2026/07/19 by Ken-ichi Tadaki
#astro-ph.GA
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\gtrsim7 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.