Interstellar Scattering as a Cosmological Probe J. Y. Koay 1* , J. -P. Macquart 1 , B. J. Rickett 2 , H. E. Bignall 1 , J. E. J. Lovell 3 , C. Reynolds 1 , D. Jauncey 4 , T. Pursimo 5 , L. Kedziora-Chudczer 6 , and R. Ojha 7,8 1 International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102, Australia *e-mail: kevin.koay@icrar.org 2 Department of Electrical and Computer Engineering, University of California, San Diego, CA 92093, USA 3 School of Mathematics and Physics, University of Tasmania, TAS 7001, Australia 4 Australia Telescope National Facility CSIRO, PO Box 76, Epping, NSW 1710, Australia 5 Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain 6 School of Physics and Astrophysics, University of New South Wales, Sydney, NSW 2052, Australia 7 NASA, Goddard Space Flight Center, Greenbelt, MD 20771, USA 8 Institute for Astrophysics & Computational Sciences, The Catholic University of America, 620 Michigan Ave., N.E., Washington, DC 20064, USA Abstract Since the discovery that the flux densities of very compact astrophysical sources are modulated by scattering in the inhomogeneous, ionized interstellar medium (ISM) of our own Galaxy through a phenomenon known as Interstellar Scintillation (ISS), these scattering effects have been used with great success as a tool to probe the physics of the ISM and the sources themselves. With the recent discovery of a redshift dependence in the ISS of quasars in a 4.9 GHz survey of about 500 sources, large statistical studies of ISS have been imbued with a cosmological significance. Possible causes of this effect include cosmological expansion, scatter broadening by the ionized intergalactic medium and evolution of quasar morphology with redshift. Since each of these hypotheses have different wavelength dependences, we have carried out dual-frequency observations of a subsample of 140 quasars to determine the origin of this redshift dependence of ISS. We are therefore using interstellar scattering, for the first time, as a cosmological probe at micro-arcsecond scales - achieving an angular resolution two orders of magnitude finer than that of Very Long Baseline Interferometry (VLBI). We discover a weaker redshift dependence at 8.4 GHz as compared to 4.9 GHz, indicating a strong wavelength scaling in the effect. We are investigating possible source selection effects and developing the theory to model the observations to enable an accurate interpretation of the data. 1. Introduction The scattering of electromagnetic waves in random media, whether terrestrial or atmospheric, presents a challenge and an indispensable tool to both scientists and engineers working in various fields. This holds true in radio astronomy as well. Radio waves from astrophysical sources are scattered as they propagate through the turbulent ionized interstellar medium (ISM) of our own Galaxy, generating interference patterns on the surface of the Earth. Relative motion between the Earth and the scattering medium or background object causes the interference patterns to drift across the Earth's surface, resulting in the phenomenon known as interstellar scintillation (ISS), where the flux densities of sufficiently compact sources appear to vary over time. ISS has been observed to modulate the flux densities of pulsars and quasars [1], though their effects in quasars are generally suppressed due to their more extended angular size - as the interference patterns originating from the different regions of the source cancel out. While some astronomers consider ISS as a nuisance, it has been used as a probe of the fine-scale structures of the ISM of our Galaxy [2] and quasar cores at micro-arcsecond scales [3], providing the resolution of a radio interferometer with baselines up to a million km across. 978-1-4244-6051-9/11/$26.00 ©2011 IEEE