Compactness of Weak Radio Sources at High Frequencies

We have obtained 8.4 GHz VLBA observations of a 31-GHz complete sample of ∼ 100 sources between 10 and 100 mJy. The main goals of these observations are: to determine the angular size, radio spectra and identification for a weak sample of high frequency sources; to find the fraction of sources which have sufficiently compact emission for use as calibrators for VLBI observations; and for design considerations of the proposed DSN Array. We find that a large fraction of observed sources have VLBI detections. A majority of these sources have most of their emission in a compact < 1 mas radio core, with remaining sources having steep radio spectra. The source list was provided from GBT observations to remove discrete sources in the CBI fields. 1. Nature of High Frequency Radio Sample Carrying out a VLBI survey of a complete and unbiased sample of weak radio sources at high frequencies provide fundamental astronomical information on the statistical and morphological properties for this class of astrophysical objects. Previous surveys of the nature and structure of weak radio sources have been carried out at relatively lower frequencies, often at 1.4 GHz (e.g. Garrett et al. 2005). At the mJy level, the proportion of AGN’s are decreasing and the population begins to be dominated by galaxies that have significant star forming regions. These are typically less than 3”, with about 30% showing milliarcsecond emission (Muxlow et al. 2005). However, the angular characteristics of sources above 8 GHz are not wellknown at the mJy level. We plan to determine the percentage of compact milliarcsecond emission, its orientation and accurate core position for better optical identification. We also plan to study spectral index correlation versus galaxy type and compare our results with similar studies carried out for brighter sources and similar surveys at lower frequencies. 2. VLBI Calibrators at the mJy Scale Differential VLBI is routinely used to determine spacecraft positions with accuracies of ∼ 1 mas using compact radio sources with flux > 300 mJy at 8.4 GHz within about 5-10 degrees of the spacecraft. Further improvement in accuracy Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, U.S.A National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, U.S.A