Radio - Loud Active Galactic Nuclei from parsec to Mega-parsec scales

The thesis covers two subjects of extra-galactic astrophysics that concern radio-loud active galactic nuclei (RLAGNs). We describe the two subjects separately, but there are deep connections between the two. The first subject concerns the Mega-parsec scale environments of z~1-2 radio galaxies. First, we describe a method (Poisson probability method, PPM) primarily introduced to search for dense Megaparsec-scale environments (i.e. galaxy clusters and groups) around a specific beacon using photometric redshifts and galaxy number counts on the basis of Poisson statistics. We test the efficiency of our method of searching for cluster candidates against simulations. Two different approaches are adopted. (1) We use two z~1 X-ray detected cluster candidates found in the Cosmic Evolution Survey (COSMOS) and we shift them to higher redshift up to z = 2. We find that the PPM detects the cluster candidates up to z = 1.5, and it correctly estimates both the redshift and size of the two clusters. (2) We simulate spherically symmetric clusters of different size and richness, and we locate them at different redshifts (i.e., z = 1.0, 1.5, and 2.0) in the COSMOS field. We find that the PPM detects the simulated clusters within the z~1-2 range with a statistical 1-σ redshift accuracy of ~0.05. The PPM naturally arises as an effective mean field theory defined on the ensemble of the photometric redshift realizations of the galaxies in the field. A differential argument embedded in the PPM theory shows that the PPM partially overcomes the limitations deriving from low number-count statistics and shot-noise fluctuations. This is ultimately achieved through the use of a solid positional prior and an accurate photometric redshift sampling. Therefore, the PPM is an efficient alternative method for high-redshift cluster searches that may also be applied to both present and future wide field surveys such as SDSS Stripe 82, Large Synoptic Survey Telescope (LSST), and Euclid. Accurate photometric redshifts and a survey depth similar or better than that of COSMOS (e.g., I<25) are required. We also apply the PPM to search for high-redshift (z~1-2) galaxy clusters around low power radio galaxies (FR I). We use a sample of 32 FR Is within the COSMOS field from the Chiaberge et al. catalog that we redefine on the basis of the radio power. The sample redefinition allows us to estimate the comoving space density of sources with L_{1.4 GHz}≅10^32.3 erg s^{-1} Hz^{-1} at z≅1.1, which strengthens the case for a strong cosmological evolution of these sources. Overdensities are found around ~70% of the FR Is. This rate is in agreement with the fraction found for low redshift FR Is and it is significantly higher than that for FR IIs at all redshifts. Cluster candidates found with our method are excellent targets for next generation space telescopes such as James Webb Space Telescope (JWST). The method reveals itself as a powerful tool to search for distant galaxy clusters. The second subject that concerns RLAGNs is focused on blazars. We build a complete sample of flat-spectrum radio quasars (FSRQs) from the WMAP 7-yr catalog and compare black hole mass estimates based on fitting a standard accretion disk model to the `blue bump' with those obtained from the commonly used single-epoch virial method. The sample comprises 80 objects flux limited at 1 Jy at 23 GHz, 55 of which (69%) have a clearly detected `blue bump'. Thirty-three of the 55 FSRQs have, in the literature, black hole mass estimates obtained with the virial method. The mass estimates obtained from the two methods are well correlated. The fact that the two totally independent methods agree so closely in spite of all the potentially large uncertainties associated with each of them lends strong support to both of them. We find evidence of mid-infrared (MIR) emitting active galactic nucleus (AGN) torus in the spectral energy distributions (SEDs) of seven of the 55 FSRQs with evidence of blue bump and an uncertain indication is found for one more. The fraction of FSRQs with evidence of AGN torus increases up to 39% if only the FSRQs at z≤1 and with evidence of blue bump are considered. Our results suggest that the detection of the AGN torus is favored in those FSRQs where the specific synchrotron flux at MIR wavelengths does not overwhelm the contribution from the torus. Conversely to BL Lac sources, which are known to lack much sign of gas, our results show that the AGN torus might be present in a large fraction of FSRQs.