We report preliminary results of a deep near-infrared search for methane-absorbing brown dwarfs ; almost five years after the discovery of Gl 229b , there are only a few confirmed examples of this type of object .
New J band , wide-field images , combined with pre-existing R band observations , allow efficient identification of candidates by their extreme ( R-J ) colours .
Follow-up measurements with custom filters can then confirm objects with methane absorption .
To date , we have surveyed a total of 11.4 square degrees to J \sim 20.5 and R \sim 25 .
Follow-up CH _ { 4 } filter observations of promising candidates in 1/4 of these fields have turned up no methane absorbing brown dwarfs .
With 90 % confidence , this implies that the space density of objects similar to Gl 229b is less than 0.012 per cubic parsec .
These calculations account for the vertical structure of the Galaxy , which can be important for sensitive measurements .
Combining published theoretical atmospheric models with our observations sets an upper limit of \alpha \leq 0.8 for the exponent of the initial mass function power law in this domain .