We report deep K-band ( 18-27 GHz ) observations with the 100-m Green Bank Telescope of HCN ( 1 - 0 ) line emission towards the two submillimeter-selected galaxies ( SMGs ) SMM J02399 - 0136 ( z = 2.81 ) and SMM J16359 + 6612 ( z = 2.52 ) .
For both sources we have obtained spectra with channel-to-channel rms noise of \sigma \leq 0.5 mJy , resulting in velocity-integrated line fluxes better than \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle < $ } \kern - 6.0 pt \lower 1.72 pt \hbox { { $% \scriptstyle \sim$ } } } 0.1 Jy km s ^ { -1 } , although we do not detect either source .
Such sensitive observations – aided by gravitational lensing of the sources – permit us to put upper limits of L ^ { \prime } _ { \mbox { \tiny { HCN } } } \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle < $ } % \kern - 6.0 pt \lower 1.72 pt \hbox { { $ \scriptstyle \sim$ } } } 2 \times 10 ^ { 10 } K km s ^ { -1 } pc ^ { 2 } on the intrinsic HCN ( 1 - 0 ) line luminosities of the two SMGs .
The far-infrared ( FIR ) luminosities for all three SMGs with sensitive HCN ( 1 - 0 ) observations to date are found to be consistent with the tight FIR-HCN luminosity correlation observed in Galactic molecular clouds , quiescent spirals and ( ultra ) luminous infrared galaxies in the local Universe .
Thus , the observed HCN luminosities remain in accordance with what is expected from the universal star formation efficiency per dense molecular gas mass implied by the aforementioned correlation , and more sensitive observations with today ’ s large aperture radio telescopes hold the promise of detecting HCN ( 1 - 0 ) emission in similar objects in the distant Universe .