VLBI Study Of Compact Steep Spectrum Sources

Active Galactic Nuclei(AGNs)have become one of the hot topics in Astronomy due to their intense activity and vast distances.The observations of AGNs in the radio bands can obtain their internal structures,magnetic fields and other physical properties.With the development of Very Long Baseline Interferometry(VLBI),the structures of AGN can be observed in milliarcsecond or even sub-milliarcsecond scales,which can further explore their physical properties and galaxy evolution.This paper is divided into four parts,the research background,observation and data processing,research results of three target sources,conclusions and prospects.Chapter one introduces the observation characteristics,classification,unified model and research purpose of AGN.Meanwhile,the VLBI technique is briefly introduced,including its equipment composition,basic principle and main VLBI arrays.The second chapter introduces the principle of observation,data processing and calibration.The polarization observations of the target radio sources were carried out by using the Very Long Baseline Array(VLBA)of the United States in the frequencies of 5 and 8 GHz.The third chapter reports our research on three target radio sources.For sources 0223+341 and 0248+430,the radio structures at 5GHz and 8 GHz and the polarization structures at 5 GHz are acquired.Source 0223+341 is composed of two likely hot spots at 5 GHz,three parts at 8 GHz.It has a distorted structure and no radio nuclei observed.The polarization is concentrated in a hot spot with neat structure and low polarization degree.Source 0248+430 is a single-side core-jet structure,and the jet has a high collimation,but the jet end has a significant turn at8 GHz,which hints a possible strong interaction between the jet and the medium.The polarization of the source is concentrated in the radio core close to the jet with neat structure and low polarization degree,and the polarization direction is almost parallel to the jet.The medium around sources 0223+341 and 0248+430 may be relatively dense,resulting in strong depolarization effect.The polarization observations of the two sources at 8 GHz did not show reliable polarization structure,so the rotation of their polarization Angle is unknown,which lead to the intrinsic polarization and magnetic field structure of the sources could not be acquired.For the source 0400+258,the radio structure and polarization structure from 5 GHz and 8 GHz observation band are obtained.The source has a single-side core-jet structure,and the jet is more collimated at 5 GHz then 8GHz.At 8GHz,the jet has an internal twisted structure.The polarization is concentrated in the radio core,and the polarization degree of 5GHz and 8GHz is relatively low.There is a linear relationship between the polarization angle and the square of the observation wavelength,which conforms to the theory.The distribution of rotation is mapped,the peak value of rotation is located at about 2 mas along the jet direction of the core.The value of rotation gradually decreases along the jet direction,while increasing from the left to the right.After the rotation is removed,the intrinsic magnetic field structure is shown.Its direction is roughly parallel to the jet direction,which hints the interaction between the medium and the jet.The fourth chapter summaries research results which understand the physical properties and evolution of AGN.Finally,the future work is prospected.