Dust-obscured star formation and the contribution of galaxies escaping
  UV/optical color selections at z~2
  A substantial fraction of the stellar mass growth across cosmic time occurred
within dust-enshrouded environments. Yet, the exact amount of star-forming
activity that took place in high-redshift dusty galaxies currently missed by
optical surveys has been barely explored. Using the Spitzer observations of
COSMOS we determined the fraction of luminous star-forming galaxies at 1.5<z<3
escaping the traditional color selection techniques because of dust extinction,
as well as their contribution to the cosmic star formation density at high
redshift. We find that the BzK criterion offers an almost complete (~90%)
identification of the 24mic sources at 1.4<z<2.5, while the BM/BX criterion
miss 50% of the MIPS population. Similarly the criterion based on the presence
of a stellar bump in massive sources (so-called "IRAC peakers") miss up to 40%
of the IR luminosity density and only 25% of the IR energy density at z~2 is
produced by Optically-Faint IR-bright galaxies selected based on their extreme
mid-IR to optical flux ratios. We conclude that color selections of distant
star-forming galaxies must be used with lots of care given the substantial bias
they can suffer. In particular, the effect of dust extinction strongly impacts
the completeness of identifications at the bright end of the bolometric
luminosity function, which implies large and uncertain extrapolations to
account for the contribution of dusty galaxies missed by these selections. In
the context of forthcoming facilities that will operate at long wavelengths
(e.g., $JWST$, ALMA, SAFARI, EVLA, SKA), this emphasizes the importance of
minimizing the extinction biases when probing the activity of star formation in
the early Universe.
