The GHz-Peaked Spectrum radio galaxy 2021+614: Detection of slow motion in a compact symmetric object

Abstract

We have analysed VSOP (VLBI Space Observatory Programme) data at 5 GHz and ground-based VLBI (Very Long Baseline Interferometry) data at 15 GHz for the GHz-Peaked Spectrum (GPS) radio galaxy 2021+614. Its morphology is consistent with it being a compact symmetric source extending over 30 h^-1 pc (H_0 = 100 h km s^-1 Mpc^-1, q_0 = 0.5). From a comparison with earlier observations we have detected an increase in the separation and a decrease in the size of the two most prominent components. We determine the projected speed with which these two components recede from each other to be 0.12+/-0.02 h^-1 c. Given the projected separation of the two components of 16.1 h^-1 pc, the infered kinematic age is 440+/-80 years, measured in the source reference frame. These results provide additional support for the contention that compact symmetric radio objects are young and the precursors of the classical FR I or FR II radio sources. The sizes of individual components appear to contract with time which is not consistent with the self-similar evolution model for peaked spectrum sources. In order to overcome problems related to the estimation of uncertainties for separation measurements between source components, we have developed and applied a method that compares two uv-data sets obtained at different epochs. This method parametrizes the most important structural change, the increase in separation between components, by rescaling the u and v axis of the amplitude interference pattern. It provides best-fit values for the parameters and uses a bootstrap method to estimate the errors in the parameters.