Hubble Space Telescope Observations of Field Ultracool Dwarfs at High Galactic Latitude

TL;DR

This study uses Hubble Space Telescope infrared imaging to discover 17 ultracool dwarf candidates at high galactic latitude, estimating their vertical distribution and discussing implications for brown dwarf populations.

Contribution

The paper presents a new sample of ultracool dwarfs and provides an estimate of their galactic scale height, highlighting differences from previous studies due to sample properties.

Findings

Estimated scale height of 290 ± 25 pc for ultracool dwarfs.

Sample contains more substellar objects than previous work.

Predicted peak in object counts at J_{125}~24 AB mag.

Abstract

We present a sample of 17 newly discovered ultracool dwarf candidates later than ~M8, drawn from 231.90 arcmin2 of {\it 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) +- 30 (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}~24 AB mag due to the exponentially-declining number density at the edge of the disc. 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.