Those galaxies with bright cusps within the central GAIA PSF can be analysed as astrometric targets, in the same way as stars. In this case, which will include many active galactic nuclei, astrometric ``jitter'', i.e. a motion of the photocenter, may be detectable, if a significant contribution to the flux is spatially variable, e.g. due to the appearance or displacement of a feature in the source or to activity in the core region.
While specific modeling will be required on a case-by-case basis, correlation of motion of the photocenter with optical variability in Seyfert nuclei and quasars can test if nuclear starburst supernovae are a significant luminosity source. In case where no jitter is seen, these sources define the reference frame. The relative location of the optical nuclei and the active nuclei can also be measured with high angular accuracy.
Photometric studies of bright galaxies allow detailed analyses of internal structures such as spiral arms and star formation regions. For fainter, and less well-resolved galacies, simper analyses of such parameters as bulge-to-disk ratio and central photometric cusp or nucleus structure are appropriate ([Okamura et al. 1999]). For the faintest useful images, model-based two-dimensional image analyses, typically involving maximum likelihood comparison of the images data with a set of simple models, have been developed and applied, especially to surveys as the HST MDS ([Ratnatunga et al. 1999]). These methodologies are capable of reliable analysis of galaxy images with signal-to-noise ratios comparable to those GAIA will produce for galaxies to magnitude . A crucial advantage of the GAIA Galaxy Survey over other studies is that it will automatically be nearly all-sky and magnitude-limited. The selection function of the observed sample of galaxies will be well-defined in terms of galaxy angular size and surface brightness.
The scientific value of this huge and homogeneous database will impact all fields of galaxy research, naturally complementing the several redshift surveys and the deeper pencil-beam studies with very large telscopes. Among the most important unique GAIA science products will be the determination of the color and photometric structure in the central regions of a nearly magnitude-limited sample of relatively bright galaxies. Recent studies of early-type ([Faber et al. 1997]) as well as late-type galaxies ([Carollo 1999]) begin to address the wealth of structure in the central regions of galaxies. Early-type galaxies are crudely distributed into cores which are flat or are steeply rising, perhaps indicative of the effects of massive central black holes ([van der Marel 1999]). Late-type galaxies, instead, show an extreme diversity of central structures, probably providing the key to bulge and central disk formation. On the whole, galaxy core structures span a surface brightness range of over 10 mag/arcsec ([Carollo 1999]). GAIA will provide three key elements: high and uniform spatial resolution, a large sample, and multi-color data.
Simultaneous multi-color light curves, albeit sparsely sampled, will naturally be obtained for every galaxy observed. Thus the statistical incidence of active galactic nuclei and related variability will be determined as a function of the photometric structure of the host bulge or inner disk. Other types of variable sources will also be detected, ranging from novae in the Local Group through Cepheids and luminous blue variables in more distant galaxies, to supernovae, and possible gamma-ray bursts. Rapid analysis of the photometric data during the mission will allow the identification of variable sources for dedicated follow-up by other telescopes.