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

Abstract

Abstract We present a search for CO (3–2) emission in SDF-26821, a $BzK$-selected star-forming galaxy (sBzK) at $z$$=$ 2.044, using the 45-m telescope of the Nobeyama Radio Observatory and the Nobeyama Millimeter Array. We did not detect significant emission, and derived the 2$\sigma$ limits: CO luminosity of $L^\prime_{\rm {CO}}$ 3.1 $\times$ 10$^{10}$Kkms$^{-1}$pc$^{-2}$, ratio of far-infrared luminosity to CO luminosity of $L_{\rm {FIR}}/L^\prime_{\rm {CO}}$$>$ 57 (Kkms$^{-1}$pc$^{2}$)$^{-1}$, and molecular gas mass of $M_{\rm {H_2}}$ 2.5 $\times$ 10$^{10}$, assuming a velocity width of 200 kms$^{-1}$ and a CO-to-H$_2$ conversion factor of $\alpha_{\rm {CO}}$$=$$0.8$ (Kkms$^{-1}$pc$^{2}$)$^{-1}$. The ratio $L_{\rm {FIR}}/L^\prime_{\rm {CO}}$, a measure of the 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 a difference in the star-formation modes between these systems, suggesting that sBzKs are not just scaled-up versions of local spirals.