Far infrared constraints on the contamination by dust obscured galaxies
  of high-z dropout searches
  The spectral energy distributions (SED) of dusty galaxies at intermediate
redshift may look similar to very high redshift galaxies in the optical/near
infrared (NIR) domain. This can lead to the contamination of high redshift
galaxy searches based on broad band optical/NIR photometry by lower redshift
dusty galaxies as both kind of galaxies cannot be distinguished. The
contamination rate could be as high as 50%. {This work shows how the far
infrared (FIR) domain can help to recognize likely low-z interlopers in an
optical/NIR search for high-z galaxies.} We analyse the FIR SEDs of two
galaxies proposed as very high redshift ($z>7$) dropout candidates based on
deep Hawk-I/VLT observations. The FIR SEDs are sampled with PACS/Herschel at
100 and 160\,$\mu$m, with SPIRE/Herschel at 250, 350 and 500\,$\mu$m and with
LABOCA/APEX at 870\,$\mu$m. We find that redshifts $>7$ would imply extreme FIR
SEDs (with dust temperatures $>100$\,K and FIR luminosities
$>10^{13}$\,$L_{\odot}$). At z$\sim$2, instead, the SEDs of both sources would
be compatible with that of typical ULIRGs/SMGs. Considering all the data
available for these sources from visible to FIR we re-estimate the redshifts
and we find $z\sim$1.6--2.5. Due to the strong spectral breaks observed in
these galaxies, standard templates from the literature fail to reproduce the
visible-near IR part of the SEDs even when additional extinction is included.
These sources resemble strongly dust obscured galaxies selected in Spitzer
observations with extreme visible-to-FIR colors, and the galaxy GN10 at $z=4$.
Galaxies with similar SEDs could contaminate other high redshift surveys.
