The Gaia reference frame amid quasar variability and proper motion patterns in the data

Sammanfattning: The astrometric satellite Gaia was launched in December, 2013. It will observe nearly one billion stars in the Milky Way and beyond along with many extragalactic objects such as quasars and galaxies. The analysis of quasar data will provide the optical counterpart of the International Celestial Reference Frame (ICRF). Also, the analysis of data for stars in our Galaxy provide a revolution in our understanding of Galactic dynamics, formation and evolution. The ICRF with its origin at the barycentre of the Solar System is based on distant quasars assumed to be static on the celestial sphere. With the expectation of a very large number of quasars from Gaia measurements, we study the effect of photocentric variability of quasars on the optical stability of the reference frame. The photocentric variability is modelled using a Markov chain model. In addition, there are many astrophysical and cosmological sources of proper motion. We review these effects of which the most significant are the secular aberration drift due to the acceleration of the Solar System, and the motion of the Sun relative to the Cosmic Microwave Background (CMB). Based on simulated data, the reference frame along with the Solar System acceleration is determined using an algorithm developed for the Gaia mission. We conclude that the photocentric variability of quasars does not have a very significant impact on the recovery of the reference frame. However, we notice a correlation between the frame parameters and the acceleration due to the inhomogeneous all-sky distribution of quasars. We also try to astrometrically determine our velocity relative to the CMB based on a cosmological model. Alternatively, if we assume that our velocity relative to the CMB is known from other missions, such as Planck, we can in principle measure the Hubble constant by astrometric means. This measurement is however very difficult and will require accurate centroiding on extended objects.