HUBBLE SPACE TELESCOPE OBSERVATIONS OF FIELD ULTRACOOL DWARFS AT HIGH GALACTIC LATITUDE

We present a sample of 17 newly discovered ultracool dwarf candidates later than similar to M8, drawn from 231.90 arcmin(2) of Hubble Space Telescope Wide Field Camera 3 infrared imaging. By comparing the observed number counts for 17.5 <= J(125) <= 25.5 AB mag to an exponential disk model, we estimate a vertical scale height of z(scl) = 290 +/- 25 (random) +/- 31 (systematic) pc for a binarity fraction of f(b) = 0. While our estimate is roughly consistent with published results, we suggest that the differences can be attributed to sample properties, with the present sample containing far more substellar objects than previous work. We predict the object counts should peak at J(125) similar to 24 AB mag due to the exponentially declining number density at the edge of the disk. We conclude by arguing that trend in scale height with spectral type may breakdown for brown dwarfs since they do not settle onto the main sequence.