Searching for T dwarfs in the ρ Oph dark cloud L 1688

We present a list of T dwarf candidates in the dark cloud L 1688 in the rho Oph star-forming region. These candidates are selected with infrared colours sensitive to T dwarf characteristics of methane absorptions and of cool atmospheres. The 1.6-mu m methane feature is diagnosed by on-off imaging using an H-band and an intermediate-band methane filter, calibrated to a set of known brown dwarfs of M, L, and T types in the field. Another methane feature at 3.3 mu m is traced with the Spitzer/Infrared Array Camera (IRAC) [3.6] - [4.5] colour. For cool atmospheres, the H - [4.5] and K - [4.5] colours are utilized. With an additional criterion of mid-infrared brightness to eliminate extragalactic interlopers, a total of 28 T dwarf candidates have been identified. A comprehensive assessment was conducted to estimate the level of contamination of our sample by young stellar variability, by extragalactic sources sharing the same colour behaviour, or by foreground T dwarfs. Though extragalactic sources may contribute up to about half of the false positives, our candidates show close spatial association with the dark cloud, rather than randomly distributed as a background population would have been. Furthermore, even though our candidates are not selected a priori by a colour-magnitude relation, they mostly follow the 1 Myr isochrones, ascertaining their youth. Our selection methodology provides guidance to search for T dwarfs in other star-forming regions. Our candidate list, when comparing with those in the literature, which often rely on a single criterion on cool temperature or methane, is more conservative but should be more secure for follow-up spectroscopic confirmation of a T dwarf sample at the early evolutionary stage.