Beyond the local volume. I. HST ultracool dwarfs

Here, we present the identification of 164 M7-T9 ultracool dwarfs in 0.6deg^2^ of deep, low-resolution, near-infrared spectroscopic data obtained with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) instrument as part of the WFC3 Infrared Spectroscopic Parallel Survey and the 3D-HST survey.

We describe the methodology by which we isolate ultracool dwarf candidates from over 200000 spectra, and show that selection by machine-learning classification is superior to spectral index-based methods in terms of completeness and contamination. We use the spectra to accurately determine classifications and spectrophotometric distances, the latter reaching to ~2kpc for L dwarfs and ~400pc for T dwarfs.