Galaxy population and molecular gas content of Planck-selected protocluster candidates at z~1.3–3

The most productive star factories in the Universe are the progenitors of galaxy clusters at the nodes of cosmic filaments, the so called protoclusters.  The extended nature of these extraordinary sites and their bright sub-millimeter emission make them detectable by the Planck satellite. We have observed a sample of 18 of these Planck-selected protocluster candidates at millimeter wavelengths with the IRAM 30-m telescope to reveal their molecular gas reservoirs and the mechanisms at the origin of their prodigious star formation rates.  We detected 40 CO lines in 24 bright Herschel sources over 14 fields at an average redshift of 2.25.  The CO redshifts support the idea that some of these fields contain protoclusters. The CO-detected sources are mainly normal SFGs, with only ∼20% undergoing a starburst phase.  Their large star formation rates, ~1000 M⊙/yr on average, are supported by large (a few 10^11 M⊙) cold gas reservoirs.  The CO spectral line energy distribution, available for ten sources implies low gas excitation in most of the cases.  These properties hint to extended and cold molecular gas reservoirs at low excitation and a steady fueling mechanism.  Millimeter and CO observations at higher spatial resolution with NOEMA and ALMA have recently been obtained for two sources to test the proposed fueling scenario, and gain more insights on the origin of their prodigious star formation activity.  The implications of these findings on the formation scenario of the most massive passive galaxies found in local clusters will also be discussed.