New Constraints On Dust Emission And UV Attenuation Of Z=6.5-7.5 Galaxies From Millimeter Observations

Context. Determining the dust properties and UV attenuation of distant star-forming galaxies is of great interest for our understanding of galaxy formation and cosmic star formation in the early Universe. However, few direct measurements exist so far. Aims. To shed new light on these questions, we have targeted two recently discovered Lyman-break galaxies (LBGs) at z approximate to 6.8 and z = 7.508 to search for dust continuum and [C II] 1158 pm line emission. Methods. The strongly lensed z approximate to 6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703 and the spectroscopically confirmed z = 7.508 LBG z8-GND-5296 in the GOODS-N field were observed with the Plateau de Bure Interferometer (PdBI) at 1.2 mm. These observations were combined with those of three z > 6.5 Ly alpha emitters (HCM6A, Himiko, and IOK-1), for which deep measurements were recently obtained with the PdBI and ALMA. Results. [C II] is undetected in both galaxies, providing a deep upper limit of L-[C II] < 2.8 x 10(7) L for A1703-zD1, comparable to the nondetections of Himiko and IOK-1 with ALMA. Dust continuum emission from A1703-zD1 and z8-GND-5296 is not detected with an rms of 0.12 and 0.16 mJy/beam. From these nondetections and earlier multiwavelength observations we derive upper limits on their IR luminosity and star formation rate, dust mass, and UV attenuation. Thanks to strong gravitational lensing, the achieved limit for A1703-zDI is similar to those achieved with ALMA, probing below the luminous infrared galaxy (LIRG) regime (L-IR < 8.1 x 10(10) L-circle dot) and very low dust masses (M-d < 1.6 x 10(7) M-circle dot). We find that all five galaxies are compatible with the Calzetti IRX-beta relation, their UV attenuation is compatible with several indirect estimates from other methods (the UV slope, extrapolation of the attenuation measured from the IR/UV ratio at lower redshift, and spectral energy distribution fits), and the dust-to-stellar mass ratio is compatible with that of galaxies from z = 0 to 3. From their stellar mass, the high-z galaxies studied here have an attenuation below the one expected from the mean relation of low-redshift (z less than or similar to 1.5) galaxies. Conclusions. More and deeper (sub-)mm data are clearly needed to directly determine the UV attenuation and dust content of the dominant population of high-z star-forming galaxies and to establish their dependence on stellar mass, redshift, and other properties more firmly.