Constraining the thermal dust content of Lyman break galaxies in an overdense field at z ≈ 5

We have carried out 870-mu m observations in the J1040.7-1155 field, known to host an overdensity of Lyman break galaxies at z = 5.16 +/- 0.05. We do not detect any individual source at the S-870 mu m = 3.0 mJy beam(-1) (2 sigma) level. A stack of nine spectroscopically confirmed z > 5 galaxies also yields a non-detection, constraining the submillimetre flux from a typical galaxy at this redshift to S-870 mu m < 0.85 mJy, which corresponds to a mass limit M-dust < 1.2 x 10(8) M-circle dot (2 sigma). This limits the mass of thermal dust in distant Lyman break galaxies to less than one-tenth of their typical stellar mass. We see no evidence for strong submillimetre galaxies associated with the ultraviolet-selected galaxy overdensity, but cannot rule out the presence of fainter, less massive sources.