Herschel reveals a T_dust-unbiased selection of z~2 ULIRGs
  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 ~ 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 1.6um. 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< log(L_IR/Lo) < 12.94), they cover a wide
range of dust temperatures (25< T_d < 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
(850-1200um) 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.
