Detection of Satellite Remnants in the Galactic Halo with Gaia - II. A
  modified Great Circle Cell Method
  We propose an extension of the GC3 streamer finding method of Johnston et al.
1996 that can be applied to the future Gaia database. The original method looks
for streamers along great circles in the sky, our extension adds the
kinematical restriction that velocity vectors should also be constrained to lie
along these great circles, as seen by a Galactocentric observer. We show how to
use these combined criteria starting from heliocentric observables. We test it
by using the mock Gaia catalogue of Brown et al. 2005, which includes a
realistic Galactic background and observational errors, but with the addition
of detailed star formation histories for the simulated satellites. We
investigate its success rate as a function of initial satellite luminosity,
star formation history and orbit. We find that the inclusion of the kinematical
restriction vastly enhances the contrast between a streamer and the background,
even in the presence of observational errors, provided we use only data with
good astrometric quality (fractional errors of 30 per cent or better). The
global nature of the method diminishes the erasing effect of phase mixing and
permits the recovery of merger events of reasonable dynamical age. Satellites
with a star formation history different to that of the Galactic background are
also better isolated. We find that satellites in the range of 10^8-10^9 Lsun
can be recovered even for events as old as ~10 Gyr. Even satellites with
4-5x10^7 Lsun can be recovered for certain combinations of dynamical ages and
orbits.
