Which are
the best QSOs to define the Gaia frame?
Summary
Gaia will survey with unprecedented accuracy approximately 500,000 QSOs, largely new detections. Besides the implications for QSO science, this sample will allow the best materialization ever of a reference frame, providing astrometry with the potential for testing General Relativity and probing modern Cosmology.
Rationale
Gaia will provide for the first time, a large, all-sky sample of QSOs (Quasi-Stellar Objects) combining astrometric accuracy with a five year-long photometric monitoring. With such an unprecedented net of fiducial objects forthcoming, it seemed all the more natural to think of a workshop addressing some of the most outstanding issues in fundamental physics and experimental gravitation.
Unlike Hipparcos, Gaia will permit the determination of the extragalactic celestial reference frame directly at optical bands, based on the QSOs with the most accurate positions. Determining this frame will be quite challenging. The state-of-art on the subject will be presented during the workshop, where questions such as: (1) the requirements for such frame (2) putative future improvements (3) the radio counterpart that will be aligned to the Gaia frame, will be addressed.
The comparison of the radio and optical positions of QSOs for a large number of objects, is quite relevant for the models that discuss the AGN (Active Galactic Nuclei) paradigm. The objects that take part of the celestial reference frame have the most accurate and stable positions, which are critical properties. Nevertheless there is evidence for a correlation between flux variation (a well known characteristic of the QSOs) and variation of the astrometric location of the QSO photocentre.The origin of such photo-centre variation might be related to different phenomena occurring in the vicinity of the central engine of the QSOs: near the bases of the relativistic jets, the broad emission line region and accretion disk.
A 6-dimensional accurate reconstruction of the individual stars across most of the volume of the Milky Way necessarily needs extremely accurate astrometric observations modelled, as first done for the Hipparcos mission, within a fully, comparably accurate, relativistic framework. Then, for a usable form of star light propagation, one has to fix the space-time geometry that star light goes through before reaching the observer. In particular, the main gravitational effects acting on the light rays for a local observer are due to the Sun and the Solar System planets, hence the natural choice for the background metric is that adopted by the IAU in 2000. Since General Relativity is the theory needed to build the reference frame at mas and sub-mas accuracy, the relativistic astrometric measurements and their link to the reference frames must be considered. The goal is to understand how (i) to treat gravity properly when compiling mas stellar catalogues, and (ii) to promote the use of highly accurate astrometry to help scrutinize among several alternative theories the one providing the route to quantum gravity or also to test locally current cosmological models.
Moreover,
high precision astrometry of extragalactic and galactic objects is a
powerful tool for providing independent experimental grounds to the
most daunting challenges in fundamental physics and cosmology.
Observing QSO at different redshifts to monitor astrometric stability
can be a powerful tool in observational cosmology: here the intrinsic
astrometric stability of (carefully selected) QSOs is checked to test
for any anisotropies of the Universe, i.e., deviations from isotropy
due to uneven distributions of dark matter and dark energy. At zero
redshift, dealing with local cosmology, accurate absolute motions of
stars within our Galaxy will provide access to the cosmological
signatures in the disk and halo. The uniqueness offered by accurate
kinematics at the scale of the Milky Way is the ability to account in
situ for the predictions of the cold dark matter model, in the case
of the halo, and eventually map out the distribution of dark matter
or other formation mechanisms required to explain signatures recently
identified in the old component of the thick disk.
The workshop attempts at putting Gaia in context by bringing together the astronomical and physical communities engaged in present and future experiments in QSO observation, gravitation and cosmology. Nowadays, thanks to the high accuracy achievable by Gaia observations, fundamental astronomy cannot be set aside from fundamental physics.
Aims
The goal of the workshop is to bring together the scientific community from different domains to work out answers for the burning questions concerning the extragalactic celestial reference frame, its determination, and its alignment to the radio frame, or concerning astrometry and relativity in the framework of Gaia:
Which are
the best QSOs to define the Gaia frame?
Are there
unseen effects related to accurate reference frame definition?
Is the
radio-optical AGN core-shift measurable in practice?
Are
astrometric measurements properly modelled within the framework of
General Relativity and other theories of gravity?
How
important is the link to the dark matter/energy problem?
What kind
of tests of fundamental physics could be done by astrometry?
To what
extent accurate astrometric monitoring of local and distant objects
can help improving our understanding of the physical law governing
birth and evolution of the Universe?