Herschel reveals a T-dust-unbiased selection of z similar to 2 ultraluminous infrared galaxies

International audience; Using Herschel Photodetector Array Camera (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) observations of Lockman Hole-North and Great Observatories Origins Deep Survey-North (GOODS-N) as part of the Herschel Multi-tiered Extragalactic Survey (HerMES) project, we explore the far-infrared (IR) properties of a sample of mid-IR-selected starburst-dominated ultraluminous infrared galaxies (ULIRGs) at z similar to 2. The selection of the sample is based on the detection of the stellar bump that appears in the spectral energy distribution 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 \textless= log(L-IR/L-circle dot) \textless= 12.94), they cover a wide range of dust temperatures (25 \textless= T-d \textless= 62 K). Galaxies in our sample range from those that are as cold as high-z submillimetre 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.