  [Abridged] We study the contribution of hot-dust to the luminosity density of
galaxies and its evolution with cosmic time. Using the Spitzer-IRAC data in the
COSMOS field, we estimate the contribution from hot-dust at rest-frame 4.2um
(from ~0 < z < ~0.2 up to ~0.5 < z < ~0.9). This wavelength corresponds to
black-body temperature of ~690K. The contribution due to stellar emission is
estimated from the rest-frame 1.6um luminosity (assumed to result from stellar
emission alone) and subtracted from the mid-infrared luminosity of galaxies to
measure hot-dust emission. In order to attempt the study of the 3.3um-PAH
feature, we use the rest-frame 4.2um to infer the hot-dust flux at 3.3um. This
study is performed for different spectral types of galaxies: early-type,
late-type, starburst, and IR-selected AGN. We find that: (a) the decrease of
the hot-dust luminosity density since ~0.5 < z < ~1 is steeper (by at least
~0.5dex) compared to that of the cold-dust, giving support to the scenario
where galaxy obscuration increases with redshift, as already proposed in the
literature; (b) hot-dust and PAH emission evolution seems to be correlated with
stellar mass, where rest-frame 1.6um luminous non-AGN galaxies (i.e., massive
systems) show a stronger decrement (with decreasing redshift) in hot-dust and
PAH emission than the less luminous (less massive) non-AGN galaxies; (c)
despite comprising < ~3% of the total sample, AGN contribute as much as a third
to the hot-dust luminosity density at z < 1 and clearly dominate the bright-end
of the total hot-dust Luminosity Density Function at ~0.5 < z < ~0.9; (d) the
average dust-to-total luminosity ratio increases with redshift, while
PAH-to-total luminosity ratio remains fairly constant; (e) at M_1.6 > -25, the
dust-to-total and PAH-to-total luminosity ratios increase with decreasing
luminosity, but deeper data is required to confirm this result.
