Four Faint T Dwarfs from the UKIRT Infrared Deep Sky Survey (UKIDSS) Southern Stripe

Abstract

We present the optical and near-infrared photometry and spectroscopy of four faint T dwarfs newly discovered from the UKIDSS first data release. The sample, drawn from an imaged area of ~136 square degrees to a depth of Y=19.9 (5-sigma, Vega), is located in the SDSS Southern Equatorial Stripe, a region of significant future deep imaging potential. We detail the selection and followup of these objects, three of which are spectroscopically confirmed brown dwarfs ranging from type T2.5 to T7.5, and one is photometrically identified as early T. Their magnitudes range from Y=19.01 to 19.88 with derived distances from 34 to 98 pc, making these among the coldest and faintest brown dwarfs known. The sample brings the total number of T dwarfs found or confirmed by UKIDSS data in this region to nine, and we discuss the projected numbers of dwarfs in the future survey data. We estimate that ~240 early- and late-T dwarfs are discoverable in the UKIDSS LAS data, falling significantly short of published model projections and suggesting that IMFs and/or birthrates may be at the low end of possible models. Thus, deeper optical data has good potential to exploit the UKIDSS survey depth more fully, but may still find the potential Y dwarf sample to be extremely rare.