Using Herschel PACS and SPIRE observations of Lockman Hole-North and GOODS-N as part of the HerMES project , we explore the far-IR properties of a sample of mid-IR selected starburst dominated ultra-luminous infrared galaxies ( ULIRGs ) at z \sim 2 .
The selection of the sample is based on the detection of the stellar bump that appears in the SED of star-forming galaxies at 1.6 \mu m. We derive robust estimates of infrared luminosities ( L _ { IR } ) and dust temperatures ( T _ { d } ) of the population and find that while the luminosities in our sample span less than an order of magnitude ( 12.24 \leq log ( L _ { IR } / L _ { \odot } ) \leq 12.94 ) , they cover a wide range of dust temperatures ( 25 \leq T _ { d } \leq 62 K ) .
Galaxies in our sample range from those that are as cold as high-z sub-millimeter galaxies ( SMGs ) to those that are as warm as optically faint radio galaxies ( OFRGs ) and local ULIRGs .
Nevertheless , our sample has median T _ { d } =42.3 K , filling the gap between SMGs and OFRGs , bridging the two populations .
We demonstrate that a significant fraction of our sample would be missed from ground based ( sub ) mm surveys ( 850-1200 \mu m ) showing that the latter introduce a bias towards the detection of colder sources .
We conclude that Herschel observations , confirm the existence of high-z ULIRGs warmer than SMGs , show that the mid-IR selection of high-z ULIRGs is not T _ { d } -dependent , reveal a large dispersion in T _ { d } of high-z ULIRGs , and provide the means to characterize the bulk of the ULIRG population , free from selection biases introduced by ground based ( sub ) mm surveys .