Detection of galaxies with

Aims. Besides its major objective tuned to detecting 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 on-board selection algorithm of Gaia. Methods. The detection of objects in Gaia will be performed in a section of the focal plane known as the Sky Mapper. Considering the Video Processing Algorithm criterion of detection and the known light profiles of disc and bulges galaxies, we assess the number and the type of extra-galactic objects that will be observed by Gaia. Results. We show that the stellar disc population of galaxies will be very difficult to observe. In contrast, the spheroidal component of elliptical galaxies and bulges having higher central surface brightness and steeper brightness profile will be easier to detect. We estimate that most of the 20 000 elliptical population of nearby galaxies inside the local region up to 170 Mpc are in a state 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. Conclusions. A 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 for studying their structural properties and their distribution