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We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of theglobular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD)clearly shows the main sequence extending towards the expected end of theHydrogen-burning limit and going beyond this point towards fainter sources. Thewhite dwarf sequence can be identified. As such, this is the deepest NIR CMD ofa globular cluster to date. Archival HST optical data were used forproper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs)from brown dwarf (BD) candidates. Detection limits in the NIR are around F110Wapprox 26.5 mag and F160W approx27 mag, and in the optical around F775W approx28 mag. Comparing our observed CMDs with theoretical models, we conclude thatwe have reached beyond the H-burning limit in our NIR CMD and are probably justabove or around this limit in our optical-NIR CMDs. Thus, any faint NIR sourcesthat have no optical counterpart are potential BD candidates, since the opticaldata are not deep enough to detect them. We visually inspected the positions ofNIR sources which are fainter than the H-burning limit in F110W and for whichthe optical photometry did not return a counterpart. We found in total fivesources for which we did not get an optical measurement. For four of these fivesources, a faint optical counterpart could be visually identified, and an upperoptical magnitude was estimated. Based on these upper optical magnitude limits,we conclude that one source is likely a WD, one source could either be a WD orBD candidate, and the remaining two sources agree with being BD candidates. Foronly one source no optical counterpart could be detected, which makes thissource a good BD candidate. We conclude that we found in total four good BDcandidates.

Deep near-IR observations of the Globular Cluster M4: Hunting for Brown Dwarfs