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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
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
.
If our understanding of the gravitational instability picture for the growth
of structure, and measurements of 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
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.
Next: 3.3.2 Galaxy Surface Photometry
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Mattia Vaccari
2000-12-05