We present CO observations of nine ULIRGs at z \sim 2 with f _ { \nu } ( 24 \mu m ) \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 318 $ } } 1 mJy , previously confirmed with the mid-IR spectra in the Spitzer First Look Survey .
All targets are required to have accurate redshifts from Keck/GEMINI near-IR spectra .
Using the Plateau de Bure millimeter-wave Interferometer ( PdBI ) at IRAM , we detect CO J ( 3-2 ) [ 7 objects ] or J ( 2-1 ) [ 1 object ] line emission from eight sources with integrated intensities I _ { c } \sim ( 5 – 9 ) \sigma .
The CO detected sources have a variety of mid-IR spectra , including strong PAH , deep silicate absorption and power-law continuum , implying that these molecular gas rich objects at z \sim 2 could be either starbursts or dust obscured AGNs .
The measured line luminosity L ^ { { } ^ { \prime } } _ { CO ( 3 - 2 ) } is ( 1.28 – 3.77 ) \times 10 ^ { 10 } K km/s pc ^ { 2 } .
The averaged molecular gas mass M _ { H _ { 2 } } is 1.7 \times 10 ^ { 10 } M _ { \odot } , assuming CO-to-H _ { 2 } conversion factor of 0.8 M _ { \odot } / [ K km/s pc ^ { 2 } ] .
Three sources ( 33 % ) – MIPS506 , MIPS16144 & Â MIPS8342 – have double peak velocity profiles .
The CO double peaks in MIPS506 and MIPS16144 show spatial separations of 45 kpc and 10.9 kpc , allowing the estimates of the dynamical masses of 3.2 \times 10 ^ { 11 } sin ^ { -2 } ( i ) M _ { \odot } and 5.4 \times 10 ^ { 11 } sin ^ { -2 } ( i ) M _ { \odot } respectively .
The implied gas fraction , M _ { gas } / M _ { dyn } , is 3 % Â and 4 % , assuming an average inclination angle .
Finally , the analysis of the HST/NICMOS images , mid-IR spectra and IR SED revealed that most of our sources are mergers , containing dust obscured AGNs dominating the luminosities at ( 3 – 6 ) \mu m. Together , these results provide some evidence suggesting SMGs , bright 24 \mu m ULIRGs and QSOs could represent three different stages of a single evolutionary sequence , however , a complete physical model would require much more data , especially high spatial resolution spectroscopy .