The Surface Densities of Disk Brown Dwarfs in JWST Surveys

TL;DR

This paper predicts the surface density and brightness distribution of ultracool dwarfs in JWST deep fields using disk models, showing they are rare at faint magnitudes and unlikely to contaminate high-redshift galaxy searches.

Contribution

It provides a simple formalism for predicting ultracool dwarf counts in JWST surveys based on disk models and luminosity functions.

Findings

Ultracool dwarfs peak around J~24 mag with ~0.3 arcmin^-2 density.

Dwarfs are very rare at J>27 mag, with densities ~0.005 arcmin^-2.

They are unlikely to contaminate high-redshift galaxy samples.

Abstract

We present predictions for the surface density of ultracool dwarfs (with spectral types M8-T8) for a host of deep fields that are likely to be observed with the James Webb Space Telescope. Based on simple thin and thick/thin disk (exponential) models, we show the typical distance modulus is mu~9.8 mag, which at high Galactic latitude is 5log(2 z_scl)-5. Since this is a property of the density distribution of an exponential disk, it is independent of spectral type or stellar sample. Using the published estimates of the ultracool dwarf luminosity function, we show that their number counts typically peak around J~24 mag with a total surface density of Sigma ~ 0.3 arcmin^-2, but with a strong dependence on galactic coordinate and spectral type. Owing to the exponential shape of the disk, the ultracool dwarfs are very rare at faint magnitudes (J>~27 mag), with typical densities of Sigma~0.005 arcmin^-2 (or ~20% of the total contribution within the field). Therefore in the very narrow and deep fields, we predict there are only a few ultracool dwarfs, and hence these stars are likely not a severe contaminant in searches for high-redshift galaxies. Furthermore the ultracool dwarfs are expected to be considerably brighter than the high-redshift galaxies, so samples near the faint-end of the high-redshift galaxy population will be the purest. We present the star-count formalism in a simplified way so that observers may easily predict the number of stars for their conditions (field, depth, wavelength, etc.).