Mid- and far-infrared deep surveys of the universe: Toward finding hidden populations of galaxies and active galactic nuclei Original Research Article

Optical and near-infrared (NIR) deep surveys have been very powerful for detecting very faint sources in the high-redshift (z) universe. However, the observed surface density of such high-z, star-forming galaxies may be lower than that expected from the observed density of luminous galaxies in the present-day (i.e., local) universe. This suggests that we are missing classes of objects with substantial reddening and have not yet sampled fully the high-z counterparts of typical nearby galaxies. In fact, growing evidence for hidden populations, perhaps at high redshift, has been reported for these years based on very deep mid- (MIR) and far-infrared (FIR) observations of the universe. We give a brief summary of these very exciting observations and then discuss future observational strategies to study new hidden populations at very high redshift.