We present a search for CO ( 3–2 ) emission in SDF-26821 , a BzK -selected star-forming galaxy ( sBzK ) at z = 2.044 , using the 45-m telescope of the Nobeyama Radio Observatory and the Nobeyama Millimeter Array .
We do not detect significant emission and derive 2 \sigma limits : the CO luminosity of L ^ { \prime } _ { CO } < 3.1 \times 10 ^ { 10 } K km s ^ { -1 } pc ^ { -2 } , the ratio of far-infrared luminosity to CO luminosity of L _ { FIR } / L ^ { \prime } _ { CO } > 57 \LO ( K km s ^ { -1 } pc ^ { -2 } ) ^ { -1 } , and the molecular gas mass of M _ { { H _ { 2 } } } < 2.5 \times 10 ^ { 10 } \MO , assuming a velocity width of 200 km s ^ { -1 } and a CO-to-H _ { 2 } conversion factor of \alpha _ { CO } = 0.8 \MO ( K km s ^ { -1 } pc ^ { -2 } ) ^ { -1 } .
The ratio of L _ { FIR } / L ^ { \prime } _ { CO } , a measure of star formation efficiency ( SFE ) , is comparable to or higher than the two z \sim 1.5 sBzKs detected in CO ( 2–1 ) previously , suggesting that sBzKs can have a wide range of SFEs .
Comparisons of far-infrared luminosity , gas mass , and stellar mass among the sBzKs suggest that SDF-26821 is at an earlier stage of forming stars with a similar SFE and/or more efficiently forming stars than the two z \sim 1.5 sBzKs .
The higher SFEs and specific star formation rates of the sBzKs compared to local spirals are indicative of the difference in star formation modes between these systems , suggesting that sBzKs are not just scaled-up versions of local spirals .