ALESS–JWST: Dust-driven Morphologies and Hidden Stellar Mass in z ∼ 3 Submillimeter Galaxies

We present JWST/NIRCam and MIRI observations of 12 z ∼ 3 submillimeter galaxies (SMGs), combined with high-resolution (0 . ″ 08–0 . ″ 16) ALMA 870 μ m imaging, enabling spatially resolved spectral energy distribution (SED) fitting on ∼kiloparsec scales. We find a resolved star-forming main sequence linking the surface densities of star formation rate and stellar mass, suggesting that star formation remains coupled to the local mass distribution even in heavily obscured systems. Our resolved SED analysis reveals a stellar mass bias in integrated fits (median ∼0.2 dex offset), despite including rest-frame ∼2 μ m MIRI imaging. Rather than classical “outshining,” this effect is driven by spatially varying dust attenuation: a “dust-obscuration bias” that causes obscured stellar mass to be missed. SMG morphologies are strongly wavelength-dependent. At rest-frame optical wavelengths, central obscuration produces large stellar-dust spatial offsets and inflated sizes, while at longer wavelengths these effects diminish. Rest-frame ∼1.5–3 μ m MIRI imaging is less affected by dust than NIRCam and reveals compact stellar structures closely matching the 870 μ m dust continuum. Centrally concentrated dust attenuation drives both the apparent spatial offsets and size variations, demonstrating that dust geometry is the primary driver of structural diversity. Consequently, morphologies inferred from rest-frame wavelengths ≲1.6 μ m can be significantly biased without longer-wavelength constraints. The intrinsic stellar mass and dust continuum sizes are consistent ( R e,870 μ m / R e,* = 1.0 ± 0.4), supporting a picture in which SMGs host compact, heavily obscured star formation that builds dense stellar cores, consistent with their evolution into massive quiescent galaxies. Such obscured structures and associated biases may be common among massive star-forming galaxies at z ≳ 1, making these effects broadly relevant.