differ from FRIs only for the paucity of extended radio emission. Different scenarios were investigated: 1) the possibility that all FR0s are young sources eventually evolving into extended sources is ruled out by the distribution of radio sizes; 2) similarly, a time-dependent scenario, where a variation of accretion or jet launching prevents the formation of large-scales radio structures, appears to be rather implausible due to the large abundance of sub-kpc objects 3) a scenario in which FR0s are produced by mildly relativistic jets is consistent with the data but requires observations of a larger sample to be properly tested.
Bent-double radio sources have been used as a probe to measure the density of intergalactic gas in galaxy groups. We carry out a series of high-resolution, 3D simulations of AGN jets moving through an external medium with a constant density in order to develop a general formula for the radius of curvature of the jets, and to determine how accurately the density of the intra-group medium (IGM) can be measured. Our simulations produce curved jets ending in bright radio lobes with an extended trail of low surface brightness radio emission. The radius of curvature of the jets varies with time by only about 25%. The radio trail seen in our simulations is typically not detected in known sources, but may be detectable in lower resolution radio observations. The length of this tail can be used to determine the age of the AGN. We also use our simulation data to derive a formula for the kinetic luminosity of observed jets in terms of the radius of curvature and jet pressure. In characterizing how well observations can measure the IGM density, we find that the limited resolution of typical radio observations leads to a systematic under-estimate of the density of about 50%. The unknown angles between the observer and the direction of jet propagation and direction of AGN motion through the IGM leads to an uncertainty of about 50% in estimates of the IGM density. Previous conclusions drawn using these sources, indicating that galaxy groups contain significant reservoirs of baryons in their IGM, are still valid when considering this level of uncertainty. In addition, we model the X-ray emission expected from bent-double radio sources. We find that known sources in reasonably dense environments should be detectable in ~100 ks Chandra observations. X-ray observations of these sources would place constraints on the IGM density and AGN velocity that are complementary to radio observations.
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