Has JWST Seen Everything? Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at z 6 - 9 with ALMA2040

JWST/NIRCam has recovered the stellar counterparts of many ALMA-detected, formerly HST-dark galaxies, yet ALMA has started identifying dusty star-forming galaxies (DSFGs) at z>6 that remain extremely faint or undetected even with NIRCam. These "NIRCam-dark" DSFGs may trace a missing rapid-growth phase connecting UV-luminous galaxies at z>10 to massive quiescent galaxies at z 4. So far, the small, heterogeneously selected NIRCam-dark sample is insufficient to determine their abundance or role in the evolution of massive galaxies. We show that a blind 1.3-mm continuum survey with ALMA2040 can provide the first high-significance census of this elusive NIRCam-dark DSFG population across z 6 - 9. We conduct sensitivity and survey-speed calculations requiring a 10σ detection of a NIRCam-dark DSFG with a dust-obscured star-formation rate of 60,M, yr-1 at z = 8. At a like-for-like 7.5-GHz normalization, ALMA2040 will reach the required 1.3-mm RMS of 21,μ Jy beam-1 about 90 times faster than current ALMA. With 100 hr on-source, ALMA2040 can cover an area of about0.5 deg squared and detect N about16 NIRCam-dark DSFGs over 7.5<z<9, assuming a fiducial number density n=3 times10-6 cMpc-3. Dividing the survey among independent fields with deep JWST coverage would mitigate cosmic variance and enable immediate NIRCam, MIRI, and spectroscopic characterization of their environments. The at least512-GHz instantaneous bandwidth expected for ALMA2040 will also detect [C II] line at a wide redshift range, supplementing the continuum discovery with redshift confirmation. Such a survey will test whether NIRCam-dark DSFGs trace a missing rapid-growth phase between the earliest UV-luminous galaxies at z>10 and the rapid emergence of massive quiescent systems at z 4.