Asymmetries of Radio Galaxies

In this work, we have studied the asymmetries of radio galaxies which are known to be related to the jet-asymmetry. They are the depolarization asymmetry and the spectral index asymmetry. The first part of the thesis is concerned with the polarization asymmetry in GRGs and B2 RGs. After the introductory chapter, we discuss the Ricean bias in the fractional polarization and suggest a hybrid solution in the second chapter. The merit of this new solution is the enhanced reliability of the pixel-to-pixel fractional polarization. Using this hybrid solution, we present DP maps of the five GRGs at the end of the chapter. We conclude that the five GRGs are highly depolarized at low radio frequencies, but the depolarization is not as significant as Burn's law predicts. NGC315 shows an interesting characteristic in that the source is NOT depolarized. The subject of the third chapter is the rotation measure of the five GRGs. After correction for the Galactic contribution to the RM, the GRGs exhibit a few hundred radians per square meters of the Faraday medium. This must be due to field reversals or to changes of the field strength on a scale length of 50 to a few hundred kilo-parsecs. This result indicates that the Faraday media of the GRGs are well resolved, despite the relatively large beam size used. In the fourth chapter, we study the depolarization asymmetry of the B2 sources. Using the hybrid solution of the second chapter, we confirm the result of Morganti et al.(1997) who discovered the `Laing-Garrington' effect in the B2 sources. The Faraday Rotation Measure (hereafter RM) of the B2 sources is estimated using a frequency range similar to that of the GRG sample, but with better angular resolution. The measured RM is about an order of magnitude lower than that of the B2 sources. We test the correlation between the depolarization asymmetry and the asymmetry of the RM dispersion. There is a trend that the more Depolarization (hereafter DP)--asymmetric source is the more RM dispersion--asymmetric. Again, the trend is not as strong as the prediction of Burn's law. The fifth chapter is concerned with the high RM that we found in the GRG sample. We discuss the possible processes producing the high RMs. Since the GRGs with a symmetric morphology, e.g. 3C236, also exhibit a high RM, we favour an `internal' mechanism. Our estimate shows that expansion losses can support the amount of magnetic energy in the Faraday media. However, an external process such as large-scale shock flows may also play a role as we saw in the case of NGC315, where the RM becomes even higher. The sixth chapter deals with the spectral asymmetry. In FR II type sources, there are conflicting results as to this asymmetry. Liu et al.(1991) and Garrington et al.(1991) find a systematic asymmetry where the jet-side spectra are flatter in a sample of FR II type sources. In contrast to these results, Dennett-thorpe(1999) reported in her thesis that there is no systematic asymmetry in a representative FR II sample. Our concern was that the spectral asymmetry could occur not only through the spectral ageing effect but also through the difference in the injection spectral index. This idea may be more relevant for FR I sources and GRGs than for FR II sources, since the latter have a highly symmetric morphology, hence similar physical conditions are expected in both double lobes. Katz-ston et al.(1997) reported that there are possibly two injection spectra in the lobes of the FR I-type radio galaxy 3C449, based on an analysis of the colour-colour diagram. We develop a new analysis, the spectral curvature (SCP) - spectral index (alpha) diagram, in order to easily distinguish different injection spectra. We apply this technique to the GRG sample for which we have multiple frequency data at our disposal. The results show that there exists more than just one injection spectrum in the individual sources. Furthermore, we test this SCP-alpha diagram for a sample of 47 CSS sources whose injection spectra had been thoroughly analyzed by Murgia et al.(1999). This test proves that the SCP-alpha analysis is highly reliable. In the GRG sample we find multiple injection spectra using our new SCP-alpha analysis. On the whole, the subject of this thesis is rather the study of the analysis of the asymmetries, rather than the asymmetries themselves. The behaviour of the two known systematic (jet-asymmetry related) asymmetries is studied. We might verify these results in another way. Recently, RGs were recognized as a promising tool for the process of cosmic structure formation. The linear polarization property of RGs enables us to probe the magneto-ionic intergalactic or intracluster medium. The theory of cosmic particle acceleration is largely based on the observation of the radio spectra of RGs. It will be interesting to investigate a larger and representative sample of RGs with the improved analysis methods suggested in this work

The relationship between radio power at 22 and 43 GHz and black hole properties of AGN in elliptical galaxies

We investigate the relationship between radio power and properties related to active galactic nuclei (AGNs). Radio power at 1.4 or 5 GHz, which has been used in many studies, can be affected by synchrotron self-absorption and free-free absorption in a dense region. On the other hand, these absorption effects get smaller at higher frequencies. Thus, we performed simultaneous observations at 22 and 43 GHz using the Korean VLBI Network (KVN) radio telescope based on a sample of 305 AGN candidates residing in elliptical galaxies from the overlap between the Sloan Digital Sky Survey (SDSS) Data Release 7 and Faint Images of the Radio Sky at Twenty-Centimeters (FIRST). About 37% and 22% of the galaxies are detected at 22 and 43 GHz, respectively. Assuming no flux variability between the FIRST and KVN observation, spectral indices were derived from FIRST and KVN data and we found that over 70% of the detected galaxies have flat or inverted spectra, implying the presence of optically thick compact regions near the centres of the galaxies. Core radio power does not show a clear dependence on black hole mass at either low (1.4 GHz) or high (22 and 43 GHz) frequencies. However, we found that the luminosity of the [OIII] $\lambda$5007 emission line and the Eddington ratio correlate with radio power more closely at high frequencies than at low frequencies. This suggests that radio observation at high frequencies can be an appropriate tool for unveiling the innermost region. In addition, the luminosity of the [OIII] $\lambda$5007 emission line and the Eddington ratio can be used as a tracer of AGN activity. Our study suggests a causal connection between high frequency radio power and optical properties of AGNs.Comment: 14 pages, 13 figures, 5 tables, Accepted for publication in A&

Simultaneous dual-frequency radio observations of S5 0716+714: A search for intraday variability with the Korean VLBI Network

This study aims to search for the existence of intraday variability (IDV) of BL Lac object S5 0716+714 at high radio frequencies for which the interstellar scintillation effect is not significant. Using the 21-meter radio telescope of the Korean VLBI Network (KVN), we present results of multi-epoch simultaneous dual-frequency radio observations. Single-dish observations of S5 0716+714 were simultaneously conducted at 21.7 GHz (K-band) and 42.4 GHz (Q-band), with a high cadence of 30-60 minute intervals.We observed four epochs between December 2009 and June 2010. Over the whole set of observation epochs, S5 0716+714 showed significant inter-month variations in flux density at both the K- and Q-bands, with modulation indices of approximately 19% for the K-band and approximately 36% for the Q-band. In all epochs, no clear intraday variability was detected at either frequency. The source shows monotonic flux density increase in epochs 1 and 3 and monotonic flux density decrease in epochs 2 and 4. In the flux density increasing phases, the flux densities at the Q-band increase more rapidly. In the decreasing phase, no significant flux density difference is seen at the two frequencies. The situation could be different close to flux density peaks that we did not witness in our observations. We find an inverted spectrum with mean spectral indices of -0.57+-0.13 in epoch 1 and -0.15+-0.11 in epoch 3. On the other hand, we find relatively steep indices of +0.24+-0.14 and +0.17+-0.18 in epochs 2 and 4, respectively. We conclude that the frequency dependence of the variability and the change of the spectral index are caused by source-intrinsic effects rather than by any extrinsic scintillation effect.Comment: 6 pages and 4 figures and 4 table

Key Science Observations of AGNs with KaVA Array

KaVA (KVN and VERA Array) is a new combined VLBI array with KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). First, we briefly review the imaging capabilities of KaVA array which actually achieves more than three times better dynamic range than that achieved by VERA alone. The KaVA images clearly show detailed structures of extended radio jets in AGNs. Next, we represent the key science program to be led by KaVA AGN sub working group. We will conduct the monitoring observations of Sgr A* and M87 because of the largeness of their central super-massive black hole angular sizes. The main science goals of the program are (i) testing magnetically-driven-jet paradigm by mapping velocity fields of the M87 jet, and (ii) obtaining tight constraints on physical properties of radio emitting region in Sgr A*.Comment: Proceedings of the 12th European VLBI Network Symposium and Users Meeting, 7-10 October 2014, Cagliari, Ital