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Title: Celestial Reference Frames: Mapping the Heavens

Speaker: Dr.  Aletha de Witt (South African Radio Astronomy Observatory (SARAO))

Date&Time: 2023-04-07, 11:00

Location: 과학관 B102호(Science Hall, B102)

Language: English

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Abstract:

Hipparchus’ catalog of stellar positions from around 130 B.C. is believed to be the first of many celestial catalogs compiled over the centuries by astronomers, using ever-better instruments and techniques, to record accurate coordinates of celestial objects. From those measurements astronomers have derived everything from basic stellar properties to estimates of the age of the Universe. Today the positions of point-like extragalactic objects, such as quasars and other radio-loud Active Galactic Nuclei (AGN), are measured with the highest accuracy using networks of radio telescopes across the globe. Since these extragalactic objects are so distant, they appear as fixed points in the sky, and form a map or grid of celestial beacons which are used as references to measure very accurately the angular displacement of other closer objects like stars and planets, which do show motions over time. Scientists also use the radio observations of these extragalactic reference points to determine the orientation of Earth in space and how the radio telescopes move relative to one another, which allows us to study the motion of the tectonic plates and the interior of the Earth. Until recently, extragalactic source positions have been measured only in the radio domain using networks of radio telescopes spread throughout the planet and operating jointly in interferometric mode. This so-called Very Long Baseline Interferometry (VLBI) technique has allowed scientists to measure the angular direction of compact AGN with a part per billion accuracy (~100 µas). The current catalog of precise positions of AGN observed with VLBI are composed of ~4500 sources and is the official IAU celestial reference frame since Jan. 1 2019. The Gaia space astrometry mission that was launched on 19 December 2013 has measured ultra-accurate positions in the sky for nearly 2 billion objects of which ~1.6 million are extragalactic objects that define the celestial reference frame in the optical domain. In order to compare the VLBI radio and Gaia optical catalog directly, it is necessary to identify common objects, i.e. objects that emit both at optical and radio wavelengths, so that the VLBI and Gaia grid of positions may be properly registered with the highest accuracy. At frequencies above the legacy standard of 8 GHz (X-band), AGN tend to become more point like allowing their positions to be measured more accurately, which should allow for improved ties of the VLBI frame to the Gaia optical frame. For this reason, in 2013, we started the construction of a radio reference frame at 24 GHz (K-band). More recently, we also started growing collaborations to extend our K-band network of radio telescopes, such as the recently started collaborations with the Korean VLBI Network (KVN) which is expected to significantly improve the accuracy of our celestial frame.