Ultracool subdwarfs: metal-poor stars and brown dwarfs extending into the late-type M, L and T dwarf regimes
Recent discoveries from red optical proper motion and wide-field near-infrared surveys have uncovered a new population of ultracool subdwarfs -- metal-poor stars and brown dwarfs extending into the late-type M, L and possibly T spectral classes. These objects are among the first low-mass stars and brown dwarfs formed in the Galaxy, and are valuable tracers of metallicity effects in low-temperature atmospheres. Here we review the spectral, photometric, and kinematic properties of recent discoveries. We also examine L subdwarf classification, and discuss how a large, complete sample of substellar halo subdwarfs could probe the star formation history of the Galaxy's Population II. We conclude by outlining a roadmap for future work to more thoroughly explore this old and very low-mass population.