A Rest-frame K-band View of Bulge Growth in Massive Star-forming Galaxies at z 2

We study bulge formation in 16 massive dusty star-forming galaxies at z 2 by combining JWST MIRI F770W imaging, NIRCam imaging in eight bands, and ALMA 870 um data. At the median redshift of z=2.18, the F770W band probes the rest-frame 2.4 um light, close to the rest-frame K band, and traces the stellar mass distribution with little effect from dust attenuation. The effective radii measured in F770W agree with those of the 870 um dust emission, with a median ratio of 0.95, and are typically 23% smaller than those in F444W. The rest-frame K-band imaging is therefore essential to measure the true extent of the stellar mass, which is as compact as the dusty star-forming regions. In stacks of the 16 galaxies at a matched resolution, the 870 um and F770W profiles agree closely out to 8 kpc, and a bulge plus disk decomposition of the F770W stack gives a bulge-to-total ratio of 0.50. Fitting the SEDs in radial bins with CIGALE, we find that the attenuation decreases from Av=2.6 mag at the center to 1.5 mag at r 7 kpc, while the specific star formation rate profile is flat. In these galaxies the bulge is already in place, and the whole system grows toward compact quiescent galaxies within about 1 Gyr. In two galaxies, by contrast, the 870 um emission is 3 to 6 times more compact than the F770W light, which suggests that they are still in the bulge-building compaction phase. Our sample may thus catch massive galaxies in several phases of bulge formation.