3.3.1 Spatial Distribution of Galaxies with GAIA

The primary scientific requirement is for a very wide angle survey, reaching into the ``zone of avoidance'' al low Galactic latitudes, with a well-defined selection function. Such data are not available from ground-based imaging surveys, as star-galaxy separation to the required reliability cannot be achieved without high spatial resolution imaging. Simulations (see 5.6) indicate that the number of galaxies which can be detected and for which useful broad-band photometry can be obtained is of order 3 millions, corresponding to a magnitude limit of $I\simeq17$ for a typical galaxy. Fainter and more compact galaxies around will instead not be detected in very large numbers due to the short integration time and relatively high readnoise. Detected galaxies will provide a measurement, through deconvolution of the measured angular power spectrum, of the spectrum of fluctuations well beyond the expected peak. Such data are both a natural complement to the ongoing redshift surveys, and also provide an input catalogue for future extensions of those surveys. The very great volume surveyed locally both makes the survey an important local normalisation and potentially allows study of the largest scale lengths, without evolutionary complications.

A primary science case for such studies arises from the difficulty in understanding the peculiar motion of the Local Group. It is well-established that the Local Group has a peculiar motion of about 600 km/s towards $(l,b)=(268^{\circ},27^{\circ})$. If our understanding of the gravitational instability picture for the growth of structure, and measurements of $\Omega_0$ and biasing are valid, this must be explicable as acceleration by identifiable galaxy clusters, or massive single galaxies. The largest of these sources, especially the Great Attractor and Perseus-Pisces, remain poorly mapped, being at low Galactic latitudes.

A second scientific goal concerns the amplitude, shape and length of structure in the Local Universe. Large filaments, walls, and the Supergalactic Plane dominate the nearby galaxy distibution. All are lost, with present data, within $20^\circ$ of the galactic Plane. It is not yet even clear if the Supergalactic Plane is a plane at all. It we are to understand the local large-scale structure, a reliable nearly all-sky galaxy survey is essential. At low Galactic latitudes random errors in star-galaxy classification, due to seeing, convolved with the numerical predominance of stars, prevent construction of such a catalogue, The combination of GAIA spatial resolution and multi-color photometry will allow substantially improved analyses.