Unrecognized Astrometric Confusion in the Galactic Centre
  The Galactic Centre is a highly crowded stellar field and frequent
unrecognized events of source confusion, which involve undetected faint stars,
are expected to introduce astrometric noise on a sub-mas level. This confusion
noise is the main non-instrumental effect limiting the astrometric accuracy and
precision of current near-infrared imaging observations and the long-term
monitoring of individual stellar orbits in the vicinity of the central
supermassive black hole. We self-consistently simulate the motions of the known
and the yet unidentified stars to characterize this noise component and show
that a likely consequence of source confusion is a bias in estimates of the
stellar orbital elements, as well as the inferred mass and distance of the
black hole, in particular if stars are being observed at small projected
separations from it, such as the star S2 during pericentre passage.
Furthermore, we investigate modeling the effect of source confusion as an
additional noise component that is time-correlated, demonstrating a need for
improved noise models to obtain trustworthy estimates of the parameters of
interest (and their uncertainties) in future astrometric studies.
