Stellar populations in the CFHTLS. I. New constraints on the IMF at low mass

15 pages The original paper with the figures at high resolution can be found under the anonymous ftp (ftp.iap.fr: pub/from_users/schulthe/cfhtls.pdf)We present a stellar populations analysis of the first release of the CFHTLS (Canada-France-Hawai Telescope Legacy Survey) data. A detailed comparison between the Besan\çon model of the Galaxy and the first data release of the CFHTLS-Deep survey is performed by implementing the MEGACAM photometric system in this model using stellar atmosphere model libraries. The reliability of the theoretical libraries to reproduce the observed colours in the MEGACAM system is investigated. The locations of various stellar species like subdwarfs, white dwarfs, late-type and brown dwarfs, binary systems are identified. The contamination of the stellar sample by quasars and compact galaxies is quantified using spectroscopic data from the VIMOS-VLT Deep Survey (VVDS) as a function of $i\'$ magnitude and $r\'-i\'$ colour. A comparison between simulated counts using the standard IMF at low masses show that the number of very low mass dwarfs may have been underestimated in previous studies. These observations favour a power law IMF following $d(n)/dm \\propto m^{-\\alpha}$ with $\\alpha=2.5$ for $m < $ 0.25 \\Msun or $\\alpha=3.0$ for $m < $ 0.2 \\Msun for single stars. The resulting LF is in agreement with the local LF as measured from the 5 or 25 pc samples. It is in strong disagreement with the Zheng et al (2001) LF measured from deep HST data. We show that this discrepancy can be understood as an indication of a different IMF at low masses at early epochs of the Galaxy compared to the local thin disc IMF