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. 
F. Boone, D. Schaerer, R. Pello, D. Lutz, A. Weiss, E. Egami, I.   Smail, M. Rex, T. Rawle, R. Ivison, N. Laporte, A. Beelen...
astro-ph.CO