We used the Plateau De Bure Interferometer to observe multiple CO and neutral carbon transitions in a z=2.2 main sequence disk galaxy , BX610 .
Our observation of CO ( 7-6 ) , CO ( 4-3 ) , and both far-infrared ( FIR ) [ C i ] lines complements previous observations of H \alpha and low-J CO , and reveals a galaxy that is vigorously forming stars with UV fields ( Log ( G G _ { 0 } ^ { -1 } ) \lesssim 3.25 ) ; although less vigorously than local ultra-luminous infrared galaxies or most starbursting submillimeter galaxies in the early universe .
Our observations allow new independent estimates of the cold gas mass which indicate M _ { \textrm { gas } } \sim 2 \times 10 ^ { 11 } M _ { \odot } , and suggest a modestly larger \alpha _ { \textrm { CO } } value of \sim 8.2 .
The corresponding gas depletion timescale is \sim 1.5 Gyr .
In addition to gas of modest density ( Log ( n cm ^ { 3 } ) \lesssim 3 ) heated by star formation , BX610 shows evidence for a significant second gas component responsible for the strong high-J CO emission .
This second component might either be a high-density molecular gas component heated by star formation in a typical photodissociation region , or could be molecular gas excited by low-velocity C shocks .
The CO ( 7-6 ) -to-FIR luminosity ratio we observe is significantly higher than typical star-forming galaxies and suggests that CO ( 7-6 ) is not a reliable star-formation tracer in this galaxy .