ALESS--JWST: Dust-driven Morphologies and Hidden Stellar Mass in $z\sim3$ Sub-millimeter Galaxies

TL;DR

This study uses JWST and ALMA imaging to analyze the morphologies and stellar mass distribution in $z\sim3$ sub-millimeter galaxies, revealing dust-driven biases and wavelength-dependent structural effects.

Contribution

It demonstrates how dust obscuration affects morphological measurements and stellar mass estimates, emphasizing the importance of longer-wavelength observations for accurate galaxy structure analysis.

Findings

Dust obscuration causes size inflation and stellar-dust offsets at shorter wavelengths.

Rest-frame 1.5-3μm MIRI imaging reveals compact stellar structures matching dust continuum.

Intrinsic stellar mass and dust continuum sizes are consistent, supporting dense, obscured star formation models.

Abstract

We present JWST/NIRCam and MIRI observations of twelve $z\sim3$ sub-millimeter galaxies (SMGs) from the ALESS survey, combined with high-resolution ($0.08''-0.16''$) ALMA 870$\mu$m imaging, enabling spatially resolved SED fitting on $\sim$kpc scales. We find a resolved star-forming main sequence linking surface densities of star formation rate and stellar mass, suggesting star formation remains tightly coupled to local mass distribution even in obscured systems. Our resolved SED analysis reveals a systematic stellar mass bias in integrated fits, even including rest-frame $\sim2\mu$m MIRI imaging. Rather than classical `outshining', this is mainly driven by spatially varying dust attenuation, indicating a `dust-obscuration bias' that causes obscured stellar mass to be missed. We show SMG morphologies are wavelength-dependent. At rest-frame optical wavelengths, central obscuration produces stellar-dust offsets and inflated sizes, while at longer wavelengths these effects diminish. The rest-frame $\sim1.5-3\mu$m MIRI imaging is less affected by dust than NIRCam and reveals compact stellar structures matching the 870$\mu$m dust continuum. We find centrally concentrated dust attenuation drives both offsets and size variations, demonstrating dust geometry is the main driver of structural diversity. Consequently, morphologies from rest-frame wavelengths $\lesssim1.6\mu$m can be biased without longer-wavelength constraints. The intrinsic stellar mass and dust continuum sizes are consistent ($R_\mathrm{e,870\mu m}/R_\mathrm{e,\ast}=1.0\pm0.4$), supporting a picture in which SMGs host compact, obscured star formation that builds dense stellar cores, consistent with evolution into massive quiescent galaxies. We suggest such obscured structures and associated biases may also be common among massive star-forming galaxies at $z\gtrsim1$, implying these effects are likely of broad relevance.