The Surface Densities of Disk Brown Dwarfs in JWST Surveys

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 that the typical distance modulus is mu approximate to 9.8 mag, which at high Galactic latitude is 5 log(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 similar to 24 mag with a total surface density of Sigma similar to 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 similar to 0.005 arcmin(-2) (or similar to 20% of the total contribution within the field). Therefore, in 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.).