The ultracool-field dwarf luminosity-function and space density from the Canada-France Brown Dwarf Survey

Context. Thanks to recent and ongoing large scale surveys, hundreds of brown dwarfs have been discovered in the last decade. The Canada-France Brown Dwarf Survey is a wide-field survey for cool brown dwarfs conducted with the MegaCam camera on the Canada-France-Hawaii Telescope. Aims. Our objectives are to find ultracool brown dwarfs and to constrain the field brown-dwarf luminosity function and the mass function from a large and homogeneous sample of L and T dwarfs. Methods. We identify candidates in CFHT/MegaCam i' and z' images and follow them up with pointed near infrared (NIR) imaging on several telescopes. Halfway through our survey we found similar to 50 T dwarfs and similar to 170 L or ultra cool M dwarfs drawn from a larger sample of 1400 candidates with typical ultracool dwarfs i' - z' colours, found in 780 square degrees. Results. We have currently completed the NIR follow-up on a large part of the survey for all candidates from mid-L dwarfs down to the latest T dwarfs known with utracool dwarfs' colours. This allows us to draw on a complete and well defined sample of 102 ultracool dwarfs to investigate the luminosity function and space density of field dwarfs. Conclusions. We found the density of late L5 to T0 dwarfs to be 2.0(0.7)(+0.8) x 10(-3) objects pc(-3), the density of T0.5 to T5.5 dwarfs to be 1.4(-0.2)(+0.3) x 10(-3) objects pc(-3), and the density of T6 to T8 dwarfs to be 5.3(-2.2)(+3.1) x 10(-3) objects pc(-3). We found that these results agree better with a flat substellar mass function. Three latest dwarfs at the boundary between T and Y dwarfs give the high density 8.3(5.1)(+9.0) x 10(-3) objects pc(-3). Although the uncertainties are very large this suggests that many brown dwarfs should be found in this late spectral type range, as expected from the cooling of brown dwarfs, whatever their mass, down to very low temperature.