Detection of Galaxies with Gaia
  Besides its major objective tuned to the detection of the stellar galactic
population the Gaia mission experiment will also observe a large number of
galaxies. In this work we intend to evaluate the number and the characteristics
of the galaxies that will effectively pass the onboard selection algorithm of
Gaia. The detection of objects in Gaia will be performed in a section of the
focal plane known as the Sky Mapper. Taking into account the Video Processing
Algorithm criterion of detection and considering the known light profiles of
discs and bulges galaxies we assess the number and the type of extra-galactic
objects that will be observed by Gaia. We show that the stellar disk population
of galaxies will be very difficult to be observed. On the contrary the
spheroidal component of elliptical galaxies and bulges having higher central
surface brightness and steeper brightness profile will be more easy to be
detected. We estimate that most of the 20 000 elliptical population of nearby
galaxies inside the local region up to 170 Mpc are in condition to be observed
by Gaia. A similar number of bulges could also be observed although the low
luminosity bulges should escape detection. About two thirds of the more distant
objects up to 600 Mpc could also be detected increasing the total sample to
half a million objects including ellipticals and bulges. The angular size of
the detected objects will never exceed 4.72 arcsec which is the size of the
largest transmitted windows. An heterogeneous population of elliptical galaxies
and bulges will be observable by Gaia. This nearby Universe sample of galaxies
should constitute a very rich and interesting sample to study their structural
properties and their distribution.
