Evidence of the unusually faint 3 mm continuum of SMGs in the 4C 23.56 field. (a): SEDs for SMGs (T_d = 35K, β = 1.5, 2, 2.8, z = 2.48) with flux scale to the stacked 850 μm flux. The observed 3 mm flux can be only reproduced when β ∼ 2.8. (b): S/N map of the stacked image for the 38 SMGs with a distance between 1′ and 4′ from the HzRG. A negative ring is shown around the central source. The white circle denotes the MUSTANG-2 beam size. (c): Azimuthally averaged S/N profile of the stacked source with the corresponding standard deviations in each annulus.