Optical-faint, far-infrared-bright Herschel sources in the CANDELS fields: Ultra-luminous infrared galaxies at z > 1 and the effect of source blending

摘要

The Herschel very wide-field surveys have charted hundreds of square degrees in multiple far-IR(FIR) bands. While the Sloan Digital Sky Survey (SDSS) is currently the best resource for opticalcounterpart identifications over such wide areas, it does not detect a large number of Herschel FIRsources and leaves their nature undetermined. As a test case, we studied seven ''SDSS-invisible'', verybright 250 m sources (S250 > 55 mJy) in the Cosmic Assembly Near-infrared Deep ExtragalacticLegacy Survey (CANDELS) fields where we have a rich multi-wavelength data set. We took a newapproach to decompose the FIR sources, using the near-IR or the optical images directly for positionpriors. This is an improvement over the previous decomposition efforts where the priors are frommid-IR data that still suffer from the source blending problem in the first place. We found that inmost cases the single Herschel sources are made of multiple components that are not necessarily at thesame redshifts. Our decomposition succeeded in identifying and extracting their major contributors.We show that these are all ULIRGs at z 1 - 2 whose high LIR is mainly due to dust-obscuredstar formation. Most of them would not be selected as sub-mm galaxies. They all have complicatedmorphologies indicative of merger or violent instability, and their stellar populations are heterogeneousin terms of stellar masses, ages and formation histories. Their current ULIRG phases are of variousdegrees of importance in their stellar mass assembly. Our practice provides a promising starting pointto develop an automatic routine to reliably study bright Herschel sources.