Constraints on the space density of methane dwarfs and the substellar massfunction from a deep near-infrared survey

We report preliminary results of a deep near-infrared search for methane-ab sorbing brown dwarfs; almost 5 yr after the discovery of G1 229b, there are only a few confirmed examples of this type of object. New J-band, wide-fie ld images, combined with preexisting R-band observations, allow efficient i dentification of candidates by their extreme (R-J) colors. Follow-up measur ements with custom filters can then confirm objects with methane absorption , To date, we have surveyed a total of 11.4 deg(2) to J similar to 20.5 and R similar to 25. Follow-up CH, filter observations of promising candidates in one-fourth of these fields have turned up no methane-absorbing brown dw arfs. With 90% confidence, this implies that the space density of objects s imilar to G1 229b is less than 0.012 pc(-3) These calculations account for the vertical structure of the Galaxy, which can be important for sensitive measurements. Combining published theoretical atmospheric models with our o bservations sets an upper limit of alpha less than or equal to 0.8 for the exponent of the initial mass function power law in this domain.