expected exponential cut-off in the luminosity function suggests that the probability of finding any such galaxy in CFBDSIR is extremely low. Balmer/4000Å break galaxies at z$\sim$2.5 do enter the fringe of our selected colour range; however, their $i'-J$ colour is typically below 5, ensuring they are detected in the $i'$-band CFBDS images and are then eliminated from our candidate list.

After this final visual check, we end up with about 1 ultracool brown dwarf candidate per square degree, which has to be confirmed with pointed NIR observations.

![Colour-magnitude diagram of all sources detected with a signal-to-noise greater than 10 in a 1 square-degree stacked WIRCam image from CFBDSIR. Since no other filtering is applied, these sources can be stars, galaxies, or detector artefacts. In this specific instance, none of the red objects is a brown dwarf. \[colpop\]](14277fg3.ps)

![Colour-colour diagram of known stars and brown dwarfs from spectral type O (deep-blue dots) to T (dark-red dots). The blue line shows the colours of high-redshift-starburst galaxies, while the red line refers to red Balmer-break galaxies. These lines follow a redshift evolution and specific redshift steps are indicated by numbers near the lines. The colours are synthesised from spectra, and take into account 