We present the discovery of three late type ( \geq T4.5 ) brown dwarfs , including a probable Y dwarf , in the WFC3 Infrared Spectroscopic Parallels ( WISP ) Survey .
We use the G141 grism spectra to determine the spectral types of the dwarfs and derive distance estimates based on a comparison with nearby T dwarfs with known parallaxes .
These are the most distant spectroscopically confirmed T/Y dwarfs , with the farthest at an estimated distance of \sim 400 pc .
We compare the number of cold dwarfs found in the WISP survey with simulations of the brown dwarf mass function .
The number found is generally consistent with an initial stellar mass function dN / dM \propto M ^ { - \alpha } with \alpha = 0.0–0.5 , although the identification of a Y dwarf is somewhat surprising and may be indicative of either a flatter absolute magnitude/spectral type relation than previously reported or an upturn in the number of very late type brown dwarfs in the observed volume .