Ojha, RoopeshFey, Alan L.Johnston, Kenneth J.Jauncey, David L.Reynolds, John E.Tzioumis, Anastasios K.Quick, Jonathan F. H.Nicolson, George D.Ellingsen, Simon P.Dodson, Richard G.McCulloch, Peter M.2020-05-082020-05-082004-06-01Roopesh Ojha et al., VLBI Observations of Southern Hemisphere ICRF Sources. I., The Astronomical Journal, Volume 127, Number 6, https://doi.org/10.1086%2F421001https://doi.org/10.1086%2F421001http://hdl.handle.net/11603/18513http://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2004AJ....127.3609Ohttps://ned.ipac.caltech.edu/cgi-bin/objsearch?search_type=Search&refcode=2004AJ....127.3609Ohttp://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/AJ/127/3609We present 8.4 GHz very long baseline interferometry (VLBI) observations of 69 southern hemisphere extragalactic sources in the International Celestial Reference Frame. These are the first in a series of observations intended to image all such sources at milliarcsecond resolution in order to determine their continued suitability for reference-frame use based on intrinsic structure. We use the resultant images to calculate a core fraction, that is, the ratio of core flux density to total flux density, for all observed sources. The resulting distribution, with a mean value of 0.83, suggests that most sources are relatively compact. However, just over half the observed sources show significant extended emission in the form of multiple compact components. These sources are probably poorly suited for high-accuracy reference-frame use unless intrinsic structure and potential variability can be taken into account. Our observations represent the first large, comprehensive VLBI imaging survey in the southern hemisphere, significantly extending the existing limited VLBI surveys and, along with some well-known objects, containing many sources that have never been imaged at milliarcsecond resolution. The overlap with Very Long Baseline Array images of sources between 0° and -35° declination helps determine the limits to imaging with the southern hemisphere–accessible telescopes.13 pagesen-USThis item is likely protected under Title 17 of the U.S. Copyright Law. Unless on a Creative Commons license, for uses protected by Copyright Law, contact the copyright holder or the author.Public Domain Mark 1.0This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. LawText