Molecular Gas in Submillimetre-Faint, Star-Forming Ultraluminous
  Galaxies at z>1
  [abridged] We present interferometric CO observations of twelve z~2
submillimetre-faint, star-forming radio galaxies (SFRGs) which are thought to
be ultraluminous infrared galaxies (ULIRGs) possibly dominated by warmer dust
(T_dust ~> 40 K) than submillimetre galaxies (SMGs) of similar luminosities.
Four other CO-observed SFRGs are included from the literature, and all
observations are taken at the Plateau de Bure Interferometer (PdBI) in the
compact configuration. Ten of the sixteen SFRGs observed in CO (63%) are
detected at >4sigma with a mean inferred molecular gas mass of ~2*10^10 M_sun.
SFRGs trend slightly above the local ULIRG L_FIR-L'_CO relation. Since SFRGs
are about two times fainter in radio luminosity but exhibit similar CO
luminosities to SMGs, this suggests SFRGs are slightly more efficient star
formers than SMGs at the same redshifts. SFRGs also have a narrow mean CO line
width, 320+-80km/s. SFRGs bridge the gap between properties of very luminous
>5*10^12 L_sun SMGs and those of local ULIRGs and are consistent with
intermediate stage major mergers. We suspect that more moderate-luminosity
SMGs, not yet surveyed in CO, would show similar molecular gas properties to
SFRGs. The AGN fraction of SFRGs is consistent with SMGs and is estimated to be
0.3+-0.1, suggesting that SFRGs are observed near the peak phase of star
formation activity and not in a later, post-SMG enhanced AGN phase. This CO
survey of SFRGs serves as a pilot project for the much more extensive survey of
Herschel and SCUBA-2 selected sources which only partially overlap with SMGs.
Better constraints on CO properties of a diverse high-z ULIRG population are
needed from ALMA to determine the evolutionary origin of extreme starbursts,
and what role ULIRGs serve in catalyzing the formation of massive stellar
systems in the early Universe.
