Conclusions

The GAIA Galaxy Survey is a nearly all-sky, magnitude-limited, multi-color astrometric and photometric galaxy survey, to be carried out with the GAIA satellite, proposed to ESA for launch in 2009 as Cornerstone 5 of its Horizons 2000 scientific programme. In the framework of the present mission design, in this study the feasibility, scientific case and optimization of the GAIA Galaxy Survey were discussed.

From both statistical considerations and numerical simulations it appears that galaxies would best be detected in the ASM1 within square areas of $2 \times 2$ arcsec$^2$ and observed in the BBPs with a sample size of $6\times4$ pixels. The first choice should yield the highest number of detected galaxies without too may false detections, whereas the second one yields the best trade-off between angular resolution and accuracy in surface photometry.

Galaxy observations could therefore be carried out in Astro 2, where a sample size of $6\times4$ pixels is not in conflict with the baseline sample size of $6\times8$ pixels adopted for the observation of stars, with only a small effort in terms of mission design and telemetry.

Under present assumptions about the instrumental performance of the satellite payload, the following measurement capabilities are expected from a 5-year mission:

• About 3 million galaxies brighter than $I\simeq17$ will be detected.
• All detected galaxies will be observed with a 0.4 arcsec angular resolution and an all-mission accuracy in surface photometry of 0.15 mag/arcsec$^2$ at 19.5 mag/arcsec$^2$ in the $V$ band.
• Multi-color (4-5 broad bands) and multi-epoch ($\simeq50$ epochs) information will be available for all observed objects.

These outstanding measurement capabilities will result in unique datasets about galaxy spatial distribution and surface photometry over a nearly magnitude-limited sample extending down to low Galactic latitudes. In turn, these data are expected to yield significant scientific results concerning the large-scale structure of the Local Universe and the multi-color photometric structure of galaxy innermost regions.

The project being in its infancy, several developments are desirable in the near future in order to further demonstrate its feasibility, including:

• The design, implementation and testing of a dedicated galaxy detection algorithm is required in order to discuss the efficiency of galaxy detection and the issues related to star-galaxy discrimination, which could result in too many false detections.
• The refinement of the adopted stacking technique so as to fully recover GAIA optical resolution, e.g. through a drizzling-like technique.
• The discussion of galaxy observation in the PSM, where the much higher sensitivity with respect to the BBPs would result in a much higher accuracy in surface photometry.
• The statistical modeling of the properties of astrophysical objects whose existence could be induced on the basis of GAIA galaxy observations, such as massive black holes at the center of galaxies.

In its present form already, however, the GAIA Galaxy Survey promises to yield fundamental contributions to the study of external galaxies, thus complementing the core scientific case of the GAIA mission.