Simulated GAIA observations of galaxies

Simulated GAIA observations of galaxies, including flux maps obtained by their stacking, are presented. The simulations are obtained for the Astro-2 BBP and are based on HST WFPC2 data and on realistic assumptions about BBP performance with respect to electron count rate, PSF and noise. A comparison between the results obtained with the baseline sample size of 6◊8 pixels and the three smaller sizes of 4◊8, 6◊4 and 4◊4 pixels is carried out, and it is concluded that these latter choices, regardless of the higher readnoise per unit area, give both better resolution and more accurate photometry. It appears for instance that flux maps reconstructed from 50 observations carried out with 6 ◊ 4 pixels/sample would have a resolution of about 350 mas. A simple preliminary aperture photometry in V of HII regions and median surface photometry of various sky zones in the background were carried out in order to characterize the observed sky area containing a part of the M100 galaxy.