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A Search for Molecular Gas toward a BzK-Selected Star-Forming Galaxy at z = 2.044


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 did not detect significant emission, and derived the 2σ limits: CO luminosity of L′co < 3.1 × 1010K kms-1 pc -2, ratio of far-infrared luminosity to CO luminosity of L FIR/L′co > 57 L⊙ (K km s-1 pc2)-1, and molecular gas mass of MH2 < 2.5 × 1010 M⊙, assuming a velocity width of 200 kms -1 and a CO-to-H2 conversion factor of αco = 0.8 M⊙ (K km s-1 pc2) -1. The ratio LFIR/L′CO, a measure of the star-formation efficiency (SFE), is comparable to, or higher than, the two z ∼ 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 ∼ 1.5 sBzKs. The higher SFEs and specific star-formation rates of the sBzKs compared to local spirals are indicative of a difference in the star-formation modes between these systems, suggesting that sBzKs are not just scaled-up versions of local spirals. © 2009. Astronomocal Society of Japan.

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