With the dawn of micro-arcsecond optical astrometry thanks to Gaia, a subject of increasing
astrophysical interest is the appearance of positional offsets between the radio and optical positions of
quasars that define the ICRF. These offsets are of profound importance to astrometry and reference frame
work, as their existence currently sets a limit for the accuracy and definition of the CRF at non-radio
wavelengths, limiting research in topics such as geodesy, the secular aberration, Earth orientation and
ephemerides, proper motions and trajectories of stars in our galaxy and its satellites, local measurements
of the Hubble constant, the stochastic gravitational wave background, and in general any field that benefits
from optimal absolute astrometry. A developing body of research has shown that these optical/radio
offsets may be due to real astrophysical processes associated with the active galactic nuclei (AGN) that
power quasars, such as jet launching/orientation, or real physical offsets between the AGN and its host
galaxy. The time is ripe for a dedicated meeting to bring together AGN and CRF researchers to discuss
outstanding questions in AGN research that may affect apparent positions. These questions include, but
are not limited to:
- Our current understanding of quasar optical/radio offsets (i.e., their physical nature) and mitigation
strategies. Relevant AGN/source galaxy physics: jet launching, obscuration,
luminosity/obscuration variability, binarity, AGN-source galaxy effects and interactions, such as
mergers and AGN feedback
- New frontiers in AGN research with current and upcoming facilities, such as Gaia/GaiaNIR, Pan-
STARRS, LSST, and the ngVLA.
- Limitations of current instrumentation and/or new methodologies with current resources.
- Strategies for maximizing all-sky samples of AGN/QSOs (e.g., multi-wavelength, time-domain
techniques, upcoming surveys).
- AGN problematic for astrometry (e.g., lensed QSOs, dual/binary AGN, dislodged AGN): how to
find and characterize them.
- Astrometry in practice: position measurement techniques, catalog cross-matching strategies (e.g.,
photometric priors, Bayesian considerations); quantifying and optimizing reliability/completeness.
This meeting will serve as a venue to discuss the status and the direction of the ICRF, as well as additional
issues that affect the ICRF, such as the need to maximize highly reliable all-sky samples of quasars and
AGN, and the need for more southern sky sources. Finally, with extremely large photometric and
astrometric catalogs making up much of modern astronomical research, discussion of astrometry in
practice is warranted, such as developing sophisticated techniques for maximizing the reliability and
completeness of catalog cross-matches. This will be of utmost value to the AGN community, which relies
intimately on the ability to produce highly reliable and statistically complete multi-wavelength samples
of AGN and their host galaxies.