Constraining the distribution of L and T dwarfs in the galaxy

We estimate the thin-disk scale height of the Galactic population of L and T dwarfs based on star counts from 15 deep parallel fields from the Hubble Space Telescope. From these observations, we have identified 28 candidate L and T dwarfs based on their i′-z′ color and morphology. By comparing these star counts with a simple Galactic model, we estimate the scale height to be 350 ± 50 pc, which is consistent with the increase in vertical scale with decreasing stellar mass and is independent of reddening, color-magnitude limits, and other Galactic parameters. With this refined measure, we predict that less than 10⁹ M_⊙ of the Milky Way can be in the form of L and T dwarfs and confirm that high-latitude, z ≃6 galaxy surveys that use the i′-band dropout technique are 97%-100% free of L and T dwarf interlopers.