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SkyMapper Protected Science Project

Photometry of flat-spectrum radio sources

Oleg Titov (Geoscience Australia), Richard Hunstead (U. Syd), Helen Johnston (U. Syd), David Jauncey (CSIRO/RSAA), Laura Stanford (GA), Tapio Pursimo (NOT), Sandor Frey (FÖMI SGO), Leonid Gurvits (JIVE)

About 3000 radio quasars are observed regularly by the International VLBI Service, IVS, to define and maintain the International Celestial Reference Frame, ICRF. We have presented evidence for large scale systematic proper motions, dipole and quadrupole effects, at a level in excess of 10 microarcsec/year. This result presents a serious challenge to the standard cosmologies. High redshifts are of particular interest because the systematic proper motions are expected to increase with redshift. However, due to the north-south imbalance in the distribution and spectroscopic completeness of the reference radio sources, the statistical analysis meets serious difficulties. To improve the statistics in the South we began a spectroscopic program in 2010 (NTT, Gemini South), but the number of high-redshift quasars is limited due to lack of modern digital sky surveys for efficient preselection of the targets. We need accurate photometric magnitudes in different optical colours to identify the strong flat-spectrum radio sources at redshifts z>3 for further observation with international VLBI networks. The SkyMapper data will be used in the same way as SDSS DR9 in the northern hemisphere to identify the stellar-like radio source identifications with large colour dropouts, e.g., g-r > 3. Based on the experience with SDSS we expect these colour dropouts to be caused by the combination of the Ly-alpha emission line at z>3 and the Ly-alpha forest absorption blueward of the emission line. The targets selected using this scheme will be proposed for spectroscopic observations at other large optical facilities.