Multi-epoch VLBI images to study the ICRF-3 Defining Sources in the Southern Hemisphere

The third realisation of the International Celestial Reference Frame (ICRF-3) is based on high precision Very Long Baseline Interferometric (VLBI) measurements of positions of 4,588 extragalactic radio sources and 4,536 sources of them are observed at 2.3 and 8.4 GHz. This includes the 303 defining sources which determine the orientation of the frame axes. These reference sources often show extended emissions on milliarcsecond scales that can vary with time, frequency and baseline projection. The effect of source structure on astrometric VLBI positions can be significant and it is therefore important to map the structures of these sources on a regular basis. However, due to the limited number of radio telescopes in the south, it has always been difficult to run dedicated imaging programmes and to monitor source structure, especially for sources below -30$^{\circ}$ declination. We present results from our efforts to image and monitor the intrinsic source structures of ICRF sources using ongoing astrometric and geodetic VLBI observations in the south.