JWST Sneaks a Peek at the Stellar Morphology of z ∼ 2 Submillimeter Galaxies: Bulge Formation at Cosmic Noon

Abstract We report morphological analyses of seven submillimeter galaxies (SMGs) at z ∼ 2 using the James Webb Space Telescope NIRCam images taken as part of the public CEERS and PRIMER surveys. Through two-dimensional surface brightness profile fitting we find evidence of compact reddened stellar structures in all the SMGs, in particular in the F444W filter, suggesting an ubiquitous presence of stellar bulges. The median size of these bulges at F444W with a bootstrapped uncertainty is found to be 0.7 ± 1.0 kpc (0.6–0.7–3.9 kpc for 14th–50th–86th percentiles) and the median Sérsic index is 0.7 ± 0.9 (0.4–0.7–2.8 for 14th–50th–86th percentiles). Structures akin to spiral arms and bars are also identified, and their asymmetric shapes, tidal features, as well as evidence of nearby galaxies at consistent redshifts as those of corresponding SMGs suggest that these SMGs are undergoing dynamical interactions, likely responsible for the triggering of their star-forming activity. Via a curve-of-growth analysis we deduce half-light radii for the NIRCam wave bands, finding that sizes are significantly smaller at longer wavelengths in all cases, in particular that the median size ratio between F444W and F150W is 0.6 ± 0.1. However, we also find that F444W sizes, roughly corresponding to rest-frame H band, are not smaller than those of submillimeter continuum as measured by the Atacama Large Millimeter/submillimeter Array, contradicting certain recent predictions from theoretical models. Our results suggest that while stellar bulges are undergoing an active formation phase in SMGs at z ∼ 2, the total stellar masses of SMGs are still dominated by their disks, not bulges.