Breathing Fire: Hot Dust in the Big Three Dragons at z = 7.15

We present new Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 (lambda obs = 0.45, mm) and 4 (lambda obs = 2.2, mm) observations towards the Big Three Dragons, a pair of merging Lyman-break galaxies (LBGs) at z=7.15. The system was previously detected in ALMA Bands 6, 7 and 8 (lambda obs = 0.73 - 1.32, mm), which, combined with our new observations, allows us to robustly constrain its dust temperature and obscured SFR. The unresolved Band 4 observations yield a 3.5 sigma detection, and the 0.4'' Band 9 observations detect the Eastern and Western LBGs at 3.7 and 3.5 sigma, respectively. Through modified blackbody fitting, we infer a global dust temperature of T d = 78-23+35, K for the system, which implies a high IR luminosity of (L IR/L) = 12.32-0.41+0.43. This makes the Big Three Dragons one of the most IR-luminous systems known at z>7, with a total SFR UV+IR = 267-153+418,M, yr-1 that is almost completely obscured (f obs = 0.94-0.09+0.04). Using resolved ALMA observations in Bands 6, 8 and 9, we confirm both LBGs have hot dust temperatures (T d about 67 - 84, K) and correspondingly high obscured fractions (f obs about 0.88-0.95). We find the Western LBG to fall about1, dex above the canonical IRX- beta UV relation, suggesting patchy dust obscuration. The compact Eastern component, on the other hand, is consistent with a Calzetti- or SMC-like dust screen within the uncertainties. Together with the similarly hot dust temperature recently reported for the z=8.31 galaxy MACS0416-Y1, our results suggest a non-negligible fraction of star formation at the bright end of the UV luminosity function is highly dust-obscured, even at z 7.