JWST sneaks a peek at the stellar morphology of $z\sim2$ submillimeter galaxies: Bulge formation at cosmic noon

Abstract: We report morphological analyses of seven submillimeter galaxies (SMGs) at $z\sim2$ using the JWST NIRCam images taken as part of the public CEERS and PRIMER surveys. Through two-dimensional surface brightness profile fittings we find evidence of bulges in all the sample SMGs, in particular at F444W filter, suggesting an ubiquitous presence of stellar bulges. The median size of these bulges at F444W is found to be 0.7$\pm$1.0 kpc and its median Sersic index is 0.7$\pm$0.9. Structures akin to spiral arms and bars are also identified, although 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 activities. Via the curve-of-growth analyses we deduce half-light radii for the NIRCam wavebands, finding that sizes are significantly smaller at longer wavelengths in all cases, in particular that the median size ratio between F444W and F150W is $0.6\pm0.1$. 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 ALMA, contrasting recent predictions from theoretical models. Our results suggest that while stellar bulges are undergoing an active formation phase in SMGs at $z\sim2$, the total stellar masses of SMGs are still dominated by their disks, not bulges.