The Canada-France deep fields survey. I: 100,000 galaxies, 1 deg2: a precise measurement of omega(theta) to I(AB) ~ 25

Using the University of Hawaii's 8K mosaic camera (UH8K), we have measured the angular correlation function omega (theta ) for 100 000 galaxies distributed over four widely separated fields totalling ~ 1deg ² and reaching a limiting magnitude of I_AB(3sigma ,3'') ~ 25.5. This unique combination of areal coverage and depth allows us to investigate the dependence of omega (theta ) at 1arcmin , A_omega (1arcmin ), on sample median magnitude in the range 19.5<I_AB-med<24. Furthermore, our rigorous control of systematic photometric and astrometric errors means that fainter than I_AB-med ~ 22 we measure omega (theta ) on scales of several arc-minutes to an accuracy of 30%. Our results show that A_omega (1arcmin ) decreases monotonically to I_AB ~ 25. At bright magnitudes, omega (theta ) is consistent with a power-law of slope delta = -0.8 for 0.2arcmin <theta <3.0arcmin but at fainter magnitudes we detect a slope flattening with delta ~ -0.6. At the 3sigma level, our observations are still consistent with delta =-0.8. We also find a clear dependence of A_omega (1arcmin ) on observed (V-I)_AB colour. In the magnitude ranges 18.5<I_AB<24.0 and 18.5<I_AB<23.0 we find galaxies with 2.6<(V-I)_AB<2.9 (the reddest bin we consider) have A_omega (1arcmin )'s which are ~ 10x higher than the full field population. On the basis of their similar colours and clustering properties, we tentatively identify these objects as a superset of the ``extremely red objects'' found through optical-infrared selection. We demonstrate that our model predictions for the redshift distribution for the faint galaxy population are in good agreement with current spectroscopic observations. Using these predictions, we find that for low-Omega cosmologies and assuming a local galaxy correlation length r_0=4.3 h<sup>-1</sup> Mpc, in the range 19.5<I<sub>AB-med</sub><22, the growth of galaxy clustering (parameterised by epsilon ), is epsilon ~ 0. However, at 22<I<sub>AB-med</sub><24.0, our observations are consistent with epsilon >~ 1. Models with epsilon ~ 0 cannot simultaneously match both bright and faint measurements of A<sub>omega</sub> (1arcmin ). We show how this result is a natural consequence of the ``bias-free'' nature of the ``epsilon '' formalism and is consistent with the field galaxy population in the range 22.0<I_AB<24.0 being dominated by galaxies of low intrinsic luminosity.

Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut des Sciences de l'Univers (INSU) of the Centre National de la Recherche Scientifique and the University of Hawaii, and at the Cerro Tololo Inter-American Observatory and Mayall 4-meter Telescopes, divisions of the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation.