出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：A direct link between a radio and an optical reference frame can be established by position measurements
of common objects. The International Celestial Reference Frame (ICRF) is dened
by the positions of 212 compact extragalactic radio sources regularly observed with Very Long
Baseline Interferometry (VLBI). At present, space- and ground-based all-sky optical astrometric
catalogues do not reach the magnitude limit needed for the detection of the majority of sources
studied with VLBI. The HIPPARCOS reference frame in the optical is linked to the ICRF indirectly
through relative astrometric measurements of a few radio stars with respect to nearby
radio-loud active galactic nuclei (AGNs). In the next decade, sensitive space-based optical astrometry
missions (e.g. Gaia) will observe thousands of VLBI-detected quasars, thus providing
a direct link between the radio and optical reference frames. Such a link is essential not only
for astrometry, but for astrophysical applications as well. The high-resolution structure of the
AGNs observed at different electromagnetic wavelengths can only be registered correctly if the
coordinates are expressed in consistent systems.
Although the Sloan Digital Sky Survey (SDSS) does not provide high-accuracy astrometric data
in the optical, its faint magnitude limit allows us to directly compare the positions of a sample of
735 radio sources (quasars or galaxies). For these objects, ICRF radio positions are available at
milli-arcsecond (mas) accuracy or better from the ICRF lists and the VLBA Calibrator Surveys
(VCS). We conrm that the SDSS coordinates are in general accurate down to 60 mas in both
right ascension and declination. We nd indications that the optical and radio brightness peaks
are not necessarily coincident for some of the objects.
