  Galaxies grow their supermassive black holes in concert with their stars,
although the relationship between these major galactic components is poorly
understood. Observations of the cosmic growth of stars and black holes in
galaxies suffer from disjoint samples and the strong effects of dust
attenuation. The thermal infrared holds incredible potential for simultaneously
measuring both the star formation and black hole accretion rates in large
samples of galaxies covering a wide range of physical conditions. Spitzer
demonstrated this potential at low redshift, and by observing some of the most
luminous galaxies at z~2. JWST will apply these methods to normal galaxies at
these epochs, but will not be able to generate large spectroscopic samples or
access the thermal infrared at high-redshift. An order of magnitude gap in our
wavelength coverage will persist between JWST and ALMA. A large, cold infrared
telescope can fill this gap to determine when (in cosmic time), and where
(within the cosmic web), stars and black holes co-evolve, by measuring these
processes simultaneously in statistically complete and unbiased samples of
galaxies to z>8. A next-generation radio interferometer will have the
resolution and sensitivity to measure star-formation and nuclear accretion in
even the dustiest galaxies. Together, the thermal infrared and radio can
uniquely determine how stars and supermassive blackholes co-evolve in galaxies
over cosmic time.
